Bulletin OF THE Illinois State Laboratory OS Natural History Urbana, Illinois, U. S. A. STEPHEN A. FORBES, Ph.D., L,L,.D., Director Vol. IX. January, 1913 Article VI. THE MIDSUMMER BIRD LIFE OF ILLINOIS: A STATISTICAL STUDY BY STEPHEN A. FORBES ERRATA AND ADDENDA Page 54, lines 3 and 2 from bottom, and elsewhere in Article III. for Cassia chainaechrista read Cassia chamaccrista. Page 62, between lines 4 and 5 from bottom of table insert Erigeron annuus. Page loi, table, after Croloii glandulosus read var. septentrionalis: and for Eijuisettim laeinyatum read Eqnisetum bycmale var. inlermedium. Page 131. line 3, for coerulea read caerulca. Page 138, last line, for Zi::a read Zizia. Page 141, line 21 from bottom, dele Diodia teres. Page l6g. between lines 3 and 4, insert as follows : Erigeron annuus (L.) Pers. An interstitial in the liunch-grass association in the Hanover area. Page 177, line 5, for casti^'ard read iveslzvard. Page 209. line 3 from bottom, for copalina read copallina. Page 210, line 13 from bottom, for Diospyrus read Diospyro'i. Page 211, line 5, for Foresteria read Foresticra. Page 256, line 3 of table, for Dr. H. M. Pepoon read H. 5". Pepoon. Page 278, line 16, the fifth word should be in Roman type. Page 286, line 6 (second column), page 295, list of secondary species (second column), and page 353. line 8 from bottom, for hiematis or hiemale read hye- mnte. Page 313, line 4 from bottom (first column), for pedicularis read pcdicularia. Page 315, line 10, second column, for Apoeynum read Apocynum. Page 323, line 3 from bottom, for Cyperus read Scirpus. Page 330, line 14, for virginianum read inrginicum. Page 336. lines 3 and 2 from bottom, for virginicum read virginianum. Page 337. line 2 from bottom, for philadelphicum read philadelphicns. Page 339» in first list of invading species, for Rhus hiria read Rhus typhiua. Page 351. line 4 from bottom, for .verophtic read .verophytic. Page 355, above line 6 from bottom, insert Scirpus heterochaetus Chase. Page 356, line 14 from bottom, for Symlocarpus read Symplocarpus. Page 360, line 14, for Pirus read Pyrus. Page 362, after line 7, insert Acer saccharinum L. Page 363, line 2 from bottom, for quadiflorum read quadriflorum. Page 365, line 14, for Ihapus read thapsus. Page 369, last line, for Tanecelum read Tanaceium. Page 417, line i. dele the. Page 497, line 9 from bottom, for neglible read negligible, and in foot-note, for Auslall read Anstalt. Page 498, line 4 from bottom, for Lockport read Chillicothe. Page 500, line 13 from bottom, after up insert in. Page 5or, line 2 from bottom, for dissolving read dissolved. Page 504, line 23, for gryina read gyrina : line 17, for dentata read knickerbockeri. Page 506, line 11, for vernata read ternala. Page 507, line 3 from bottom, for Macon read tt'asoi. Page 513, line 19, for Nepa read Zaitha; line 18, and page 517, line 13 from bot- tom, page 520, line 12 from bottom, and page 532, line 4, read naid or naids for naiid or naiids. Page 517, line 6 from bottom, for pondiveed read pickcrel-iveed. Page 519. for first sentence of last paragraph read as follows: We have no exactly comparable chemical data for July; but analyses for August give percentages of saturation for Morris and Marseilles as follows : 20.4 per cent, at Morris on the nth and 11 per cent, at Marseilles on the 12th; 16.35 P<^r cent, at Morris on the 22d and 23d and 7.4 per cent, at Marseilles on the 24th and 25th. Page 521, line 6 from bottom, and page 529, line g, for chrysoleucas read cryso- leticas. Page 525, line 22, and page 536, lines 21 and 24, for Ekmann read Ekmnn. Page 532, line i, for Ancyclus read Ancylus. Page 551, line 7, for 00 read 572. Page 615, second line above foot-note, for 106 read 94. Page 616, line I, for the second Biindeln read Bilndel; line 2, for Biindeln read Biindcls; line 3, for ausscrn read ausscren; line 6, for sweierlie read sweierlei. Page 629, line 12, for kciii read kcincn. Page 634, line 9, for untcrnommcn read untcninntincnen; and in line 14 from bottom, after ;;/$ insert is fig. Plate III, Fig. i, after the word mixed in legend insert consocies of the. Plate IX, Fig. 2, dele the legend and read instead ; Root-system of Tephrosia virgiiiiana, exposed by blowing of the sand. Plate X, Fig. 2, dele the legend and read instead ; A blowout almost stabilized by bunch-grasses, especially Leptoloma cognatum. Plate XXXIX. for Calainogrostis read Calamagrostis. Plate LIV. exchange places of cuts, but not the legends. Plate LXXXV, for 7 read ye. Article VI. — The Midsummer Bird Life of Illinois: A Statis- tical Study * By Stephen A. Forbes. In the course of a statistical survey of the bird population of the State of Illinois, begun with a view to a better knowledge of the significance of birds in the economy of nature, two field observers, A. O. Gross and H. A. Ray, engaged in this work as assistants on the State Natural History Survey, spent virtually a month of the summer period of 1907 in each of the three principal sections of the state—June in southern, July in central, and August in northern, Illinois. Selecting in each section a locality typical for that part of the state, they made regular trips on foot in various directions and to various distances, traveling always thirty yards apart, and noting as they went the species and numbers of all birds flushed by them on a strip fifty yards in width, including likewise those flying across this strip within a hundred yards to their front. They kept record, also, by means of mechanical counters, of the distances traveled over each distinguishable kind of area, commonly marked by the crop which is borne. The present paper is a report of a few of the more general re- sults of a study of the materials thus brought together, illustrating the numbers and ecological distribution of the birds of Illinois dur- ing the relatively stable period of their summer residence—the time between the conclusion of the spring migration and the beginning of the fall movement to the southward. It is a period of breeding and steady habitation for our most permanent and characteristic bird population, and will best help us to an understanding of the main normal ecological significance of Illinois birds. The Area of Observation The total distance traveled by my observers on these various midsummer trips was 428 miles (omitting fractions), of which 141 miles was in southern Illinois, 112 in central, and 175 in northern. The total area covered by this strict census of the bird population was a trifle over 12 square miles, or 7,693.5 acres—33 per cent, of this acreage being in the southern, 26 per cent, in the central, and 41 per cent, in the northern, part nf the state—or approximately a *Reprinted from the American Naturalist, Vol. XLII, August, 1908. 374 third of this area in southern, a fourth in central, and two fifths in northern, Ilhnois. The field observations began in the south June 4, and ended at the north August 23, with the idea of avoiding, so far as possible, by this order of progress, differences due to different seasonal conditions. It was not possible, of course, to eliminate these wholly, with only one pair of observers ; and it will tax our ingenu- ity, and sometimes perhaps oxertax it, to detect these differences and to distinguish them from those due to mere difference of latitude and of climate corresponding. The total surface on which these precise midsummer observa- tions were made was 1/4720 part of the whole state, and the cjues- tion at once arises, Was this area sufficient to give these results any general value for the state at large, and, if so, how may we be sure of it? There is, I believe, no mathematical method of determining the sufficiency of these data for generalization purposes, and I know of no test at present applicable except that of the general consistency and reasonableness of the totals, averages, and ratios, for the differ- ent districts and seasons, the presence or absence of which each can readily see for himself as this discussion proceeds. If the data of observation are insufficient for the uses made of them, there will be a random variability and inexplicable irregularity in my statistical summaries which we shall not fail to notice. General Product op the Survey Gross and Ray identified during the summer, on the territory covered by their data, 7,740 birds, belonging to 85 species. This is at the rate of 645 birds per square mile, or almost precisely i per acre, including the so-called English sparrow. If we omit the 1,414 interloping English sparrows observed—which is a little more than 18 per cent, of the entire number of birds—we have remaining 527 native birds to the scjuare mile. The total for Illinois,* on this basis, is 30,750,000 native birds and 5,536,000 English sparrows, or ap- proximately 14 summer resident birds to each person in this state living in the country or in towns of less than 25,00x3 inhabitants. Of the 85 species represented by the 7,740 birds recognized on these trips, the 21 most abundant species were represented by 6,596 birds. That is to .say, 85 per cent, of the birds belonged to 25 per cent of the species. The 21 more abundant species numbered,, taken *A cnmliination of the averages for the three sections of the state, computed separately, the data for the sections being differently weighted to compensate for differences in area. 375 together, 550 to the square mile, and the 64 less abundant species, taken together, numbered 95 birds to the square mile, or i to every 6% acres. The latter species are evidently negligible as general factors in the ecological system, and attention need be given, in dis- cussing the birds of the state as a whole, only to the 21 species common enough to produce some appreciable general effect. Given in the order of their abundance they are as follows. A. 0. U. Nos. 376 not sufficient to offset the increase in the native species, the total numbers per square mile for all summer birds in the three sections of the state being 6io, 650, and 682—or 100 birds in northern for 107 in central and 112 in southern Illinois. This same gradation was much more pronounced in the record of the zvinfer residents. From the last of November to March 15, birds averaged 384 to the square mile in northern Illinois; from December 23 to March 21, 582 to the mile in central Illinois; and from February 6 to February 21, 832 to the mile in southern Illinois, — numbers related to each other as 100, 151, and 217. Indeed, we find birds more abundant in extreme southern Illinois in the midwinter period of 1906-07 than in the midsummer period of 1907, averag- ing at the rate of 122 birds in the former season to each hundred in the latter. If we take into account the numbers for the whole year, there are for every hundred birds in the northern part of the state, 133 for central and 181 for southern Illinois. Birds by Sections 377 suits of a trip across central Illinois made by Gross, and Ray during the fall of 1906. A comparison of the general average of the bird population, determined from the data of this trip for the period of the fall migrations, with the midsummer average for the same section of the state, as determined July, 1907, shows an inter- esting difference which leads us to consider the effect of the autum- nal movement to the south on the numbers of the local bird popula- tion. On the above trip across the state, made between August 28 and October 17, 1906, a general average of 579 native, birds to the square mile was found, while the corresponding midsummer average for the year 1907, is 537 native birds to the square mile — a difference of 42 birds to the mile, or nearly 8 per cent, in favor of the fall population. Native Birds per Square Mile, Faix (1906), Summer (1907) 378 and more rapid development at the north of the annual fall migra- tion. This contrast of the number of the resident summer population with that of the fall migration period is still more clearly and strongly shown by a comparison of the totals of all our central Illinois ob- servations in midsummer and in fall, respectively. These average 1.07 birds to the acre for the period from July 9 to September 21, and 2.31 per acre for the interval between the ist and the 26th of October. That is, more than twice as many birds per acre were seen in October, 1907, as in July, August, and September. The data of the spring migration of 1907 are unsatisfactory ow- ing to the extraordinary character of the season, and the consequent repeated interruption and remarkable prolongation of the movement. Nevertheless, they indicate a larger population during the early part, at least, of this migration period also than either before or after it. A trip down the eastern side of the state from Cook to White county, begun March 26 and ending April 11, gave an average of 1.34 birds to the acre—a number to be compared with our midsummer aver- age for the whole state, which is 1.03. That is, the average early spring population of this exceptional year was 30 per cent, greater than the average of the summer following. On the other hand, a trip across central Illinois between April 20 and May 29, still within the migration period, gave us, for 53^ square miles of area, an average of only .89 per acre—less than even the midwinter average of .91 for the same part of the state. Vegetation op the Inspection Area As a basis for a more precise account of the distribution of birds as a whole and of the more important species, it will be necessary to consider the vegetable covering of the soil, since there is little else in Illinois by which different portions of its area may be dis- tinguished. The territory traversed by my observers, it need hardly be said, was almost wholly under cultivation. Excluding only for- ests in which the trees were too high, or the undergrowth was too dense, to permit a full and accurate census of the birds, the terri- tory reported upon was chosen wholly at random, and the total for each division of the state seems sufficient to give us, with the exception just mentioned, a fair sample of its crops and surface con- ditions. The areas from which all the birds were determined were 3,172 acres for northern Illinois, 2,117 acres for central, and 2,504 acres for southern. 379 In the upper third of the state, 95 per cent, of the surface was in corn, small grain, and grass—31 per cent, in corn, 27 per cent, in small grain (nearly all of it oats), and 37 per cent, in the pasture and meadow crops, about equally in each. In the central region the area in corn rises to 46 per cent, of the whole, that in small grains was about 26 per cent, (again nearly all oats), and that in the forage crops was 27 per cent, (the pasture lands nearly twice as extensive as the meadows)—a total of 99 per cent, of the area examined which was devoted to these great farm crops. In the lower third of Illinois only 23 per cent, of the land was in corn, an almost equal area (21 per cent.) was in small grain—more than half of it wheat—and 44 per cent, was in grass, clover, and similar forage plants, rather equally divided between pastures and meadows. That is to say, the areas in corn and small grains were nearly the same, and these to- gether were barely equal to the meadows and pastures. The total Crop Areas. Per cent., 1907 Corn Grain Grass Miscellaneous Northern Illinois 31 27 37 5 Central Illinois 46 26 27 I Southern Illinois 23 21 44 12 in all these crops was 88 per cent, of the area inspected, the remain- ing 12 per cent, covering the orchards, the more open woods, the waste and unfilled lands, and a few additional minor items. Numbers op Birds by Crops Illinois is still a prairie state in the predominance of birds which prefer a grassy turf as an abiding place. Almost exactly half of those recorded for the state in the summer of 1907 were from pastures and meadows, although the total acreage in these lands was but 36 per cent, of the entire area inspected. These figures are equivalent to a density ratio on pastures and meadows of 1.39 for all the birds of the state.* Corn is an exotic crop in Illinois, and birds were only about a third as abundant in corn fields as in grass lands, while in small grains they were nearly twice as abundant as in corn. The acreage in these crops was such that 15 per cent, of all the birds of the season were found in corn fields and 22 per cent, were in small *That is, taking an average density of the bird population for the whole area of the state as r, the density in pastures and meadows only is 1.39. 380 CROP AREAS AND BIRDS 381 of the more pronounced results for one species of bird throughout its range in summer, and for one kind of crop area as visited or inhabited by midsummer birds. The Mkadow-lark One thousand and twenty-five meadow-larks were identified by my observers in their work on the summer residents of the state, an average of 85 to the square mile for the whole area traversed by them. As these birds were unequally distributed, never occur- ring, for example, in woodlands or among shrubbery, their numbers rose in some situations far aboxe this general average, amounting to 266 to the square mile in stubble, 205 in meadows, 160 on un- tilled lands, 143.5 '" pastures, and 131 on waste lands, and falling to 10 to the square mile in fields of corn. Meadow-larks per Square Mile. Summer, 1907 Stubble 266 Meadows 205 Fallow 160 Pastures 143.5 Waste 131 Corn 10 Woods — Shrubs — State 85 They varied also in abundance, in a very interesting way, from the north to the south. One hundred of them in northern Illinois were represented by 175 in central and by 215 in southern Illinois. This variation was evidently independent of any difference in the extent of surface covered by the kinds of vegetation which they most prefer, since the, ratio of pasture, meadow, waste and untilled lands taken together was considerably less for central than for north- ern Illinois, although the meadow-larks were 75 per cent, more nu- merous; and it was only a fourth greater for southern Illinois than for northern, although the meadow-larks were more than twice as abundant. The cause of the greater numbers southward, so far as I can see, can be accounted for only rather vaguely as climatic. Much more difficult of even general or hypothetical explanation is a curious difference in the observed abundance of meadow-larks in pastures and meadows respectively, in the three divisions of the state. In northern Illinois there were 87 larks per square mile in pastures to 129 in meadows; in southern Illinois there were 125 in 382 pastures to 297 in meadows; while in central Illinois this relation was reversed, the number in pastures being 274 to the mile, and that in meadows 189. That is, while 100 pasture birds were represented in northern Illinois by 148 in meadows, and in southern Illinois by 242, in central Illinois they were represented by only 69. Since the southern Illinois observations were made in June, those for central Illinois in July, and those for northern Illinois in August, one nat- urally looks to differences in season, in the advancement of the crops, or in agricultural operations as related to the haunts and habits of these birds, for an explanation of their apparent shift from meadows to pastures in July in central Illinois, and a seemingly plausible ex- planation is suggested by the fact that haying was mainly done dur- ing July in the central part of the state, but was not yet fairly be- gun in southern Illinois in June and was nearly over in northern Illinois in August. Pasture Birds per Square Mile. Summer, 1907 Meadow-larks 383 five kinds of birds in meadows, there were, in the northern section, 25 of them in pastures, in the central section 41, and in the southern section 32. The cause of this apparent change in the preference of the meadow-larks of central Illinois seems, therefore, something peculiar to themselves, and is still to seek. Birds of the Pastures The birds of a given situation may be discussed from two quite different standpoints, both interesting and pertinent, and both really necessary to a complete understanding of the facts. We may con- sider the members of an assemblage of species there with first ref- erence to their relative importance to the situation itself—-with ref- erence, that is, to their comparative numbers, or to the nature and effect of their activities; or we may consider the situation with first reference to its relative importance in the economy and life of each species of bird which inhabits or visits it. If this situation is wood- land, for example, a bird found only in forests might, if a compara- tively rare species, have very little importance—might produce very little effect in the situation because of its infrequent occurrence there, while to the species itself the forest situation would be all-important, as the sole place of its habitation. Its own significance in forests might be easily overbalanced by a very abundant species which should visit woodlands only occasionally, but whose average numbers there might be twice or thrice as large to the unit of area and time as those of the less abundant species inhabiting forests exclusively. Time will not permit me to illustrate this division of my topic from both these points of view, and I will limit myself to a few words in conclusion on the pasture birds as a group and on some of the more prominent pasture species with reference to their importance in pastures. Pasture lands were the preferred resort of our most abundant midsummer birds. That is, more birds were seen in pastures than in any other of the larger crop areas of the state—2,107 in that situation as against 1,814 in meadows, 1,752 in fields of small grain, and 1,169 '" fields of corn. Indeed, 27.2 per cent, of all the mid- summer birds determined by my observers were seen in pastures, 23.4 per cent, in meadows, 22.6 per cent, in small grain, and 15.1 per cent, in corn. The area in pastures was larger than that in meadows, however, and on this account, if we consider the number of birds per square mile, we must change this order of precedence. With a general midsummer average of 645 birds to the square mile 384 for the whole state, we have 920 to the mile for meadows, 878 for pastures, 562 for small grain, and 300 for corn. Or, if we take the number per square mile for the entire state as i, 1.36 will be the density ratio for pastures, 1.43 for meadows, .87 for grain fields, and .47 for corn fields. Summer Birds in Crops, 1907. 385 birds of northern Illinois pastures, 80 per cent, of those of central Illinois, and 64 per cent, of those of southern Illinois. Besides the five species already mentioned, these were the flicker, the robin, the mourning-dove, the red-headed woodpecker, and the red-winged blackbird. One general impression made by this preliminary examination of the present bird population of the State of Illinois is that of a re- markable flexibility and tenacity of the associate and ecological re- lationships of birds in the face of revolutionary changes in their en- vironment. Apart from the results of the introduction of the English sparrow, and the direct destruction of game birds and birds of prey, the main effect of human occupation seems to have been the with- drawal of most of the prairie birds from the area devoted to Indian corn, and their concentration in pastures, meadows, and fields of small grain—situations which most nearly resemble their original habitat.