Zoodiversity_01_2021.indb UDC 598.2:574:504.37(6-13) AVIAN DIVERSITY ALONG A PRECIPITATION GRADIENT IN SOUTHERN AFRICA G. Kopij Department of Integrated Environmental Science Ogongo Campus University of Namibia Private Bag 5520 Oshakati NAMIBIA E-mail:g.kopij@unam.na G. Kopij (https://orcid.org/ 0000-0001-7614-1983) Avian Diversity along a Precipitation Gradient in Southern Africa. Kopij, G. — In April 2013, a transect c. 1100 km long with 100 points arranged in 5 sections was designed in northern Namibia between Ruacana Watefall (W) and Kongola (E) to study avian diversity in relation to gradual changes in precipitation (c. 350 mm in W to c.600 mm in E). In total, 81 bird species were recorded in all 100 points, but in particular section, the numbers were low, ranging from 22 to 38 species. In overall, the most frequent and most numerous (dominant) were the following species: Cape Turtle Dove, Blue Waxbill, Grey-headed Sparrow, Laughing Dove and Fork-tailed Drongo. Together they comprised 42.2 % of all individuals recorded. All of them, except for the Fork-tailed Drongo, were granivores. Only nine species were recorded in ten or more points. Besides the above-mentioned dominant species, the following other were in this group: Black-chested Prinia, Black-throated Canary, Pied Crow and White-browed Scrub Robin. In particular section the number of dominant species ranged from 5 to 7, without a gradient. Two species, the Cape Turtle Dove and Blue Waxbill, remained dominant in all fi ve sections. Th e Laughing Dove and Fork-tailed Drongo — in four sections, and the Grey-headed Sparrow — in three sections. Although, no gradient in species diversity, evenness, and dominance structure was recorded, signifi cant diff erences between two most western and two most eastern sections were apparent. Two species were more frequent and numerous in the eastern than in the western sections: White-browed Scrub Robin and Cape Turtle Dove; while the following species were more frequent and numerous in the western than the eastern sections: African Palm Swift , Black-chested Prinia, Black-throated Canary, Blue Waxbill, Grey- headed Sparrow and Pied Crow. K e y w o r d s : community ecology, ecological gradient, point count method. Zoodiversity, 55(1): 17–24, 2021 DOI 10.15407/zoo2021.01.017 18 G. Kopij Introduction Ecological gradients are quantifi ed patterns describing the ways of change of some ecological parameters, e. g. biodiversity, change with latitude, altitude, humidity, air pressure, air temperature etc. Th ey are among the most widely recognised patterns in ecology and may exist almost in each imaginable ecosystem in the world, and may refer to almost all ecological factors (Smith, 1996). However, the best known are longitudinal, altitu- dinal and precipitation gradients. Th ey are usually linked to gradual changes in vegetation, and these in turn are caused by gradual changes in precipitation and air temperature (Begon et al., 2006). It is well known that, biodiversity is, in general, increasing gradually southwards in northern hemisphere, and northwards in the southern hemisphere; and it is decreasing with the increase of the altitude in most places in the world (Smith, 1996). Less obvious are such changes along a precipitation gradient (e. g. Kopij, 2006, 2014 a). So, in this paper, the precipitation gradient was investigated in the central part of the southern African subcontinent. A subject of this investigation was avian diversity. Since precipitation increases eastwards in central part of southern Africa (Mendesohn et al., 2009), a gradient in avian diversity was also expected. Methods Th e point count method was employed in this study (Bibby et al., 2012; Sutherland, 1996). Points were arranged along a transect running along a highway in extreme northern part of Namibia. Th e transect starting point was near Ruacana Waterfall in the Kunene Region, and the ending point was in Kongola in the Zambezi Region. Five sections (A, B, C, D, and E) were designed in the transect, and in each section 20 points were located. Th ere were therefore together 100 points. Points were separated by a distance of 5 km in section A–D, and by 10 km in section E (fi g. 1, table 1). All birds seen and/or heard around each such point were recorded. At each point counting lasted about fi ve minutes. Counting was conducted in the morning under sunny and calm conditions. Points were accessed by a car. Th e dominance is expressed here as the percentage of the total number of individuals of a given species in relation to the total number of all individuals of all species recorded. Th e dominant species is defi ned as that comprising 5 % and more of all individuals of all species recorded, while subdominant — that comprising 2–4.99 %. Th e cumulative dominance is defi ned as the sum of dominance value of all dominant species. Th e community dominance index was calculated as follow: DI = (n1 + n2)/N, where n1, n2 — number of pairs of two most abundant species, N — total number of pairs of all species. Th e following indices were used to characterize the diversity and evenness of the communities: 1) Shannon’s diversity index: H’ = –∑ pi ln pi, where: pi is the proportion of individuals belonging to the ith species; 2) Simpson’s diversity index: D = ((∑n(n-1))/N(N-1), where: n — total number of individuals belonging to a given species, N — total number of breeding pairs of all species; Fig. 1. Location of the transect (broken red line in the map above, yellow line in the map below). 19Avian Diversity along a Precipitation Gradient in Southern Africa 3) Pielou’s evenness index: J’ = (–∑ pi ln pi)/ln S, where pi is the proportion of individuals belonging to the ith species; S — total number of species. J’ varies between 0 and 1. Th e less variation between species in a community, the higher J’ is. Similarity among avian communities (study plots) was investigated using the Sörensen’s Coeffi cient: I = 2C/A+B, where A — the number of bird species in one plot, B — the number of bird species in another plot, C — the number of bird species common to both plots. Systematics and nomenclature of bird species follow Hockey et al. (2005). Scientifi c names of all bird species are listed in Appendix 1. Results A total of 81 bird species were recorded in all 100 points. In the particular section, the numbers were, however, much lower, ranging from 22 to 38 species (table 2). In overall, the most frequent and most numerous (dominant) were the following species: Cape Turtle Dove (recorded in 40 % of points, and comprising 11.7 % of all individuals recorded), Blue Waxbill (47 % vs. 9.3 %), Grey-headed Sparrow (38 % vs. 8.3 %), Laughing Dove (37  % vs. 7.6  %) and Fork-tailed Drongo (29 % vs. 5.3 %). Together they comprised 42.2  % of all individuals recorded. All of them, except for the Fork-tailed Drongo, were granivores. Only nine species were recorded in ten or more points. In addition to the above-mentioned dominant species, the following other were in this group: Black-chested Prinia, Black- throated Canary, Pied Crow and White-browed Scrub Robin (App. 1). In the particular section, the number of dominant species ranged from 5 to 7, without a gradient. Two species, the Cape Turtle Dove and Blue Waxbill, remained dominant in all fi ve sections. Th e Laughing Dove and Fork-tailed Drongo — in four sections, and the Grey-headed Sparrow — in three sections. Nine other species were recorded as dominant only in two (Black-throated Canary, White-browed Scrub Robin) or in one section (Black- chested Prinia, African Palm Swift , Cattle Egret, Pied Crow, Rattling Cisticola, Red-billed Hornbill and Red-eyed Dove) (App. 1). However, the cumulative dominance has decrease eastwards, while no gradient was recorded in the community dominance index (table 3). Th e study suggests that there is an eastward increase in the population density of the following species: Brubru, Burchell’s Starling, Emerald-spotted Dove, Red-billed Oxpecker, Ground Hornbill, Schalow’s Turaco, Red-eyed Dove, Magpie Shrike, Southern Black Tit, T a b l e 1 . Th e division of the latitudinal transect into sections, their length, number of count points and date of counts No Section Mean annual precipitation, mm Length, km No. of points Data of counts A Ruacana Waterfall-Outapi 350 100 20 13.04.2013 B Ogongo-Ondangua 450 100 20 15.04.2013 C Eenhana-Mpingu Vlei 500 100 20 22.04.2013 D Rundu-Kaukuwa 550 100 20 08.04.2013 E Divindu-Kongola 600 200 20 08.04.2013 T a b l e 2 . Characterisation of breeding bird community in fi ve sub-transects (sections) of a latitudinal transect in northern Namibia Parameter A B C D E Total Number of species and pairs Number of species 30 32 22 38 27 81 Number of breeding pairs 120 157 74 123 88 591 Dominance Number of dominant species 6 7 5 6 6 5 Cumulative dominance (%) 57.0 62.7 67.8 64.8 58.2 42.2 Community dominance (DI) 0.28 0.28 0.65 0.24 0.44 0.21 Indices Shannon’s Diversity Index (H’) 2.93 2.94 2.55 3.53 2.84 2.14 Pielou’s Evenness Index (J’) 0.86 0.85 0.82 0.97 0.86 0.49 20 G. Kopij A p p en d ix 1 . R es ul ts o f p oi nt c ou nt s o n bi rd s a lo ng a p re ci pi ta ti on g ra di en t i n no rt he rn N am ib ia Sp ec ie s Se ct io n A Se ct io n B Se ct io n C Se ct io n D Se ct io n E T ot al F N % N F N % N F N % N F N % N F N % N F N % N A ca ci a Pi ed B ar be t, Tr ic ho la em a le uc om el as 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 8 1 1 1. 1 2 2 0. 34 A fr ic an G ol de n O ri ol e, O ri ol us a ur at us 0 0 0. 0 0 0 0. 0 0 0 0. 0 2 2 1. 6 2 2 2. 3 4 4 0. 68 A fr ic an G re y H or nb ill , L op ho ce ru s n as ut us 1 1 0. 8 0 0 0. 0 4 5 6. 8 1 1 0. 8 3 3 3. 4 9 10 1. 69 A fr ic an H oo po e, U pu pa a fr ic an a 0 0 0. 0 1 1 0. 6 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 17 A fr ic an O pe nb ill , A na st om us la m m el lig er us 0 0 0. 0 1 4 2. 5 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 4 0. 68 A fr ic an P al m S w ift , C yp siu ru s p ar vu s 1 3 2. 5 5 12 7. 6 2 2 2. 7 0 0 0. 0 0 0 0. 0 8 17 2. 88 A fr ic an P ip it, A nt hu s c in na m om eu s 0 0 0. 0 3 3 1. 9 0 0 0. 0 0 0 0. 0 0 0 0. 0 3 3 0. 51 A rr ow -m ar ke d Ba bb le r, Tu rd oi de s j ar di ne ii 0 0 0. 0 0 0 0. 0 0 0 0. 0 2 2 1. 6 3 4 4. 5 5 6 1. 02 Bl ac k- ba ck ed P uffb ac k, D ry os co pu s c ub la 0 0 0. 0 0 0 0. 0 1 1 1. 4 0 0 0. 0 0 0 0. 0 1 1 0. 17 Bl ac k- ch es te d Pr in ia , P ri ni a m au lo sa 5 5 4. 2 9 9 5. 7 0 0 0. 0 2 2 1. 6 0 0 0. 0 16 16 2. 71 Bl ac k- co lla re d Ba rb et , L yb iu s t or qu at us 0 0 0. 0 2 2 1. 3 0 0 0. 0 2 2 1. 6 0 0 0. 0 4 4 0. 68 Bl ac k- he ad ed H er on , A rd ea m el an oc ep ha la 0 0 0. 0 1 1 0. 6 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 17 Bl ac k- sm ith L ap w in g, V an el lu s a rm at us 0 0 0. 0 2 3 1. 9 0 0 0. 0 1 1 0. 8 0 0 0. 0 3 4 0. 68 Bl ac k- th ro at ed C an ar y, C ri th ag ra a tr og ul ar is 10 10 8. 3 9 9 5. 7 0 0 0. 0 0 0 0. 0 0 0 0. 0 19 19 3. 21 Bl ue W ax bi ll, U ra eo gi nt hu s a ng ol en sis 14 18 15 .0 14 17 10 .8 7 8 10 .8 8 8 6. 5 4 4 4. 5 47 55 9. 31 Br oa d- bi lle d R ol le r, Eu ry st om us g la uc ur us 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 8 0 0 0. 0 1 1 0. 17 Br ow n- cr ow ne d T ch ag ra , T ch ag ra a us tr al is 0 0 0. 0 0 0 0. 0 1 1 1. 4 0 0 0. 0 0 0 0. 0 1 1 0. 17 Br ub ru , N ila us a fe r 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 3 3 3. 4 3 3 0. 51 Bu rc he ll’ s S ta rl in g, L am pr ot on is au st ra lis 0 0 0. 0 0 0 0. 0 1 1 1. 4 0 0 0. 0 2 2 2. 3 3 3 0. 51 C ap e St ar lin g, L am pr ot or ni s n ite ns 2 2 1. 7 0 0 0. 0 0 0 0. 0 2 3 2. 4 0 0 0. 0 4 5 0. 85 C ap e T ur tle D ov e, S tr ep to pe lia ca pi co la 3 4 3. 3 3 3 1. 9 11 22 29 .7 18 33 26 .8 5 7 8. 0 40 69 11 .6 8 C ar di na l W oo dp ec ke r, D en dr op ic os fu sc es ce ns 0 0 0. 0 0 0 0. 0 0 0 0. 0 3 3 2. 4 0 0 0. 0 3 3 0. 51 C at tle E gr et , B ub ul cu s i bi s 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 3 18 20 .5 3 18 3. 05 C in na m on -b re as te d, B un tin g Em be ri za ta ha pi si 1 1 0. 8 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 17 C om m on , Q ue le a, Q ue le a qu el ea 1 8 6. 7 0 0 0. 0 0 0 0. 0 1 6 4. 9 0 0 0. 0 2 14 2. 37 C ri m so n- br ea st ed S hr ik e, L an ia ri us a tr oc oc ci ne us 0 0 0. 0 0 0 0. 0 0 0 0. 0 4 4 3. 3 0 0 0. 0 4 4 0. 68 21Avian Diversity along a Precipitation Gradient in Southern Africa D ar k- ca pp ed B ul bu l, Py cn on ot us tr ic ol or 0 0 0. 0 1 1 0. 6 0 0 0. 0 2 3 2. 4 0 0 0. 0 3 4 0. 68 Em er al d- sp ot te d D ov e, T ur tu r c ha lc os pi lo s 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 1. 1 1 1 0. 17 Fo rk -t ai le d D ro ng o, D ic ru ru s a ds im ili s 4 6 5. 0 2 2 1. 3 6 6 8. 1 9 9 7. 3 8 8 9. 1 29 31 5. 25 G ab ar G os ha w k, M ic ro ni su s g ab ar 0 0 0. 0 0 0 0. 0 2 2 2. 7 0 0 0. 0 0 0 0. 0 2 2 0. 34 G ol de n- br ea st ed B un tin g, E m be ri za fl av iv en tr is 0 0 0. 0 0 0 0. 0 0 0 0. 0 3 3 2. 4 2 2 2. 3 5 5 0. 85 G ol de n- ta ile d W oo dp ec ke r, C am pe th er a al bi ng . 0 0 0. 0 0 0 0. 0 1 1 1. 4 1 1 0. 8 0 0 0. 0 2 2 0. 34 G re at er S tr ip ed S w al lo w , C er co pi s c uc ul la ta 1 2 1. 7 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 2 0. 34 G re en W oo d H oo po e, P ho en ic ul us p ur pu re us 0 0 0. 0 0 0 0. 0 2 3 4. 1 0 0 0. 0 0 0 0. 0 2 3 0. 51 G re en -b ac ke d M el ba , P yt yl ia m el ba 0 0 0. 0 0 0 0. 0 1 1 1. 4 0 0 0. 0 0 0 0. 0 1 1 0. 17 G re y G o- aw ay -b ir d, C or ith ai xo id es co nc ol or 0 0 0. 0 0 0 0. 0 0 0 0. 0 2 2 1. 6 1 1 1. 1 3 3 0. 51 G ro un ds cr ap er Th r us h, T ur du s l its its ir up a 0 0 0. 0 1 1 0. 6 1 1 1. 4 2 2 1. 6 1 1 1. 1 5 5 0. 85 La ug hi ng D ov e, S tr ep to pe lia se ne ga le ns is 10 12 10 .0 9 12 7. 6 6 6 8. 1 12 15 12 .2 0 0 0. 0 37 45 7. 61 Le ss er G re y Sh ri ke , L an iu s m in or 3 3 2. 5 2 2 1. 3 0 0 0. 0 0 0 0. 0 0 0 0. 0 5 5 0. 85 Le ss er S tr ip ed , S w al lo w , C er co pi s a by ss in ic a 3 4 3. 3 1 1 0. 6 0 0 0. 0 1 1 0. 8 0 0 0. 0 5 6 1. 02 Li la c- br ea st ed , R ol le r, C or ac ia s c au da tu s 2 2 1. 7 0 0 0. 0 0 0 0. 0 1 1 0. 8 0 0 0. 0 3 3 0. 51 Lo ng -b ill ed , C ro m be c Sy lv ie tt a ru fe sc en s 1 1 0. 8 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 17 M ag pi e Sh ri ke , U ro le st es m el an ol eu cu s 0 0 0. 0 0 0 0. 0 0 0 0. 0 5 5 4. 1 1 1 1. 1 6 6 1. 02 M ar ic o Su nb ir d, C in ny ri s m ar iq ue ns is 2 2 1. 7 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 2 2 0. 34 M au ri co F ly ca tc he r, M el ae no rn is m ar iq ue ns is 1 1 0. 8 1 1 0. 6 0 0 0. 0 0 0 0. 0 0 0 0. 0 2 2 0. 34 M ey er ’s Pa rr ot , P oi ce ph al us m ey er i 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 1. 1 1 1 0. 17 N am aq ua , D ov e, O en a ca pe ns is 1 1 0. 8 1 1 0. 6 0 0 0. 0 0 0 0. 0 0 0 0. 0 2 2 0. 34 Pi ed C ro w C or vu s a lb us 4 4 3. 3 13 14 8. 9 0 0 0. 0 0 0 0. 0 1 1 1. 1 18 19 3. 21 Q ua ilfi n ch , O rt yg os pi za a tr ic ol lis 0 0 0. 0 2 2 1. 3 0 0 0. 0 2 2 1. 6 0 0 0. 0 4 4 0. 68 R at tli ng C is tic ol a, C ist ic ol a ch in ia na 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 5 5 5. 7 5 5 0. 85 R ed -b ill ed B uff a lo W ea ve r, Bu ba lo nr is ni ge r 1 3 2. 5 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 3 0. 51 R ed -b ill ed H or nb ill , T oc ku s e rt hr or hy nc hu s 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 8 4 5 5. 7 5 6 1. 02 R ed -b ill ed O xp ec ke r, Bu ph ag us e ry th ro ry nc hu s 0 0 0. 0 0 0 0. 0 1 1 1. 4 0 0 0. 0 0 0 0. 0 1 1 0. 17 R ed -b re as te d Sw al lo w , C er co po is se m ir uf a 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 8 0 0 0. 0 1 1 0. 17 22 G. Kopij R ed -e ye d Bu lb ul , P yc no no tu s n ig ri ca ns 1 1 0. 8 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 17 R ed -e ye d D ov e, S tr ep to pe lia se m ito rq ua ta 0 0 0. 0 0 0 0. 0 0 0 0. 0 8 9 7. 3 0 0 0. 0 8 9 1. 52 R ed -f ac ed M ou se bi rd , U ro co liu s i nd ic us 1 1 0. 8 4 5 3. 2 0 0 0. 0 2 2 1. 6 0 0 0. 0 7 8 1. 35 R oc k K es tr el , F al co ru pi co lu s 1 1 0. 8 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 17 R uf ou s- ve nt ed W ar bl er , S yl vi a ca er ul ea 0 0 0. 0 5 5 3. 2 0 0 0. 0 3 3 2. 4 0 0 0. 0 8 8 1. 35 R ed -c ap pe d La rk , C al la nd re a ci ne re a 0 0 0. 0 1 1 0. 6 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 17 Sc al y- fe at he re d W ea ve r, Sp or op ip es sq ua m ifr on s 1 1 0. 8 2 4 2. 5 0 0 0. 0 0 0 0. 0 0 0 0. 0 3 5 0. 85 Sc ha lo w ’s T ur ac o, T au ra co sc ha lo w i 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 1. 1 1 1 0. 17 So ut he rn B la ck , T it Pa ru s n ig er 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 8 2 2 2. 3 3 3 0. 51 So ut he rn G re y- he ad ed S pa rr ow , P as se r d iff us us 13 15 12 .5 20 27 17 .2 0 0 0. 0 5 7 5. 7 0 0 0. 0 38 49 8. 29 So ut he rn G ro un d H or nb ill , B uc or vu s l ea db ea te ri 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 2 2 2. 3 2 2 0. 34 So ut he rn M as ke d W ea ve r, Pl oc eu s v el at us 5 5 4. 2 4 4 2. 5 0 0 0. 0 0 0 0. 0 0 0 0. 0 9 9 1. 52 So ut he rn Y el lo w -b ill ed H or nb ill , T oc ku s l eu co m el 0 0 0. 0 0 0 0. 0 1 1 1. 4 0 0 0. 0 3 3 3. 4 4 4 0. 68 Sw ai ns on ’s Fr an co lin , F ra nc ol in us sw ai ns on ii 0 0 0. 0 1 1 0. 6 0 0 0. 0 2 2 1. 6 0 0 0. 0 3 3 0. 51 Sw am p Bo ub ou , L an ia ri us b ic ol or 0 0 0. 0 0 0 0. 0 0 0 0. 0 6 6 4. 9 0 0 0. 0 6 6 1. 02 T aw ny E ag le , A qu ila ra pa x 0 0 0. 0 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 1. 1 1 1 0. 17 Th re e- ba nd ed P lo ve r, C ha ra dr iu s t ri co lla ri s 0 0 0. 0 3 3 1. 9 0 0 0. 0 0 0 0. 0 0 0 0. 0 3 3 0. 51 V io le t- ba ck ed S ta rl in g, C in ny ri ci nc lu s l eu co ga st er 0 0 0. 0 0 0 0. 0 1 1 1. 4 1 1 0. 8 0 0 0. 0 2 2 0. 34 V io le t- ea re d W ax bi ll, G ra na tin a gr an at in a 0 0 0. 0 0 0 0. 0 1 1 1. 4 0 0 0. 0 0 0 0. 0 1 1 0. 17 W hi te -b el lie d Su nb ir d, C yn ni ri s t al at al a 0 0 0. 0 0 0 0. 0 2 2 2. 7 0 0 0. 0 0 0 0. 0 2 2 0. 34 W hi te -b ro w ed S cr ub R ob in , C er co tr ic ha s l eu cu co p 1 1 0. 8 0 0 0. 0 6 6 8. 1 2 3 2. 4 8 8 9. 1 17 18 3. 05 W hi te -c ro w ne d Sh ri ke , E ur oc ep ha lu s a ng ui tim en s 1 1 0. 8 0 0 0. 0 1 1 1. 4 0 0 0. 0 0 0 0. 0 1 2 0. 34 W ir e- ta ile d Sw al lo w , H ir un do sm ith ii 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 8 0 0 0. 0 1 1 0. 17 Y el lo w -b ill ed E gr et , E gr et ta in te rm ed ia 0 0 0. 0 1 1 0. 6 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 1 0. 17 Y el lo w -b ill ed K ite , M ilv us a et io pi cu s 1 1 0. 8 2 2 1. 3 1 1 1. 4 0 0 0. 0 0 0 0. 0 4 4 0. 68 Y el lo w -b ill ed O xp ec ke r, Bu ph ag us a fr ic an us 0 0 0. 0 1 3 1. 9 0 0 0. 0 0 0 0. 0 0 0 0. 0 1 3 0. 51 Y el lo w -f ro nt ed T in ke rb ir d, P og on iu lu s c hr ys oc om 0 0 0. 0 0 0 0. 0 0 0 0. 0 2 2 1. 6 1 1 1. 1 3 3 0. 51 N o te . F — F re qu en cy o f o cc ur re nc e of a g iv en s pe ci es in 1 00 p oi nt s, N — to ta l n um be r of r ec or de d in di vi du al s in a ll po in ts ; % N — p er ce nt ag e of th e to ta l n um be r of in di vi du al s o f a ll sp ec ie s r ec or de d. V al ue s f or d om in an t s pe ci es a re in di ca te d w ith b ol d ca se . 23Avian Diversity along a Precipitation Gradient in Southern Africa White-browed Scrub Robin, Golden-breasted Bunting, Arrow-marked Babbler, African Golden Oriole and Yellow-fronted Tinkerbird. Th e data suggest also westward increase in the population densities of the Yellow-billed Oxpecker, Namaqua Dove, Lesser Striped Swallow, Scaly-feathered Finch and Grey-headed Sparrow (App. 1). Contrary to expectation, no gradient was recorded in the number of bird species, neither in the number of individuals recorded. Also no gradient was recorded in the Diversity Index and the Evenness Index (table 3). Although, no gradient in species diversity, evenness, and dominance structure was recorded, signifi cant diff erences between two most western and two most eastern sections were apparent. Two species were more frequent and numerous in the eastern than in the western sections: White-browed Scrub Robin and Cape Turtle Dove; while the following species were more frequent and numerous in the western than the eastern sections: African Palm Swift , Black-chested Prinia, Black-throated Canary, Blue Waxbill, Grey-headed Sparrow and Pied Crow. All the diff erences were statistically signifi cant (table 3). As expected, the most similar were bordering sections laying at the two ends of the transect: A vs. B, and D vs. E; the most dissimilar — sections laying at opposite ends of the transect: A vs. E, and B vs. E (table 4). Discussion Th e point count method employed in this study could underestimate some elusive, not active, and silent species, e. g. tits (Paridae), fi nches (Estrildidae) or korhaans (Otidae). Of cause, nocturnal birds were omitted in this study altogether. Th e points were not ar- ranged near lakes or marshlands, so also species associated with these habitats are omitted here. Th e point count method employed in this study enables, however, a rough estimate of abundance of common and easily detected species, such as doves, shrikes or drongos. In the northern hemisphere a precipi- tation and temperature gradients overlap to large extent with the latitudinal gradi- ent. In some tropical regions of the world, e. g. in southern Africa, such overlap does not exist, and more oft en such changes oc- cur along the same latitude, i.e. from the east to the west. Such situation was also confi rmed in this study. Ta b l e 3 . Diff erence in the frequency of occurrence and in the numerical percentage in bird species be- tween eastern (D, E) and western (A, B) parts of Namibia Species Based on frequency of records Based on number of indi- viduals recorded x2-value Level of sig-nifi cance x 2-value Level of sig-nifi cance African Palm Swift E < 5 – 15 0.01 Black-chested Prinia 9 0.01 9 0.01 Black-throated Canary 19 0.01 19 0.01 Blue Waxbill 6.4 0.05 11.2 0.01 Cape Turtle Dove 9 0.01 23.2 0.01 Cattle Egret E < 5 – 9 0.01 Fork-tailed Drongo 5.2 0.05 1.6 > 0.05 Grey-headed Sparrow 20.6 0.01 25 0.01 Laughing Dove 1.6 > 0.05 2.1 > 0.05 Pied Crow 14.2 0.01 15.2 0.01 Red-billed Quelea E < 5 – 0.1 > 0.05 White-browed Scrub Robin 7.2 0.01 8.3 0.01 Total 2.1 > 0.05 2.6 > 0.05 N o t e . All species listed occur commonly both in the eastern (D, E) and the western (A, B) part of the transect. T a b l e 4 . Sorensen index of similarity between particular sections (A–E) of the transect A B C D E A x 0.55 0.35 0.44 0.18 B X 0.26 0.43 0.17 C x 0.30 0.33 D x 0.46 E x 24 G. Kopij One of the main assumption about the precipitation biodiversity gradient is that the list of species in a given point is complete. In this, as in most other studies, such assump- tion was, however, not met. Th is means that the gradient steepness may, in fact, be more pronounced. Such gradient is also more pronounced in a macro (within a few degrees) than in a micro-scale (within a degree), hence the points were grouped in sections to create macro-scale scenario. Cape Turtle Dove, Laughing Dove, Grey-headed Sparrow and Blue Waxbill recorded in this study as dominant species are probably the most numerous species in northern Na- mibia at large, in both natural and human-transformed habitats (Kopij, 2014 a, b, 2015 a, b, 2017, 2018 a, b, 2019 a, b). Except for the Cape Turtle Dove, they also appear to be good indicators of disturbed land, as they dominate both urbanized and agricultural landscape of Namibia (Kopij, 2014 a, b, 2015 c, 2017, 2018 b, 2019 a, b). Most of them were more nu- merous in the western than in the eastern sections. It is probably because, granivores, are better adopted to live under savannah (western sections) than woodland (eastern sections) conditions, as savannas produce much more seeds (especially grass seeds) than woodlands (Smith, 1996; Begon et al., 2006). Th e western or eastern border of geographical range of some bird species run through north-central Namibia (Hockey et al., 2005), hence these species were common either in the eastern or western sections of the transect, but virtually absent in the opposite ones. Th e group includes the following ‘eastern’ species: Magpie Shrike, Schalow’s Turaco, Red-billed Oxpecker, Broad-billed Roller, Emerald-spotted Dove, Swamp Boubou, Burchell’s Starling. Th e group of ‘western’ species includes the Yellow-billed Oxpecker and Red-eyed Bulbul. References Begon, M., Townsend, C. R, Harper, J. L. 2006. Ecology. From Individuals to Ecosystems. 4th ed. Blackwell Publishing. Bibby, C. J., Burgess N. D., Hill, D. A., Mustoe, S. 2012. Bird Census Techniques, 2nd ed. Academic Press, London. Hockey, P. A. R., Dean, W. R. J., Ryan, P. G, Maree, S., eds. 2005. Roberts’ Birds of Southern Africa. John Voelcker Bird Book Fund, Cape Town. Kopij, G. 2006. Th e Structure of Assemblages and Dietary Relationships in Birds in South African Grasslands. Wydawnictwo Akademii Rolniczej we Wrocławiu, Wrocław. Kopij, G. 2014 a. Avian Assemblages in Urban Habitats in North-central Namibia. International Science & Technology Journal of Namibia, 3 (1), 64–81. Kopij, G. 2014 b. Avian communities of a Mixed Mopane-Acacia Savanna in the Cuvelai Drainage System, North-Central Namibia, During the Dry and Wet Season. Vestnik Zoologii, 48 (4), 269–274. Kopij, G. 2015 a. 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Population density and structure of birds breeding in an urban habitat dominated by large baobabs (Adansonia digitata), Northern Namibia. Biosystem Diversity, 27, 354–360. Kopij, G. 2019 b. Structure of avian communities in a mosaic of built-up and semi-natural urbanised habitats in Katima Mulilo town, Namibia. Welwitschia International Journal of Agricultural Sciences, 1, 68–75. Mendesohn, J., Jarvis, A., Roberts, C., Robertson, T. 2009. Atlas of Namibia. A Portrait of the Land and its People. Sunbird Publishers, Windhoek. Smith, R. L. 1996. Ecology and Field Biology. 5th ed. Addison-Wesley Longman, Menlo Park (CA, USA). Sutherland, W. J., ed. 1996. Ecological census techniques. A handbook. Cambridge University Press, Cambridge. Received 23 June 2020 Accepted 5 January 2021