OIKOS 33: 121-126, Copenhagen 1979 d
l: I 1e Morphological correlates of habitat selection in a community ,r of shorebirds (Charadriiformes) I) n h Myron Charles Baker a e 1- Baker, M. C. 1979. Morphological correlates of habitat selection in a community of shorebirds (Charadriiformes), - Oikos 33: 121-126.
Habitat selection by six species of shorebirds (Least Sandpiper1 Semipalmated Sandpiper, Dunlin, Short-billed Dowitcher, Lesser Yellowlegs, and Semipalmated Plover) was studied on wintering and breeding ranges. Features of the terrain that r characterize the feeding m icrohabitat were compared to the surrounding habitat and I. examined in relation to culmen and tarsal length. The main results were, (a) The shorebirds tend to forage along edges adjacent to talleryegetation and deeper water than are typical of the feeding habitat; (b) The diversity of the feeding microhabitat is greater in summer than in winter; (c) Culmen length is positively correlated with vegetation height and diversify of the feeding mici-ohabitat; (d) Tarsal length is positively corr-elated with water depth in the feeding microhabitat; and (e) The degree to which the utiliZed microenvironment is distinct from the surrounding area ' varies among the species. The bill of a shorebird is a versatile instrument and this is I, not obvious from morphology alone. An increase in bill length appears to produce an i ,r increase in versatility and exploitation of a more diverse environment. M. C. Baker, Dept of Zoology and Entomology; Colorado State Univ,, Fort Collins, " CO 80523, USA. . j
Hccn~Barrn: 5HOT0!1W{€Ci(YJ() H30Mpa1'e.nbHOCTh Y 6 BM,tl;OB 6eperol3blX n~ neco:rnHKa Kp::(lJl oo.rtbl:tJ8 JieTCM, \J:eM 3HM.'..)A: 1 B/ lVIJ1Ha Ha,rl;K.TllOBbrI ITOJJO)l{HT€J11)H0 KoPpeJIHpyeT C BbCQTOA: pacTHTe.rJI:,HOCTH H )?a3Hoo6pa3Ho.1. KOI=M)BbDC yqaCTI I I Accepted 14 November 1978 I © OIKOS 0030-1299/79/040121-06 $ 02.50/0 ') I9 OIKQS 33: I (1979) 121 l. Introduction values up to and including the mean length of the feed ing transect for that species. This operation was per To develop insight into the relationship of bill and leg formed for the A, B, C, and D transects, and the total length to foraging ecology in shorebirds, l made a de profile was then assembled. tailed study of the microhabitats of six species on their -·E breeding and wintering ranges. In this report, I describe .'! 2.3. Computation of microhabitat diversity the microhabitat features that characterize and distin guish the terrain where shorebirds forage, and relate Vegetation height and water depth were grouped into these features to culmen and tarsal length. sixteen categories: wet mud or marl surface, water 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and> 10 cm deep, dry mud or marl surface, vegetation 1-5, 6-10, and >10 cm high. Each 2. Methods transect constitutes a collection of height/depth categories consisting of a number of 20 cm units. Esti 0 Populations of Least Sandpiper Calidris minutilla, mation of microhabitat diversity must take into account Semipalmated Sandpiper C. pusilla, Dunlin C. alpina, two facts: (1) not all transects are of equal length and I Short-billed Dowitcher Limnodromus griseus, Lesser thus the number of height/depth categories may in I Yellowlegs Tringa flavipes, and Semipalmated Plover crease with transect length and (2) not all transects <.9 w Charadrius semipalmatus were studied in winter contain all categories. Pielou (1966) has provided a I -1 1968-69 on coastal habitats in the Everglades National procedure for this problem. Applied to microhabitat di z Park near Flamingo, Monroe Co., Florida. Summer· versity this entails the following steps; calculate Bril o' I- observations were made on the breeding grounds near louin's H for the feeding transect from H = 1/N (log ! - <( I- I Churchill, Manitoba, during the months of June, July, :1:log n, !) in which N = total transect units (total feeding w <.9 -, and August 1968-69. transect/20), and n1 = number of units in each w Winter habitats were intertidal mudflats and coastal height/depth category. Next, H is calculated succes > -1, marsh with a few study areas containing mangrove and sively, each time adding on another feeding transect. saltgrass vegetation. Summer habitats were tundra and When the cumulative H becomes relatively stable, sue... taiga composed of muskeg, sedge marshes, upland cessive estimation of H' (true average diversity) can be lichen plains, small shrubs such as dwarf willow and made. This is accomplished by calculating h, as succes tram birch and some larger trees. sive estimates then using the average hk as the estimate sider of H', h• = (N•H• - N,_,Hk_,)/(Nk - N._1) in which the may subscript k indexes the number of transects accumu~ 2.1. Measurement methods talle1 lated. This method of estimating H'allows calculation of speci Foraging birds were observed for 2-min periods. the standard error. I ser 'J Observations interrupted by the bird flying were dis watei carded. The two points which were farthest apart on the To foraging path were connected by running a string, 3. Results rohal knotted at 20 cm intervals, between stakes set at the two midli 3.1. Pro!iles points. The height of the vegetation and the depth of i the Ji water were measured with a ruler every 20 cm along the The average terrain features of microhabitats where stand feeding transect. Four additional sets of measurements each shorebird species forages on the breeding grounds The were taken. A set was taken along a Jine extending from have several interesting properties (Fig. 1). Average Sandi each end of, and equal in length to, the feeding transect vegetation height and water depth along the feeding abOVl (A and B transects). Lines equivalent in length to the transect tends to be conspicuously less than the sur Dowi feeding transect were extended out at right angles from rounding areas. This is most extreme in the smaller ctatio the center of the feeding transect (C and D transects). three species, Least Sandpiper, Semipalmated are te Data were collected on 50 or more individuals of each Sandpiper, and Semipalmated Plover, whereas Dunlin down species in summer and 50 or more individuals of each feeding transects are least differentiated from the sur depth species in winter. rounding habitat. No consistent differences are seen in Pro comparing the zone of habitat ahead of the trajectory of were: the foraging bird (A transect) and the zone behind (B 2.2. Preparation of microhabitat 1>rofilcs A and transect). The terrain to the right and left of the forag the fe, Profiles of the average vegetation height and water ing bird (C and D transects) is characterized by deeper much depth were constructed to evaluate each species' forag water and taller vegetation with increasing distance I legsw ing microhabitat. The seasonal profile was constructed from the feeding transect. Semipalmated Sandpipers . summ1 in the following way. The average vegetation height and forage along the edge of deeper water. To a lesser ex- · For water depth in the first 20 cm unit of the transect was tent, this is also true for Semipalmated Plovers, and j differe calculated for all individuals and this operation was re Least Sandpipers. Dunlin and Short-billed Dowitchers l habita peated sequentially along the feeding transect for all show some of the water edge property in their C and D contai: 122 OIKOS 33: 1 (1979) I ,cl. Fig, 1, Summer profiles of !f# foraging microhabitats of six tal ! species of shorebird!!, Above lhe horizontal line is vegetation relief and below E the line is water, Feeding -'=- f------"'-- transects are stippled, A ,,_ II.~ transects are ahead of the trajectory of the feeding to Q_ s pp w bird, B behind, C to the 2, 0 18 right, and D to the left. SPP lfl 0: 12 Semipalmated Plover, w = ch I·- 6 L YL = Lesser Yellowlegs, th <( D D C SBD = Short-billed :,: 0 Dowitcher, Sps = Li· Semipalmated Sandpiper, nt DUN = Dunlin, LST = 1d Least Sandpiper. 11· :ts a Ii- DUN 12 ii- f- 6 <( f- 0 D ,g uJ (') ·6 ch w is- > ·12 ct. 100- cm 0- be :s- I transects but Lesser Yellowlegs has almost none. Con contained in the average of the A and B transect pro te sidering the vegetation of the C and D transects, one files (Fig. 1) and expressed this figure as a percent. he I may also see that most species forage along the edge of Separate ratios were calculated for vegetation and water u- I taller vegetation. This is rather pronounced in all portions of the profiles (Tab. 1). of " species except Dunlin. It is interesting to note that Les ser Yellowlegs ,.show little evidence of foraging along water edges, but definite vegetation edges are present. 3.2. Feeding transect diversity To visualize a bird's eye view of the foraging mic The profiles of the feeding transects give a qualitative rohabitat, a short horizontal line is placed above the idea of the diversity of microenvironments encountered midline of the feeding transect (Fig. 1). The height of by an individual bird. To provide a quantitative as well the line corresponds to the top of a bird's head as it as a graphic representation of this diversity, cumulative re stands in the water at the center of the feeding transect. diversity curves were constructed for all the species of ds The smaller shorebirds, Scmipalmated and Least shorebirds. A curve that rises rapidly to a fairly flat ge Sandpipers, Semipalmated Plover, tend to project plateau indicates that all the individual feeding transects ng above the vegetation whereas Dunlin, Short-billed are similar and each new bird added into the cumulative 1l If Dowitcher and Lesser Yellowlegs forage through veg I er etation at or above head height. These generalizations Ii eel are tempered by the fact that birds do not always walk Tab. 1. Distinctness1 of profiles of feeding transects from sur 11 in down the center of the feeding transect nor is the water rounding habitat, comparing six species of shorebirds in breeding season. I: If· depth constant along the transect. i in } Profiles of microhabitat transects taken in winter Species Vegetation Water Total2 of were relatively homogeneous within each species. The '.B I A and B profiles usually did not differ dramatically from Semipalmated Plover 8.5 33.5 15.5 g- the feeding transect nor did the C and D profiles reveal Semipalmated Sandpiper . " 14.4 24.5 20.1 ,er much of an edge property except in the Lesser Yellow Short-billed Dowitcher ..... 49.6 72.8 55.0 l Lesser Yellowlegs ...... , , ce legs where vegetation height appeared very similar to its 41.8 102.9 55.5 Least Sandpiper .... , ...... 50.9 72.5 57.5 ~rs summer C and D profiles. Duntin ...... 71.5 63.l 68.2 x For a comparison among species on the degree of nd difference between feeding transect and surrounding 1. Ratio of area cqntaincd in feeding tranfiect profile (Fig. 1) to average of A and B profiles expressed as a percent. High ~rs habitat in summer, I calculated the ratio of the area values indicate less contrast. D I contained in the feeding transect profile to the area 2. Vegetation and water considered together. 179) I9• OIKOS 33: 1 (1979) 123 1.0 Fig. 2. Cumulative diversity (H) of feeding transects of 0.8 -summer six species of shorebirds in ..... ,winter summer and winter. H was ~, 0.6 calculated ·after each E succeeding transect was ·) u added onto the total. 0.4 .....···'·• ...... Estimates of H' and ~ ; ... /"" .. ,• standard errors were I 0.2 calculated for the last I SPP LST SPS twenty-five transects. 2 0 0 >- >- I- 1.2 ' if> <( 0c: I- w 1.0 w ' > (D ' a 0.8 w > C 0.6 0.4 Fig. 3. 0.2 transe( DUN SBD LYL shoreb o-"-~--~---~ e-~~~~-~~~ 0 IO 20 30 40 0 IO 20 30 40 0 IO 20 30 40 ACCUMULATED NUMBER OF TRANSECTS Rar seasor estimate of diversity does not appreciably affect the es average diversity was estim~ted from the last twenty rank c timate. A curve rising slowly to a plateau indicates that five points in each plot (Fig. 2). The value of hk was each new transect contributes new diversity. The successively calculated and a mean taken to represent 3.3, M plateau implies that the sample of transects is large an estimate of H'. Five such runs were performed, one charact enough to include most of the diversity present. Early with the transects accumulated in the order gathered in dips in the curve imply a run of low diversity transects. the field (Fig. 2) and four additional runs in random , On !iv, In the case of shorebirds, the diversity curves leveled order to see how sensitive H' was to variation in trans- I ten nu off sufficiently well that it is probably safe to make some ect accumulation. Medians of the five values for each \ mcasui generalizations. In all species, feeding transect diversity species were selected to represent J-1' (Tab. 2). These were p is lower in winter than in summer. To attach a numerical ·values confirm the visual result that feeding transect etation value to these plots of cumulative diversity, the true diversity is greater in summer thall winter. Tab. 2. Estimates of feeding transect diversity and standard errors for five different orderings of transects, comparing six species of ;, shorebirds in ~ummer and winter. Semipalmated Least Semipalmated Lesser Short-billed I- Plover S~ndpiper S- z I- Summer <( By Date .. ' .... ' ...... ' ...... 0.36 0.11 0.78 0.08 0.62 0.07 1.05 0,08 l.12 0.03 1.13 0.04 n: (f) C Random 1 ... ' ... ' .. ' ..... ' ...... 0.28 0.10 0.66 0.07 0.82 0.04 1.06 0.09 1.08 0.03 l.07 0.03 i I- 5 Random 2 ...... ' ...... 0.36 0.09 0.72 0.07 0.68 0.09 0.86 0.07 1.07 0.03 1.09 0,02 > Random 3 ...... ' ...... ' ... ' .... 0.29 0.08 0.74 0.07 0.76 0.09 0.88 0.06 1.05 0.02 1.09 0.04 ' (..') - C I zo Random 4 .... 0.24 0.06 0.64 0.05 0.78 0. 10 1.08 0.08 1.09 0.03 1.15 0.04 ' Medians .... ' .. ' ... ' .. ' .. ' ...... 0.29 0.09 0.72 0.07 0.76 0.09 1.05 0.08 1.08 0.03 1.09 0.04 ' 0 w 0 Winter w I LL By date ...... 0.22 0.08 0.47 0.05 0.59 0.04 0.88 0.04 1.27 0.05 1.06 0.06 Randmrt 1 ...... ' . ' ' . . ' . . ' . ' . . . . . ' ' 0.31 0.09 0.34 0.03 0.52 0.06 0.76 0.()3 0.99 0.04 0,98 0.05 l Random 2 ... ' ..... ' ' ..... " . ' ..... 0.24 0.08 0.40 0.05 0.63 0.06 0.81 0.05 1.01 0.06 0.95 0.05 i Random 3 .... ' ..... ' ...... 0.25 0.09 0.39 0.06 0.58 0.06 0.85 0.08 0.99 0.06 0.98 0.()4 I Random 4 ...... ' .... ' ...... ' 0.22 0.07 0.43 0.06 0,60 0.05 0.85 O.o7 1.08 0.06 0.96 Cl.OS Mediuns . . . ' . . ' ...... ' . . . ' ...... 0.24 0.08 0.40 0.05 0.59 0.06 0.85 0.05 1.01 0.06 0.98 0.05 Fig. 4. R ··-----· length fo1 ing hnbiu 124 OU (1979) \ O!KOS 33, I (1979) 125 (Dunlin). Such differences in insularity could influence feeding range. Dunlin frequently reach high above their 01 other behavior. At the extremes, for example, a heads for prey and also peck from underwater surfaces Semipalmated Plover when foraging can be in visual or deeper than Least and Semipalmated Sandpipers. Les ser Yellowlegs appear to outreach Dunlin in the vertical l auditory contact with few other conspecifics whereas a •) Dunlin, with a less restricted view of surrounding ter plane. The problem in this interpretation is the longest rain, can have contact with numerous conspecifics. The billed species, the Short-billed Dowitcher. For this detection of predators would also vary with the isolation species, the depth of water probed is the greatest of all N of the foraging patch. six species, which conforms to the general pattern. This Several studies have suggested functional relation species seldom pecks high onto plant stems, however, so ships between morphology and foraging behavior in this dimension does not fit. the general pattern. T. (Osterhaus 1962, Fretwell 1969, Burton 1974). In the In conclusion, the present study suggests that the present case, it is clear that leg length is positively cor versatility of the bill as a tool may not be obvious from related with the depth of water in which the six species its form, so behavioral variables should be considered in forage, as most naturalists would have supposed. This, conjunction with morphological ones in community however, is only part of the story since shorebirds spend studies. a. major portion of foraging time out of the water (Baker and Baker 1973). Possible functiona,1 relationships between bill length Acknowledgements - l thank N. P. Ashmole for advice and encouragement during the course of this study and A. E. M. and diversity of the feeding transect or vegetation Baker for field assistance. Financial help was provided by Yale height are less clear. Probably the environment is less University, The Frank M. Chapman Fund, and the Society of restrictive to a long-billed bird than to a short-billed The Sigma Xi. one. A long bill can be used to capture prey from the bottom of a pool over a greater range of water depths than can a short bill. A long bill can also peck prey from References high on plant stems as well as close to the ground. Thus, to the extent that foraging behavior is matched to the Baker, M. C. and Baker, A. E. M. 1973. Niche· relationships among six species of shorebirds on their wintering and vertical range of feeding substrate, the bill length vs. breeding ranges. - Ecol. Monogr. 43: 193 ..... 212. diversity and bill length vs. vegetation height correla Burton, P. J. K. 1974. Feeding and the feeding apparatus in tions are understandable. Field observations of foraging waders: a study of anatomy and adaptations in the are consistent with this interpretation. The Semipal Charadrii. - British Museum (Natural History), London, Fretwell, S. D, 1969, Ecotypic variation in the non-breeding mated Plover seldom pecks prey from standing vegeta season in migratory populations: a study of tarsal length in tion or from pond bottoms. Least and Semipalmated some Fringillidae. - Evolution 23: 406-420. Sandpipers peck from plant stems in the range of their Osterlrnus, M. B, 1962, Adaptive modificulions of the leg head height and slightly higher and peck into pond structure of some North American warblers, - Am. Midi. Nat. 68:-474-486. bottoms more deeply than Semipalmated Plovers. Pielou, E, C. 1966. The.measurement of diversity in different Without considerable data, it is not possible to say if I types of biological collections. - J. Theor. Biology 13'. Least and Semipalmated Sandpipers differ in vertical 131-144. ' Accer \ © om 1:26 OIKOS 33: I (1979) OlKOS l