m 'M' I.*,r: .·.."'T r. :,2 :lfr~TAT. I B'll . A , I . I. of the FLORIDA STATE MUSEUM Biological Sciences Volume 26 1981 Number 3 BIOLOGY OF THE YELLOW-SHOULDERED BLACKBIRD - AGELAIUS ON A TROPICAL ISLAND WILLIAM POST - 11 - ~ - ./=.* . - 4 '* 'a .' f"F r : & *ftit.t{ 99,~r. m. m 4 9 4' ft*/, /2' » »X, 8 /~ 'pi'../. 5 2 ..04 0 0 Fl ilk, 1~ . 911} F {i, f 4*1* 1'10 «, 4 4,~, A - » ..4 , A 26«4*1 . UNIVERSITY OF FLORIDA GAINESVILLE Numbers of the BULLETIN OF THE FLORIDA STATE MUSEUM, BIOLOGICAL SCIENCES, are published at irregular intervals. Volumes contain varying numbers of pages and. are not necessarily completed in any one calendar year. OLIVER L . AUSTIN , JR ., Editor RHODA J. BRYANT, Managing Editor Consultants for this issue: FRANCES C. JAMES EDWIN O. WILLIS Communications concerning purchase or exchange of the publications and alr manuscripts should be addressed to: Managing Editor, Bulletin; Florida State Museum; University of Florida; Gainesville, Florida 32611. Copyright © 1981 by the Florida State Museum of the University of Florida. This public document was promulgated at an annual cost of $4,074.00 or $4.07 per copy. It makes available to libraries, scholars, and all interested persons the results of researches in the natural sciences, emphasizing the circum-Caribbean region. Publication date: November 23, 1981 Price: $4.10 BIOLOGY OF THE YELLOW-SHOULDERED BLACKBIRD-AGELAIUS ON A TROPICAL ISLAND WILLIAM POSTI SYNOPSIS: The Yellow-shouldered Blackbird (Agelaius xanthomus) nests in mangroves, seat- tered trees in pastures, and in suburbs. In the most frequently used habitat, mangroves, birds use isolated cays or scattered trees in salt pans. Nests are either open cups placed on branches or are built in cavities. The breeding season starts with the spring rains in April and May. These rains stimulate new vegetative growth, which may result in an increase in the numbers of insects on which the blackbirdi feed. Most young are in the nest during the relatively dry summer period and fledge before the fall rains begin in September or October. Pairs form 6-10 weeks before egg laying within groups of birds that visit nesting areas of previous years. During the mate acquisition period males display at old nest sites, which the females visit. Males follow and guard females. The extended period of affiliation before mating may be advantageous to the female if it assures male loyalty, as male parental care is important in a species with such widely separated feeding and nesting sites. All males and females studied during a 2-year period were monogamous. The average clutch is three eggs. The incubation period is 12.5 days, and hatching is asyn- chronous. Only females incubate and brood, but some males deliver food to their incubating mates. Males guard the nest when the female is away. Both sexes deliver food at the same rate. Delivery rates are like those of North Temperate icterids. The growth pattern of A. xan- thomus is the same as that of Red-winged Blackbirds (A phoeniceus). After fiedging, young follow adults for at least 24 days. Nest success (proportion of nests producing at least one young) was 46 % (54 nests). Nests on offshore cays had higher success than those in adjacent mainland areas, because they were parasitized less by Shiny Cowbirds (Molothrus bonariensis). Pairs nesting in cavities fledged about three times as many young per nest as did those nesting in the open. Mortality during the egg stage outweighed that during the nestling stage, mainly because of disruption by cowbirds. The average distance between nests in colonies was 16 m. Two pairs occasionally nested in the same tree. Nesting aggregations seemed to result from active attraction among birds, rather than common response to limited nest sites. Communal mobbing was a conspicuous behavior of the nesting groups. The space around the nest consistently defended.against con- specifics and other species was small, extending 3 m in any direction. Males defended the nest vicinity more often than females. The species forages mainly in the upper strata of trees, obtaining arthropods by probing and gleaning epiphytes, leaf clusters, and the surfaces of branches and trunks. This use of foraging behaviors more typical of orioles and woodpeckers may reflect the scarcity of com- petitors in habitats where the blackbirds are common, in combination with lack of suitable ground-foraging sites (marshes) in Puerto Rico. Most food brought to nestlings is gathered up to 2 km from nests, but commuting pairs do not follow each other to scattered foraging sites. 1The author is an Adjunct Associate Curator of Ornithology at the Florida State Museum. University of Florida. Gaines- ville, Florida 32611 POST, WILLIAM. 1981. Biology of the Yellow-shouldered Blackbird-Agelaius on a tropi- cal island. Bull. Florida State Mus., Biol. Sci. 26(3):125-202. 126 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 The Yellow-shouldered Blackbird feeds on the ground as do other Agelaius, but unlike its marsh-dwelling congeners, when ground-feeding it gathers mostly vegetable material rather than arthropods. When delivering to nests, adults carry more than one item in their beal0.1, ld.f.) The survival to adulthood of 29 individuals that were marked as juveniles was 65.5 % (Table 15). Again birds that were not seen at least once after capture were excluded from the analysis, but if all the juveniles captured during this period are included, the survival rate is 59.4 % . The juveniles considered here were caught in their first winter or spring, and the survival value is for one year from capture, i.e. through their second year. During the same period that I caught these 32 juveniles (February- May 1974; Table 15), I also took 158 adults. This gives a juvenile:adult ratio of 0.203/1.000, which agrees fairly closely with the estimated an- nual adult mortality of 17.6 %. This ratio is also close to the 0.210/1.000 ratio among the 92 birds collected throughout Puerto Rico that I exam- ined in museums. In the nesting seasons of 1974 and 1975 I color-marked 29 fledglings. Eight of these were later seen flying about, independent of their parents. Although the sample is small, it gives an estimated fledgling to juvenile survival rate of 27.6 %. The product of the survival rates of fledglings to 172 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 juvenilehood and of juveniles to adulthood is 18.1%, which again is close to the average annual adult mortality rate of 17.6 %. Determining survival by sighting color-marked individuals is more ac- curate than any system dependent on recaptures, which are often biased by the vagaries of trapping (Grosskopf 1964; Coulson and Wooller 1976). This system is particularly effective with Yellow-shoulders because they are sedentary and conspicuous, and the census stations, widely spaced in nesting and feeding areas, were manned throughout the year. DISCUSSION. - The success of open, unparasitized nests at La Parguera is similar to that Marchant (1960) reported for nine open-nesting passerines in an arid tropical region of Ecuador, where nest success was 51 % and overall egg success was 44 % (versus 63 % and 35 % for Yellow- shoulders). Open, cowbird-parasitized nests at La Parguera had a lower success than the above, and their success was similar to that of some species that occupy cleared edges of humid tropical forests, where partial losses related to such factors as brood parasitism are high. For example, the Clay-colored Robin had nest success of 25 % and egg success of 16 %, while, as with parasitized Yellow-shoulder nests, its fledgling/nestling success was relatively high: 53% (Skutch 1966). No information is available on the reproductive success of cavity- nesting birds in the arid tropics, but the success of cavity-nesting Yellow- shoulders at La Parguera exceeds the average success of 16 species of hole nesters in the humid tropics (Ricklefs 1969). These species had a nest sue- cess of 54 %, and 44 % of their eggs produced fledglings, in comparison to 86 % and 63 % for the Yellow-shoulder cavity nests, In comparison to icterid nests in the temperate zone of North America, Yellow-shouldered Blackbird nests not parasitized by cowbirds have lower success in the percent hatched of eggs laid, but higher fledg- ling/nestling success. In addition, overall nest success was higher in the Yellow-shoulder. Robertson (1972) found that Red-winged Blackbirds nesting in marshes (where there was no cowbird parasitism) had nest sue- cess of 53%, fledgling/nestling success of 65 % and fledgling/egg success of 44 %. In continental icterid populations, nestlings appear to be subjected to more predation, and because of larger clutch size may experience greater starvation, both of these effects increasing with age of nestlings (Smith 1943, Young 1963, Robertson 1972). Egg loss in the Yellow- shoulder greatly outweighs loss 6f nestlings, while in temperate zone Red- winged Blackbird populations mortality is fairly constant throughout the nest period. Red-winged Blackbirds nesting in fresh water marshes in Costa Rica had a nest success of 21.5 % (Orians 1973), in comparison to 27 % for Yellow-shouldered Blackbird open nests (Table 10). As with Costa Rican 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 173 Red-wings, most of the Yellow-shoulder losses during the nestling stage were due to starvation. Whole nest losses, occurring during both egg and nestling stages, and usually by predation, were more important in Costa Rica than in Puerto Rico, but partial losses in Puerto Rico, directly or in- directly attributable to brood parasitism, nearly equaled whole-nest loss in Costa Rican Red-wings. As expected, cavity-nesting Yellow-shouldered Blackbirds at La Parguera had much higher nest success than open- nesting Red-winged Blackbirds in Costa Rica. In comparison to estimates of survival rates based on similarly gathered data for Red-winged Blackbirds, Yellow-shoulders have a higher rate of survival. Data for 325 Red-wings marked and retrapped in coastal Massachusetts gave an annual (weighted) adult survival rate of 53.4 % (Fankhauser 1967). Significantly lower than my weighted estimate of 69.8% (xe = 9.6; p<.005, 1 d.f.) for the Yellow-shoulder. Another estimate of Red-wing survival generated from shootings of banded birds throughout North America gave a figure of 51.1% (Fankhauser 1967) . High survival rates appear to be characteristic of some tropical bird species, although further research, particularly in disturbed neotropical areas, needs to be done. On Sarawak, Fogden (1972) found a minimum annual survival rate of 86 % for a composite sample of species occupying forest habitats. Snow and Lill (1974) estimated minimum annual survival for the White-bearded Manakin (Manacus manacus), a species occupying tropical forests in Trinidad, to be 89 %. Although my estimate is for a tropical species occupying open, disturbed habitat, for which category there appear to be no other survival estimates, some of the factors con- tributing to the increased longevity of tropical forest birds must apply to species in disturbed habitats, i.e. stable climate, buffered food supply, and lack of migration. BEHAVIOR HOME RANGE-The marked birds that I studied seem to be permanent residents around La Parguera, and I detected no seasonal movem6nts by any significant proportion of the population. Some individuals that I had marked on the coast were seen inland during the non-breeding season; 11 birds (6 males and 5 females) appeared occasionally in flocks of 50-70 Yellow-shoulders that fed around the cattle barns at the Lajas Experimen- tal Station, 7.3 m N of La Parguera. During this time several of these birds were also seen on the coast, where they apparently returned ilightly to roosts. Individual birds ranged widely (Table 16; Fig. 7), but I detected no seasonal differences in size or usage pattern of their home ranges. This is probably due to (1) the year-round use of communal roosts, which were usually distant from nesting and feeding areas (Fig. 1); (2) year-round use 174 B U LLE T IN F LO R ID A S TATE M U S E U M V ol. 26, N o. 3 TABLE 16. - Home ranges and distances between captures. Males Females N x Range N x Range Home ranges (kmz) 26 2.56*0.502 0.27-13.34 12 1.73=i: 0.36 0.39-3.70 Distances between recaptures (m)1 200 1325 f 95.22 25-5440 200 1630*131.5 25-7260 'Only distances between sightino or recaptures at successively different localities counted. 2Standard error of the mean. b * 067 *a ~___0~5-062 <:s~*-DI 31/1 - dJ 0 0£32 /$/ 00 -ho0 .c, -~:° \ 0 11 1981 PO ST: Y E LLO W -S H O U LD E R E D B LA C K B IR D B IO LO G Y 175 2 1. 8 8 9 * .f-Za d.4 A" «/.r-77 10~ fo 0 9 B 0% 5 1 km | FIGURE 7.-Home ranges of Yellow-shouldered Blackbirds. For localities, refer to Fig. 1. Black dots indicate sighting or capture points. Single dots mean one instance; multiple instances are indicated by numbers. Open arrows indicate nesting sites, with year; solid arrow is roost site. (a) male GYG-A; (b) male BA-RG; (c) female YA-XY; (d) female RYR-A. 176 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 31 of communal feeding grounds such as those on La Cueva and El Guayachn; and (3) distant feeding behavior. The large sizes of the home ranges are reflected in the distance be- tween successive recaptures. Distances between recaptures (Table 16) are reasonable estimates of daily movements: birds using the La Cueva and El Guayachn feeding grounds often flew to one of.the La Parguera roosts (Fig. 1), making daily round-trip flights of 10-12 km. Other individuals used feeding station in La Parguera and flew nightly 5.5 km to a roost in Bahia Montalva E of the study area. Most of the marked birds whose nests I located used winter feeding grounds near their nest sites (Fig. 7). The main nesting places, Salinas Arcelay and Salinas Carlos, were 1600 m and 800 m from the nearest monkey feeders on El Guayachn and La Cueva. At least seven birds marked as nestlings used one of these feeding grounds the winter after they hatched. The average distance between their hatching places and winter feeding grounds was 1549 m, range 980-2680 m. FOOD AND FORAGING. - During the nesting season Yellow-shouldered Blackbirds brought two types of food to dependent young; the bulk of the food was arthropods, the remainder vegetable matter (Table 17). Arth- ropods were mainly gathered in the canopy and subcanopy layers of trees, while vegetable matter was obtained by birds visiting feeding sites of domestic animals. The main bulk of the 25 food samples that I took from young birds consisted of large items such as wood-boring beetles (Buprestidae), with an average length of 11 mm, tree crickets (Gryllidae), averaging 20.6 mm, larvae and pupae of moths (Olethreutidae and Noc- tuidae), the larvae averaging 11.9 mm and pupae averaging 14.1 mm, and arboreal spiders (Anyphaenidae) with a mean length of 9.1 mm. Observations of foraging sites and tactics of the blackbirds (Table 18) and the behavior of the arthropods showed that the blackbirds get most of their food by probing into crevices in trunks and branches, epiphytes, and leaf clusters. Most of the forms brought to the young are either always hidden (Buprestidae and immature Olethreutidae) or are nocturnal and hide during the day in crevices, leaf clusters, or epiphytes (Noctuidae, Gryllidae, and Anyphaenidae). Among the noctuids collected from nest- ling A. xanthomus, Melipotis sp. were common. Larvae of these moths feed in the tree canopy at night and during the day move down the trunk to hide in crevices in the bark, which is also screened by surrounding shrubs. They also pupate in these sites (D. H. Habeck, pers. comm.). Im- mature Olethreutidae, the most common food delivered to young blackbirds, infested leaf buds of red mangroves, where I often saw Yellow-shoulders probing. Small arthropods such as leafhoppers (Fulgoridae), about 2 mm, were occasionally taken (Table 17). These insects infested patches of scrub such 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 177 as Bumelia spp. One leafhopper, Petrusa epilepsis, was utilized by groups of wafblers and was so abundant that foraging birds flushed 5-10 insects with each m6ve (Post 1978). I saw Yellow-shoulders take these only four times. The long distances that nesting blackbirds often flew to gather food for their young may have precluded their bringing such small items to the -- nests (Emlen 1966), and they may have been used mainly by the adults that encountered them while searching for larger items. Yellow- shouldered Blackbirds may have delivefed small items more frequently than my samples indicate because small pieces of food may slip past neck collars (Orians 1966). When delivering to the young, adults usually carried more than one arthropod at a time. They also regurgitated food from their gullets, mainly plant material and occasional small fragments of arthropods. I determined this by collecting food from the throats of some collared young immediately after I saw a parent regurgitate material to them. Vegetable matter appeared in 15 of the 25 food samples I collected (Table 17): rice or grain (7 out of 25 samples), bread or flour products (5 of 25), and monkey chow (4 of 25). The average representation bj) volume of vegetable material in the 15 samples was 29 %, range 5-100 %. Birds nesting offshore usually foraged in trees in pastures along the mangroves, flying up to 2 km between these sites and their nests. I also saw Cay-nesting birds foraging in red mangroves near their nests. Birds nesting in red mangroves occasionally fed on the roots and trunks, but a cursory examination of these sites revealed few arthropods, possibly because of the high tannin content of red mangrove bark. In salinas the birds did not forage on the open mud around their nests. Birds nesting in salinas occasionally foraged on the trunks and branches of dead mangroves, although they most often flew relatively long distances , (0.5-1 km) to forage in trees in pastures. These birds, as well as those nesting in the pastures themselves, did not forage for arthropods on the ground there either. The foraging behavior of Yellow-shoulders varied. On trunks and branches they used their bills to probe crevices, flake off pieces of bark, and enlarge holes. Yellow-shoulders gaped inside crevices, fruit, buds, and cocoons. Individuals clung to the sides of vertical trunks or to the undersides of branches to probe or to glean surfaces. Birds gleaned leaves and twigs as they walked along branches, but more often they flew be- tween leaf clusters , They commonly probed epiphytes ( Tillandsia recur- vata), usually standing on top of the tangled balls of vegetation and inserting their beaks up to the base of their skulls in the clumps, but sometimes they hung under the clumps to probe the undersurfaces. Other species I saw probing the Tillandsia recuruata clumps were Adelaide's Warblers (Dendroica adelaide) and Black-cowled Orioles (Icterus TABLE 17. - Food brought to young in nests at La Parguera, 1974-1975. 178 B U LLE T IN FLO R ID A S TATE M U S E U M Vol. 26, N o. 3 'Length (mm)Order or Group Number of samplesi Total No. Family and stage in which found found Average Range Lepidoptera Olethreutidae, larvae 14 (56)2 46 (28.2) 10.4*3.6 5-22 pupae 4 (16) 4 (2.5) 7.0 7 Noctuidae, larvae 4 (16) 10 (6.1) 18.7*1.6 15-20 pupae 3 (12) 31 (19.0) 15.0*0 15 Unidentified pupa 1 (4) 1 (0.6) 7.0 - Araneae Anyphaenidae 10 (40) 16 (9.8) 9.1=i= 1.5 6-12 Salticidae 5 (20) 6 (3.7) 7.4 5-12 Araneidae 2 (8) 2 (1.2) 7.5 5-10 Unidentified 5 (20) 5 (3.1) 7.6 6-10 Orthoptera Gryllidae 7 (28) 16 (9.8) 20.6*7.0 5-30 Blattidae 1 (4) 1 (0:6) 6.0 - Homoptera Fulgoridae 2 (8) 17 (10.4) 2.0 2 Tropiduchidae 1 (4) 1 (0.6) piece - Coleoptera Buprestidae 2 (8) 3 (1.8) 11.0 10-13 Elateridae 1 (4) 1 (0.6) 10.0 - Unidentified 1 (4) 1 (0.6) piece - 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 179 , 0 1 (0. 6) pi ec e 1 1 e ' 8 E S (09) ST-- 25 sa m pl e is th e fo od m a ll yo un g ne st d u ng 1 h p er io d. ar en th es es a rc en ta ge s o to la |. ,ER. ric e an d 1- no nk ey ~ w , S ~E .8 F.-M E 1 K# 1 : 7 5 C ~2 5i 5 7 0 y 7 ,le 92# TABLE 18.- Foraging behavior of the Yellow-shouldered Blackbird during breeding season (May-September).1 Foraging Zone Canopy Subcanopy Herb Ground Total and Foraging Tactic Outer Middle Inner Outer Middle Inner Layer Layer (Percent) Gleaning Leaf 282 3 2 25 28 9 5 0 100 (15.4)3 Cleaning Twig 3 1 0 17 17 5 0 0 43 (6.6) Cleaning or Probing Branch 15 1 1 22 29 44 0 0 112 (17.3) Flycatching and Hovering 18 0 0 3 2 32 0 0 55 (8.5) Chasing 5 0 0 0 0 0 0 12 17 (2.6) Probing Flower 1 0 0 0 0 0 38 0 39 (6.0) P.robing Fruit 1 0 0 2 5 5 0 14 27 (4.2) Probing Terminal Bud 1 2 1 21 6 16 0 0 47 (7.3) Probing Epiphyte 46 0 0 96 27 8 \ 0 0 177 (27.3) Pecking Ground - - - - 31 31 (4.8) Total and 118 (18.2) 7 (1.1) 4 (0.6) 186 (28.7) 114 (17.6) 119 (18.4) (Percent) 129 (19.9) 419 (64.7) 43 (6.6) 57 (8.8) 648(100.0) 'Does not include foraging at human food sources. See methods section for procedures. ENumber of times behavior was performed in each foraging zone. 3Number in parentheses are percent of·total observations. 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 181 dominicensis), both of which were rare in the pastures where these epiphytes were common. I saw Yellow-shoulders flycatch, most often after they had flushed insects as they moved, but occasionally they sat on exposed perches and sallied. When foraging on the ground, Yellow-shoulders walked, occasionally scratching with one foot. They gaped under monkey biscuits to turn them over, thereby pushing the food away from the body. Orians (1961) saw Red-winged Blackbirds turn rocks by this method. Yellow-shoulders also turned the food over with their closed mandibles. Individuals also in- serted the lower mandible under monkey biscuits and lifted them toward the body. Yellow-shoulders used the sides as well as the tip of the beak to break off material from large pieces of food, the sides of the beak being moved back and forth in a shearing motion. The birds probed for nectar commonly in January and February when Aloe vulgaris was blooming, and during this period small groups were in constant attendance at patches of aloe. They probed flowers by inserting their beaks up to the bases of their skulls, gaping to enlarge the entrances. On one occasion I noted three Yellow-shoulders probing Yucca flowers. Wetmore (1916) saw Yellow-shoulders probe bucare (Erythrina sp.) blossoms, and Danforth (1926) reported Yellow-shoulders taking nectar from guama (Inga laurina), as well as fruits of several species of cacti, e.g. Selenicerus sp. and Cephalocereus rogenii. During the nesting period, adult A. xanthomus foraged mainly in the subcanopy (64.7 % of 648 observations; Table 18) and canopy (9.9%) layers of trees. Within these layers most foraging maneuvers were per- formed in the outer zones. Probing epiphytes, the most common foraging tactic, composed 27.3% (177 of 648) of the maneuvers observed. Of the total, 21.9 % were preformed in the outer canopies and subcanopies. Similarly leaf gleaning, the third most common foraging tactic, occurred mainly in the outer zones of the canopy and subcanopy (8.2% out of a total of 15.4 %). Cleaning and probing of branches occurred mainly in the subcanopy (14.7 % out of 17.3%), the zone of vegetation which has the greatest mass of branches. Flycatching was a relatively important ac- tivity, and it occurred mainly (4.9% out of 8.5%) in the inner subcanopy, which was relatively open in vegetation such as red mangroves, and where lower turbulence may have allowed easier capture of flying insects. Flycatching and hovering also took place in the outer canopy and sub- canopy (2.3 % of total). Probing of terminal buds was accomplished mainly in the subcanopy (6.6 % of total), and most of this activity was seen in red mangroves. Most flower probing, a relatively uncommon behavior in the breeding season (6 %), involved aloe, and therefore oc- curred in the herb zone. 182 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 k Hz 8- A B n.+48.F*9.-* =-.. .*.- 2- 1 1 1 0 0.25 0.50 0.75 1.00 SECONDS FIGURE 8.-Sound spectrograms of Yellow-shouldered Blackbird vocalizations. (a) growl, wide band filter; (b) same as preceding, but with narrow band filter. VOCALIZATIONS GROWL, - Lasting about 1 sec, this sound is composed of a short in- troductory note and a buzzy trill. The click-like introduction extends beyond 16 kHz, and resembles a chwip or check call (below). The in- troductory figure is probably imperceptible to humans, as it is overlapped temporally by the more audible buzz component, which has a frequency range of 1.5-5.0 kHz, with energy concentrated at 4-5 kHz (Fig. 8). I noted some individual variation in the structure of the growl, such as in- terruption in the trill, abbreviated and lengthened trills, as well as incor- poration of other vocalizations such as gueea in the beginning of the vocalization. The growl is probably homologous with those vocalizations given dur- ing "song-spread" (Nero 1956a) by other icterids. The structure of the growl resembles the trill part of the buzzing song of the Yellow-headed Blackbird Xanthocephalus xanthocephalus (Orians and Christman 1968; Figure 14b). Both sexes utter growls during the song-spread and during wing-raise. With song-spread the growl is given as the wings reach their full height, the sound continuing as the wings are lowered. No head movements are associated with the utterance of growl, but the beak is opened (see Action Patterns). The rate of singing varied from 1.2 to 2.3/min by some birds perched near their nests, while others in communal roosts wing-raised (with growls) 12 to 17 times/min. Juveniles sometimes rapidly uttered a short, muted growl accompanied by a wing-raise. These may have been examples of subsong (Thorpe 1961). On nesting areas growls as well as pee-puus (see below) were ex- changed by neighboring birds that were out of each other's sight. In April 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 183 TABLE 19,- Contagion of growls on nesting grounds. No. of growls initiated per 10 consecutive sec intervall: 0 1 2 3 or more Observed 267 83 51 17 Expectedz 238 134 38 7 'Observation periods. totalling 70 min, were each 300 sec long. 'Calculated from the Poison series. The observed number of calls in each category is significantly different from the random distribution (x'=41.7. P<.001: d.f. -3). 1975 I recorded the incidence of growling by individuals in groups of 6-8 that were visiting nesting sites in the red mangroves on W. Matita. I found a significant contagion between growls: a growl from one bird was usually followed within an unexpectedly short interval by one or more growls from other individuals (Table 19). Birds sitting next to each other and engaging in song spread occasionally initiated growls and pee-puus before neighbors had terminated their growl, and in several instances 3 or 4 individuals overlapped their growls to form a continuous sound lasting about 4 sec. As discussed under wing-raise, growls as well as gueeas were sometimes repeated erradically in mass by flocks of birds that were mob- bing humans, or at least reacting to their presence. These may have been instances of group performance of antipredator strategy (Smith 1977), as the "indefinably confusing, all-pervading sound" (Grinnell 1903) made localizing any one individual difficult. , RAsp. -This call, rendered uuut, is structurally similar to growl, although its fundamental is lower, at about 1.5 kHz, with discernible overtones at 3 and 4.5 kHz. To humans the call sounds thinner and less resonant than a growl. The rasp was heard only during agonistic en- counters, and it was uttered by flying birds or by ones about to fly, as when preparing to dive at predators. Yellow-shoulders also gave the call when supplanting and being supplanted from feeding positions. GREEAH. - This call, which sounds to me like the scold of the Red-eyed Vireo (Vireo olivacus), has a complaining, nasal quality. I heard it on numerous occasions, but only in the vicinity of nests, and most often from parents around the time young were fledging. It was associated with a tendency to fly or move. With disturbances near the nest, both sexes gave the call as they moved around, often accompanying each other. In addi- tion , I once heard greeahs given by a fledgling that was being fed near its nest. The female that was feeding it also gave greea/is. On another occa- sion a female that was mobbing me wing-trailed while giving greeahs. The nearby young gave pink calls (below) and moved toward the greeah- calling female . Individuals often uttered greeah when they engaged in 184 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 k Hz 8- B C ' 4944 4 -I6 - .1~'11 *4411~1,7 5\\ 1,1/ , r447 OF 4- 2- ~2 .-- 1 -t ~- 1 10 0.25 0.50 0.75 1.00 SECONDS FIGURE 9.-Sound spectrograms of cut-zee type vocalizations. (a) cut-zee, made with wide band filter; (b) same as preceding, but with narrow band filter; (c) a tslink variants, wide band filter. moth flight . On one occasion I noted that a male's greeahs merged into growls, given with song-spread. SCREAM. - This harsh noise is similar in structure to screams given by other icterids (Orians and Christman 1968, Figure 2ld). The scream was most often uttered by birds that were being handled, but also by those confined in nets or traps. CUT-ZEE. - A common alarm call with much individual variation, cut- zee is composed of two parts: a short introduction followed by a slightly longer, falling element. There is temporal overlap in the two figures, in- dicating the use of two or more syringeal membranes (Fig. 9). Both sexes utter cut-zee when mobbing of predators around nest sites, and I also heard it throughout the year in feeding and roosting areas. Cut- zee is given with or without wing-raise and tail flip. Wing-trailing birds also give cut-zee. It is occasionally combined with other vocalizations: on 25 May 1974 female XAGR, 4 m from her nest, in 3 min gave eight cut- zees that intergraded into growls . Some of these combinations were repeated rapidly with only a 5-sec lapse. During mobbing cut-zee is often associated with checks and queeas. The information provided by cut-zee is probably similar to that made available by queea, but cut-zee appears to be used in more intense mob- bing, as indicated by the closer approach of cut-zee calling birds to predators. Nestling Yellow-shoulders may recognize cut-zee as an alarm. On 25 June 1974 a female perched under her nest repeatedly gave chu,ips, while the young continued to beg loudly. The female then began uttering cut-zees, and the young immediately became quiet. Occasionally only part of cut-zee is given; the second note, zee, or a close variant sounding like tslink, was recorded (Fig. 9). On other occasions this sound was repeated rapidly in flight, sounding like zeenk zeenk zeenk. 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 185 QUEEA. - Queea is composed of two figures, the fundamental of the first being about 3.5 kHz, with two discernible harmonics. The second note, which temporally overlaps the first, falls slightly in pitch, and has its fundamental at about 5 kHz (Fig. 10). This variable call is occasionally repeated in series with wing-raises, and is often combined with other vogalizations such as growl and chwip. Queea is usually given with tail flip. - --I Queea is frequently given by biFds mobbing predators, such as humans, monkeys, or cats, or by birds that are disturbed in any way. The call is also given by females that are being pursued persistently during sex- ual chases. Average rate of calling was 18/min, range 7-28/min (602 calls from 13 birds timed for 30 min). Queea is given in association with other sounds such as check. For example, on 18 February 1975 male RARG, sitting with two other Yellow-shoulders, gave 13 queeas, 20 checks, and 1 growl as I approached, switching from check to queea each time I moved. Although queea-type calls appeared to be used most often as alarms, I also noted that they were occasionally used when birds seemed to be un- disturbed. At these times the call was often given rapidly with wing- raises, 2-6 times in succession, to form what sounded like a chatter. When repeated rapidly as a chatter, queea may function as a social or contact signal between members of feeding flocks or nesting groups. This chatter may be homologous with the female song of the Red-winged Blackbird (Nero 1956a). CHWIP. - Relatively simple in structure (Fig. 10), chwip resembles check of the Yellow-headed Blackbird (Orians and Christman 1968; Fig. 2Op-r). Young in the nest begin to give chwip when they are 6-7 days old, k Hz 8- A B C 6- 4- 10'' 1114, 2- //~ , b 1 1 1 0 0.25 0.50 075 1.00 SECONDS FIGURE 10.-Sound spectrograms of Yellow-shouldered Blackbird vocalizations, all made with wide band filter ; (a) queea, variant 1 ; (b) queea, variant 2; (c) check; (d) chwip: (e) 1)co (first part of pee-puu) 186 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 so after begging calls and pink (below), it is one of the first vocalizations young Yellow-shoulders give. Chwip is used in practically all situations, by birds in flocks or alone, flying or sitting, and may indicate that a bird is seeking association, may fly, escape, or is indecisive. Chwip is usually ac- companied by tail-flip. Average rate of calling was 14/min, range 1-41/min (36 birds timed for 75 min). CHECK. - Somewhat more structured than chu@, check shows a distinct harmonic, and the call lasts about twice as long. The energy of check is centered around 4 kHz, but the call extends over a wide fre- quency (Fig. 10). Check is given by birds that are scolding predators, are engaged in agonistic encounters, or possibly are disturbed in some way. Check probably provides information about the individual's readiness to attack, escape, or fly, or indicates that it is acting indecisively. It is often uttered in association with other vocalizations that function as alarms, such as queea, cut-zee, and chwip, The associated action pattern is the tail-flip. Average rate of calling was 13/min, range 1-47/min (16 birds timed for 30 min). PEE-PUU. - The pee-puu is composed of two components: the introduc- tory pee is a clear, slightly rising note, while puu, also unslurred, falls in pitch. This vocalization, for which I have a spectrograph for only the pee component (Fig. 10), probably functions mainly in communication be- tween paired individuals. I frequently heard it on nesting grounds, and pee-puus from females were usually answered by growls from their males. On 5 August 1974 male GABR inspected his nest. His female, XAGR, perched 4 m away, gave a pee-puu, which was immediately followed by the male's song spread. The female repeated her pee-puu 30 sec later. On 11 April 1975 male GYGA, perched next to a 1974 nest, in 3 min gave nine song-spreads, each immediately followed by a pee-puu from the female with whom he was associated. These pee-puus were also returned by neighboring birds. PINK. - To my ear, pink sounds remarkably like the flight note of the Bobolink. It was given as a contact call between young and their parents. I first heard the call given by nestlings when they were six days old. Free- flying young gave pinks as they followed their parents about, either singly or as a double call. Adults leading their young in flight also gave the call. Rate of utterance by one young just fledged on 1 July 1974 was 122 in 3 min. FLIGHT SERIES. - A flight series, usually uttered when birds were first air-borne, appeared to have no unique components, but rather was a composite of different calls, such as cut-zee, queea, pee-puu, and chwip. I noted a large amount of individual variation in the composition of the call. The call was given all year, in the winter from birds leaving feeding flocks or secondary roosts, but most commonly during the nesting period 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 187 from birds that were leaving their nests on distant feeding flights. The flight vocalization may be important as a means of contact between mated birds traveling between nesting and feeding areas. ACTION PATTERNS SONG-SPREAD. - Initially the head may be pointed upward (Fig. 11). After about 2 sec the bird lowers its beak and begins to raise its wings, the tail becoming increasingly fanned and the body plumage fluffed. The wing elevation phase takes about 1 sec, at the end of which the beak touches the breast and the ventral plumage is ruffled (Fig. 11). The wings are held up for about 2-3 sees, and lowered in about 1 sec, then the head is again pointed up. During wing elevation the carpus is rotated forward, providing maximum frontal exposure of the epaulets. I noted much in- dividual variation in song-spread, mainly in asymmetry of wing and leg positions and in angle of body tilt. The bill-up at the start of the song- spread was sometimes omitted, but the terminal bill-up was nearly always present. Both sexes sing in many situations, but most commonly in (1) close ~ quarter agonistic encounters with conspecifics, usually near the nest, but also in feeding flocks and roosts; (2) the presence of predators in the nest vicinity; (3) instances when mated birds meet near their nest; (4) agonistic encounters with other bird species; and (5) nest-site advertisement by the male (see below). Representative rates of singing are (1) 9 times in 3 min by a male near his nest and mate; (2) 18 times in 15 min by a male near his nest; and (3) 17 times in 7.5 min by a female near her nest. The only vocal signal that I heard given with song-spread, and one that accompanied it invariably, was the growl, which lasted about 1 sec, and was initiated as the wings reached their full elevation. WING-RAISE. -Less complex than song spread, wing-raise may corre- spond to a less ritualized stage of the former display, in which some com- ponents are lacking (bill-up and bill-down) and others are less exag- gerated (wings and body plumage not as elevated). In the less extreme form, the carpus may be rotated upward only slightly, with the tips of the remiges remaining in contact with the body. In the more extreme form, the wings may be spread and raised more fully, but the tips are seldom raised more than 30° above the horizontal. Wing-raise is usually repeated rapidly, every 3-5 sec; in April 1975 a juvenile Yellow-shoulder sitting alone in the mangroves wing-raised 55 times in 4 min, giving an abbreviated growl with each wing-raise. This may have been an instance of practice singing (see p. 182). Wing-raise occurred in the following situations: (1) during close- quarter intraspecific and interspecific agonistic encounters on and off 188 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 FIGURE 11.-Song-spread of the Yellow-shouldered Blackbird, showing lateral (a) and frontal (b) aspects. nesting grounds; (2) during mobbing of predators; and (3) when a bird was alone, either on or off nesting areas. Rapidly repeated wing-raises, given with growls and queeas, were occasionally uttered in mass by birds sitting close together in roosts. These massed displays seemed to be elicited by disturbances and may have been instances of group mobbing or confu- sion chorus (Grinnell 1903). Vocal signals given with the wing-raise were growl, pee-puu, queea, chatter (queea in series), and cut-zee. When repeated rapidly, the wings may be lowered and raised with each utterance, as usually occurred with growls, or they may be held up continuously and fluctuated only slightly with the repetition of each vocalization, as usually occurred with chatter. BILL-up. -In this posture the head is rotated upward, near the ver- tical. The neck may be extended, normal, or contracted. The body plumage is normal or fluffed, while the wings are folded normally or drooped slightly. The displaying bird may gape. I saw bill-up throughout the year, but only in close-quarter interactions, in feeding, roosting, and nesting areas. Yellow-shoulders gave bill-ups to other Yellow-shoulders as 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 189 190 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 well as to grackles, cowbirds, and Troupials. Mated birds also gave bill- ups when near each other. Bill-up appears to have become incorporated into the beginning and ending of the song-spread. Bill-up displays are widespread among icterids and other passerines (Nero 1956a, Andrew 1961). BILL-DOWN. -The head is lowered and the bill is pointed toward the abdomen. The head plumage may be ruffled. Yellow-shoulders give the bill-down when near conspecifics in situations similar to those in which they gave bill-ups, but the posture may indicate a greater tendency to escape or a greater conflict between escape and attack than does bill-up. Bill-down is also often given by birds that are moving or about to move while in the presence of conspecifics. Unlike bill-up, the display is given by birds that are alone. Andrew (1961) suggested that bill-down ("bill lowering") is an example of a "reverse movement" to bill-up ("bill raising") in species in which the latter has a threat function, e.g. it may act as an appeasement gesture. In this regard it is interesting that bill- down has been incorporated in song-spread and wing-raise. HEAD-FORWARD. - In this posture, similar to that described for other passerines (Andrew 1961), the head is extended toward an opponent, while the legs may be flexed and plumage normal, fluffed, or sleeked (see illustrations in Post and Wiley 1976). The wings may be raised slightly at the shoulders and the bird may gape. Head-forward was seen most often in agonistic encounters at feeding sites, and it was frequently preceded by bill-up. HEAD-IN. -The bird usually crouches, bringing its head in to the body. The body plumage may be fluffed or ruffled. The beak is often directed at the nearby individual, and perhaps gaped. This posture is seen most often during agonistic encounters in the feeding areas, but occa- sionally near'nests by birds that are challenged upon entering the activity spaces of others. Males also occasionally give the head-in when they ap- proach females that are giving wing flutter. WING-FLUTTER. -The body feathers are fluffed, and belly feathers ruffled, while the wings are held out from the body and vibrated. The legs are flexed. The tail is spread, and may be elevated or held normal. Wings are occasionally raised asymmetrically when they are fluttered. I saw this display from females that were begging food from their mates or soliciting, usually near their nests. Once a male gave wing- flutter repeatedly to a female that had just been released from a trap. The female was trying to remove her bands and exhibited no special behavior. Another time a male wing-fluttered in the presence of a female that was nest-building. In many species wing vibration in male courtship is prob- ably homologous with female precopulatory display (Andrew 1961). Wing-flutter of both male and female Yellow-shoulders resembles the 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 191 precopulatory displays of Red-winged Blackbirds and other icteri(is (Nero 1956a). On those few occasions when I saw copulation, prior to it the female's tail was above the horizontal, while on other occasions males were re- pulsed by wing-fluttering females whose tails were held normally, The head-up tail-up variant is homologous with the female precopulatory display of other female passerines (Andrew 1961), but other than the posi- tion of the tail, there is little difference between this form of the display and that given when a female begs for food. The wing-flutter probably functions as a distance decreasing mechanism when mated birds are at- tempting to approach one another. Some males brought food to brooding or incubating females, and the females wing-fluttered and gaped for food only when they both were at the nest. In the case of males that did not regularly deliver food to females, their mates often food-begged and wing-fluttered when the males arrived at the nest with food for the young and the females were present. The females took the food, sometimes man- dibulated it, and then gave it to the nestlings. WING-TRAIL. -As the bird walks slowly, the wings are lowered at the carpals and the remiges are spread so that the feathers may touch the ground. Body feathers are raised to varying degrees, but the rump feathers are usually ruffled. The rectrices are spread and may also drag the ground. The beak is horizontal or pointed slightly down. Wing-trail was given by both sexes when I approached nests containing young. On several occasions I handled young without their parents' wing-trailing, but when the young screamed, the adults began to wing-trail and also utter screams. Other vocalizations given with wing-trail were queea, chwip, greeah, cut-zee, and check. When used by parents whose young are threatened, the wing-trail may be a relatively unritualized form of distraction display, as described for many other passerines, including Red-winged Blackbirds (Orians 1973) and Bobolinks (Dolichonyx oryzivorous) (Nero 1955). MOTH FLIGHT. -This display takes the form of short flights in which the wings are moved slowly and with small amplitude. I saw this peculiar flight pattern given only by females around their nests. Females gave moth flights when either approaching or leaving the nest and the male was near it. Once moth flight preceded the female's wing-trailing, and several times it preceded wing-fluttering. Females gave the display fre- quently around the time the young fledged, especially when a human was near. At these times many short moth flights were accompanied by vocalizations such as greeah and cut-zee. As pointed out by Andrew (1956), moth ("impeded") flight may occasionally form part of distraction display, and in general appears in situations of strong conflict between escape and approach tendencies. On the other hand, it may also be used 192 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 during courtship, as by male Red-winged Blackbirds during symbolic nest-site selection (Nero 1956a). MALE NEST ADVER'rISEMENT. - Although females actually construct nests, during the period of early pairing males stand in the cups of old nests and pull or jab at nest material. They also try to form the nest cup by crouching and pushing against its edge with their breasts. Sometimes males carried nest material away from the nest, but usually dropped it nearby. All of these activities are associated with the presence of a female with which the male is attempting to pair. Symbolic nest-building by males of species in which only the female constructs the nest has been recorded for several other icterids (Nero 1956a). According to Nero's descriptions, in the Red-winged Blackbird this behavior seems to be more fully developed than in the Yellow-shoulder, and seems to be associated with the period of egg-laying rather than that of pair establishment. TAIL-FLIP. - The tail is moved rapidly up and down (duration about 1.3 sec). During tail-flip other body parts are usually positioned nor- mally. Tail-flip is given in all situations, with nearly all vocalizations. Typically, a call such as chwip or check was uttered as the tail reached its highest point or was just beginning to be lowered. Rate of tail-flipping varied; for example, a bird that was mobbing humans gave 54/min, while a preening bird gave 4/min. SLEEK. - The body plumage is compressed, while the bird may flex its legs, indicating a tendency to fly. Occasionally the bird assumes an erect posture and sleeks the neck and breast feathers, in which case sleek may indicate readiness to fly at an opponent. Sleek is probably homologous with similar postures of other icterids (Orians and Christman 1968). FOOD-BE:GGING. - Begging postures of young Yellow-shoulders are similar to those described for other icterids. The head is held in, the beak is pointed up and gaped. Wing tips are held into the body, while the wing is vibrated at the carpus. I saw food-begging by young that had been out of the nest 24 days. BREAST-TO-BREAST FIGHTING. - Fighting occurs frequently in ter- ritorial disputes, and also at feeding locations. Opponents rise in the air, clawing at each others' breasts and beating their wings against one another. Occasionally protagonists fall in the water while fighting. Breast-to-breast fighting is sometimes preceded by a form of pouncing, in which an attacking individual hops with outstretched feet on the oppo- nent's back. SEXUAL CHASING. - Infrequently I saw group sexual chasing, such as described for Red-winged Blackbirds (Nero 1956a). On the nesting cays 2-4 birds occasionally chased a female, ending in one bird's pursuing her down into the mangrove roots, and even into the water. These chases were accompanied by loud calls of queea, check, and cut-zee. 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 193 BILL-WIPING. - Under relevant conditions, bill-wiping serves to clean the bill, usually after feeding or preening. The bird simply wipes one side of the beak from base to tip on a branch, then perhaps repeats on the other side. These movements also occurred often in seemingly irrelevant circumstances, and more frequently than body maintenance alone seemed to warrant. On 15 February 1973 a bird waiting to feed inside a monkey cage bill-wiped 24 times in 105 sees. It was probably showing strong con- flict between approach and escape tendencies, as during the same period it gave 33 queeas, each with a tail-flip. Bill-wiping was also common dur- ing mobbing. DISCUSSION The distances that nesting Yellow-shoulders commute to gather food suggest that food distribution is not the determining factor in nest place- ment. Considering: (1) the energy costs of flying to foraging sites, (2) that birds do not seem to exchange information about the location of food, and (3) that food is not highly concentrated within the large areas (e. g. groves of trees in pastures, scattered trees in savannahs, mangroves) foraging blackbirds visited, one would expect nests to be spaced uniformly within the feeding areas (Horn 1968; Waser and Wiley 1979). Wiley and Wiley (1980) found a similar discrepancy between food distribution and ideal nest locations in their study of the Yellow-hooded Blackbird in Venezuela. These birds nested in small patches of suitable marsh vegetation and flew, independently of each other, long distances outside the nesting marshes. Similar examples of limited nest sites and widespread feeding grounds occur in populations of Clay-colored Sparrows (Spizella pallida) (Knap- ton 1979) and Seaside Sparrows (Ammospiza maritima) (Post 1974). In these cases the critical factor may be finding secure nest sites, and the energy budget model (Horn 1968), which considers nest distribution a function of food supply, is not appropriate, or food may not be limiting in any of these situations. Adult Yellow-shouldered Blackbirds usually delivered 3-6 items per trip to the nest, indicating that food was readily available. In contrast, female Red-winged Blackbirds in Costa Rica (Orians 1973) and Yellow-hooded Blackbirds in Venezuela (Wiley and Wiley 1980) usually delivered one item per nest visit. The varied foraging behavior of the Yellow-shoulder may be at- tributable to ecological release (Crowell 1961, 1962). Reduced competi- tion on islands may allow a species to use niches occupied by other forms on continents. The Yellow-shoulder uses some foraging maneuvers and foraging sites that in adjacent continents are preempted by woodereepers (Dendrocolaptidae), woodpeckers (Picidae), and wrens (Troglodytidae): probing and gleaning the branches and trunks in the inner zones of trees. Only one member of these families, the Puerto Rican Woodpecker 194 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 (Melanerpes portoricensis) is sympatric with A, xanthomus. In the SW coastal zone where the Yellow-shoulder was most common, the Puerto Rican Woodpecker was rare. Another niche A. xanthomus occupied is that of orioles: gleaning leaves and probing leaf clusters and terminal buds in the outer zones of trees, and probing fruit and flowers in all strata. Only one species of oriole, Icterus dominicensis, is native to Puerto Rico, and it is uncommon in the habitats A. xanthomus occupies. An in- troduced oriole, Icterus icterus, was fairly common in the SW coastal zone, but it foraged in the herb layer and was mainly frugivorous (Post, unpubl. data). The phenomenon of expanded niche use in the absence of competition is well illustrated by the behavior of the Jamaican Blackbird (Nesopsar nigerrimus), closely related to Agelaius (Bond 1950, Cruz 1978, Wiley and Cruz 1980). Like A. xanthomus, N. nigerrimus is arboreal, and a prober of epiphytes (58 % of its observed foraging activity, Cruz 1978) and branches and trunks (10 %). The Jamaican Blackbird forages more on the trunks and inner branches and less in the outer zone of trees than does the Yellow-shouldered Blackbird. In the Jamaican habitats Cruz (1978) studied, the Jamaican Oriole (Icterus leucopteryx) foraged nnainly in the outer parts of trees. The foraging behavior of the Yellow-shoulder has diverged from that of marsh-dwelling Agelaius, most species of which are ground feeders in the breeding season. This difference may be a result of reduced competi- tion combined with scarcity of marsh habitat on Puerto Rico. In North America Red-winged Blackbirds occasionally feed in trees (pers. obs.). Another West Indian Agelaius, the Tawny-shouldered Blackbird, is prob- ably arboreal in the breeding season (Lack 1976) and also nests in palms (Barbour 1923). That the two insular species, which presumably evolved from a marsh-dwelling form similar to A. phoeniceus, are partly arboreal may be attributable to the elimination of Cuban marshes during the post- Pleistocene rise in sea levels, though Bond (1950) considers it more likely that the ancestor of A. humeralis and A. xanthomus evolved arboreal foraging behavior during a period of competition with the Red-winged Blackbird (A. p. assimilis), which occupies marshes on Cuba. Whether the ancestor of xanthomus was arboreal when it arrived on Puerto Rico is conjectural, but there may have been some large marshes such as those that existed until recently in the Yauco-Boquer6n Valley, covering an area of about 150 kme (Danforth 1926). The Yellow-shoulder uses at least 26 distinct displays and vocaliza- tions. This repertoire appears to be larger than that of other monogamous icterids that have been studied (Orians and Christman 1968), and its size is closer to that of polygynous species such as the Tricolored and Red- 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 195 winged Blackbirds. Male Red-wings, for instance, have 18 vocalizations and 12 action patterns, females 6 and 9 (Orians and Christman 1968). Estimates of the number of displays of various species vary according to authors' procedures and preferences (Smith 1977). As I lack adequate spectrographic material in some cases, I reserve judgement on variants of some vocal signals. More detailed study of the vocal repertoire of the Yellow-shoulder may reveal it to be larger. Unlike Red-winged and Tricolored Blackbirds, the Yellow-shoulder has few intersexual differences in its display repertoire. Each sex appears to have only one unique visual display, and all the remaining displays, visual and vocal, are shared by both. Orians and Christman (1968) note a tendency for males of polygynous species to have a larger number of displays than females, this disparity being correlated with their advertis- ing for and holding more mates than monogamous species. The sexual dimorphism reflected in display repertoire is also found in body size and plumage of polygamous species. Hamilton (1961) suggested that as those species breeding farthest from the equator are generally migratory, and on returning to their breeding grounds have little time for pairing, dimorphism is also advantageous in reducing intersexual strife and allow- ing rapid pair formation. Intersexual differences in display repertoire facilitate rapid mating in these northern species, but species such as the Yellow-shoulder, that pair well in advance of mating, may be under little selective pressure to segregate displays sexually. The ancestral condition was presumably one of intersexual equality in display repertoire size, but with the evolution of non-monogamous mating systems, sexual segrega- tion of displays was accentuated, and eventually each sex appropriated certain displays. Some differential use of certain displays may occur in the Yellow- shoulder , such as has been shown for the Black-capped Chickadee, Parus atricapillus (Ficken et al., 1978). Examples of different degree of usage by each sex probably occur in pee-puu and queea in series (chatter), both used more often by females, and (single) queeas used more often by males in mobbing. Among species of North Temperate icterids that are not cryptic, those having large territories usually have more displays than others that defend only a small area around the nest (Orians and Christman 1968). Yellow- headed and Red-winged Blackbirds have more displays than Brewer's Blackbirds or Common Grackles. This difference is related to the larger number of displays transmitting messages over long distances that the first two species haye, such as flight displays and song. More needs to be learned about the influence of winter social organization on size of display repertoire, but Orians and Christman (1968) suggested that the large number of displays of the Tricolored 196 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 Blackbird may be due in part to year-round association of both sexes. A similar effect may occur in the Yellow-shoulder, whose display repertoire is about the same size as that of the Tricolor. As Wiley (1976a) found for the Common Grackle, few of the vocal signals of the Yellow-shoulder transmit unequivocal information about the internal state of the transmitter, but most vocalizations are given in close-quarter interactions, whether in nesting or feeding places, and more precise information is available to the receiver by reference to context or to accompanying visual display. Most of the Yellow-shoulder sounds are abrupt, broad spectrum signals that are easily located in noisy en- vironments such as exist in nesting colonies, feeding flocks, or roosts. The growl vocalization given with song-spread has a simpler structure than similar sounds of other icterids. There has probably been little selec- tive pressure for further elaboration of song, because of the small territory size of the Yellow-shoulder, and of relaxation of selection for specific distinctiveness in the absence of closely related species (Grant 1972). In species occupying large territories, song must be transmitted without distortion over relatively long distance. The song (growl) of the Yellow-shoulder is highly modulated, and it covers a wide frequency range, so it is probably of little use in long-range communication. Rather, the growl is used repetitively in close-quarter interactions, and coupled with action patterns such as song-spread and wing-raise. Song-answering, a form of antiphonal singing that Wiley (1976e) considers to be important in the vocal coordination of Common Grackle pairs, was also characteristic of Yellow-shoulders during their period of pairing and nest site selection. The timing of song-answering is thus similar to that of grackles. In the Yellow-shoulder, song-spread is always accompanied by song (growl), and growl is invariably accompanied by some form of wing elevation. In the Red-winged Blackbird, song-spread and song are not always coupled (Peek 1972). Red-wing song is given most often with incipient song-spreads, and also without any wing elevation. The Red-wing's song- spread is used mainly in close-quarter interactions, while its song is used for both long and short range communication. In the Red-wing, the vocal component of song-spread is perhaps diverging in function from the visual component, because of its importance in long-range communication. Most of the vocal signals of the Yellow-shoulder are emphasized at high frequencies, have wide frequency ranges and are highly modulated. These characteristics make them easily localized and identified, but mainly by nearby individuals in the open, as these characteristics also lead to rapid attenuation and distortion over distance or through dense vegeta- tion such as mangroves (Morton 1975). One vocalization, pee-puu, ap- 1981 POST: YELLOW-SHOULDERED BLACKBIRD BIOLOGY 197 pears to be composed of pure tones, and was used frequently for long- range communication by birds nesting in red mangroves. Correlated with the historic rarity of predators on Puerto Rico, the Yellow-shoulder has no specialized predator ("hawk") alarm calls as do several continental icterids. Instead, the Yellow-shoulder uses general alarm calls such as queea, that appear to have functions besides predator warning. Cut-zee seems to be the one vocalization most closely associated with predator mobbing response, and its use is largely restricted to the nest vicinity. Even this degree of specialization is interesting, and possibly cut-zee has evolved primarily in response to aerial predators such as gulls and frigatebirds that have probably been present in Puerto Rico since the blackbird's ancestors arrived. Unlike some open-country icterids, such as the Red-winged and Yellow-headed Blackbirds (Orians and Christman 1968), the Yellow- shoulder does not have stereotyped flight displays, and its flight vocaliza- tion is composed of a recombinable series of other signals. Displays related to bonding are also few: wing-flutter, probably homologous with precopulatory displays of other passerines; male nest-building display; and pink call, given between parents and young. Displays relating to establishment of pair bonds may be more important for polygynous, usually highly dimorphic species, in which male dominance is well developed, but mollified by the use of these displays to facilitate pairing. In the presence of predators Yellow-shoulders use a variety of visual displays, but as with vocal displays, no single one has become specialized as a predator warning. As with cut-zee, one visual display, wing-trail, oc- cum mainly in the presence of predators, and it may be homologous with distraction displays given by other passerines in the vicinity of their nests. CONCLUSIONS The Yellow-shouldered Blackbird, probably derived from marsh- dwelling and ground-feeding blackbirds of North America, has diverged from the behavior of its closest mainland relatives. Its foraging behavior is similar to that of orioles and woodpeckers. This may be a result of re- duced competition, and also lack of suitable feeding sites on the ground, particularly marshes, in Puerto Rico. The species does not engage in coactive feeding as do some relatives, nor exhibit cooperative breeding, though it aggregates when nesting. As colony sites are isolated from feeding grounds, and the types of food delivered to the young are widely distributed within relatively large patches, it would be predicted, on the basis of maximum foraging effi- ciency, that nests should be uniformly distributed in the food field. As this is not the case, nest dispersion is probably a result of selective pressure ex- 198 BULLETIN FLORIDA STATE MUSEUM Vol. 26, No. 3 erted by predation. This conclusion is supported by the frequency with which colony members engage in communal mobbing, the large number of displays and vocalizations used in the presence of predators, the inaccessible sites used for nesting, and the relatively high prevalence of nest predation. Yellow-shouldered Blackbirds were monogamous in all the habitats in which they were studied. Among other icterid species that nest in colonies and visit remote feeding grounds, polygamy is seen at least occasionally. In the Yellow-shouldered Blackbird, I conclude that the conditions for male emancipation are not realized because of the crucial role of the male in parental care. The mate acquisition and pair maintenance behavior of the species, in which birds affiliate long before breeding, and in which pairing takes place around nest sites of previous years, assures high levels of intra- and interseasonal mate loyalty. A long period of affiliation before breeding may also be advantageous if it allows pairs to respond rapidly to unpredictable events (spring rains). The size and complexity of the display repertoire has probably evolved as a response to pressure for a closely coordinated monogamous pair bond of relatively long duration. The species' social behavior, in the context of its high reproductive sue- cess, lack of dispersal, high annual survival, and especially delayed breeding of both sexes implies that a conservative reproductive strategy has been evolved. LITERATURE CITED Andrew, R. 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