Intraclutch Variation in Egg Weights of Pied-Billed Grebes

Abstract

Much discussion has recently focused on the effects that quality of eggs, and hatching order of chicks, have on chick survival within avian broods. Chicks which hatch from larger eggs are heavier (e.g., Howe 1976), grow faster (e.g., Ankney 1980), and survive better (e.g., Lundberg and Viiis~inen 1979). Chicks which hatch later, however, generally have poorer survivorship (e.g., Parsons 1975). Thus there potentially exists either additive or opposing influences of egg quality and hatching order of chicks on chick survival. One rationale for examining whether or not such effects are additive has been an attempt to resolve controversy concerning the adaptiveness of hatching asynchrony (review by Clark and Wilson 1981). The only work on intraclutch variation in egg quality for grebes has been Fugle and Rothstein's (1979) examination of the effects of egg removal, from two clutches of Pied-billed Grebes (Podilymbus podiceps), on clutch size and egg-size determination. Grebe broods, however, show considerable hatching asynchrony (Cramp and Simmons 1977). For example, Forbes and Ankney (1987) found that broods of P. podiceps from clutches of five to eight eggs hatched over 3 to 7 days. Thus chicks within grebe broods may exhibit considerable differences in size at hatch completion due to hatching order (Nuechterlein 198 1, Forbes and Ankney 1987). Our objective was to determine whether or not variation in egg weights within clutches ofPied-billed Grebes was related to laying order of eggs. For both altricial (e.g., Richter 1984) and semiprecocial species (e.g., Horsfall 1984), laying order of eggs was a good predictor of hatching order of chicks and thus may be used for comparisons with egg quality (review by Slagsvold et al. 1984).