Egg dimensions and neonatal mass of shorebirds

-Eggs of 10 species ofshorebirds (Charadriidae, Scolopacidae, Phalaropodinae) collected at Churchill, Manitoba, were measured and artificially incubated. Neonates were weighed within 6 h of hatching. This paper reports variation in egg size and neonate mass among clutches, and the relationship between egg dimensions and neonate mass. Coefficients of variation were 4 to 9% for egg volume and 6 to 15% for neonate mass. In most species, a significant proportion (48-84%) of the variance in egg volume could be attributed to differences between clutches. Neonate mass was significantly related to egg size in six of nine species. Coefficients of variation in the volume of eggs within populations of shorebirds (Charadriidae, Scolopacidae, and their allies) are on the order of 4 to 7% (Viisanen et al. 1972, Viisinen 1977). Much of this variation arises from differences in the sizes of eggs laid by different females, which are detected as between-clutch variation (e.g., Viis~nen et al. 1972, Miller 1979, Baker and Cadman 1980). The relationship between the sizes of eggs and neonates has been described for many species of birds, including some members of Charadriiformes (e.g., Ricklefs et al. 1978, Lundberg and Vdisinen 1979, Lloyd 1979), but not for shorebirds. Because shorebird chicks must feed themselves shortly after hatching, often in cold environments, larger size, more advanced developmental state, and greater energy reserves of neonates presumably carry a selective premium, all else being equal. Indeed, the relative egg size (percent adult mass) of shorebirds is large compared to most other groups of birds (Rahn et al. 1975). If egg size is under strong selection (i.e., most unfavorable genes have been removed), variation within populations must be either nongenetic in origin or compensated by variation in other, balancing traits that are correlated with egg size (e.g., Rose 1982). The large variance between clutches found in most populations suggests genetic variation in egg size, although age and developmental effects have not been ruled out. As a step towards understanding the evolution of large egg size in shorebirds, and interpreting variation in egg size among birds in general, I report here on variation in egg size and neonate mass among clutches, and on the relationship between egg size and neonate mass in 10 species of shorebirds at Churchill, Manitoba. MATERIALS AND METHODS Ten species were included in this study: Lesser Golden-Plover (Pluvialis dominica); Semipalmated Plover (Charadrius semipalmatus); Whimbrel (Numenius phaeopus); Hudsonian Godwit (Limosa haemastica); Lesser Yellowlegs (Tringaflavipes); Stilt Sandpiper (Calidris himantopus); Short-billed Dowitcher (Limnod us griseus); Dunlin (Calidris alpina); Least Sandpiper (C. minutilla); Red-necked Phalarope (Phalaropus lobatus). These were the speci s most readily available in the study area and, fortuitously, represent much of the variation in size and habits among arctic shorebirds. Clutches were collected at various stages of incubation from wet and dry tundra habitats to th east of Churchill, Manitoba (500N, 940W), during midand late June 1979. Eggs were marked with indelible ink and their lengths (L) and breadths (B) were measured to the nearest 0.1 mm with dial calipers. I used th product of the length and the square of breadth (LB2) as an index to volume (Viisanen 1969, Vdisanen et al. 1972). Regressions of volume on LB2 had coefficients of determination of between 88 and 98% in eight species of waders studied by VdtisAnen (1977). Eggs were incubated in a small incubator (Hovabator, Inc.) at 370C and approximately 60% rela ive humidity. Neonates were individually marked and weighed soon after hatching, usually within 6 h. I could match neonates to their eggs unambiguously in most cases. Where there was doubt, the data were omitted from the pertinent analyses. Additional data on lengths and breadths of eggs in 5 clutches of Hudsonian Godwits, 8 clutches of Whimbrels, 7 clutches of Shortbilled Dowitchers, 24 clutches of Least Sandpipers, and 33 clutches of Stilt Sandpipers (all from the Churchill area) were generously provided by J. R. Jehl. I analyzed these measurements separately, rather than pooling them with my data, because neonatal masses were not available. I used the SAS statistical package (Helwig and Council 1979), primarily the MEANS and GLM procedures, to analyze the data. Univariate statistics presented are the