Composition of Wood Duck Eggs in Relation to Egg Size, Laying Sequence, and Skipped Days of Laying

Abstract

-We collected 138 freshly laid Wood Duck (Aix sponsa) eggs from 13 nests to determine the effects of egg size, laying sequence, and skipped laying days on egg composition. All components except yolk ash and eggshell, both wet and dry, increased in direct proportion to fresh-egg mass. Egg size increased during the first half of laying and decreased thereafter. The effects of laying sequence were component-specific. Absolute levels of neutral lipids (comprising 65.6% of dry yolk) were near average until about 75% of the clutch was completed and then declined. Mass of albumen components tended to increase sharply with the first few eggs and then decrease as laying progressed. Fat indices (egg lipid/lean dryegg content) tended to be highest in the first 40% of the clutch. We interpret laying-sequence effects on egg composition as adaptations that enable female Wood Ducks to initiate incubation before clutch completion and still allow for synchronous hatching without compromising the hatching success of first-laid eggs. Hens frequently skipped a laying day between the penultimate and ultimate egg, which tended to increase the mass of the yolk component in last-laid eggs compared with those that were laid the day following the penultimate egg. Received 16 August 1996, accepted 19 February 1997. VARIATION IN AVIAN EGG SIZE is largely attributable to differences among species, and contrasting developmental modes dictate primary differences in constituent proportions (Ricklefs 1977, Carey et al. 1980, Sotherland and Rahn 1987). Egg size also varies considerably within species, typically varying more among than within clutches (Ricklefs 1984, Rohwer 1986, Hepp et al. 1987). Intraspecific variation in egg size is related to proximate factors such as laying date (Runde and Barrett 1981, Bancroft 1984), food supply (Horsfall 1984, Pierotti and Bellrose 1986), and age (Ryder 1975, Crawford 1980, Gratto et al. 1983). Such factors however, exercise their influence within the confines of a large heritable component of egg size (Ojanen et al. 1979, Van Noordwijk et al. 1980). Recent attention has been focused on intraspecific variation in the composition of eggs. As with egg size, egg composition typically varies more among than within clutches (Ricklefs 1984, Bancroft 1985, Alisauskas 1986, Hepp et al. 1987) and has been shown to vary with re' E-mail: kennamer@srel.edu 2 Present address: W1396 Stokdyk Ingelse Road, Oostburg, Wisconsin 53070, USA. 3Present address: National Park Service, Southeast Support Office, 100 Alabama Street Southwest, Atlanta, Georgia 30303, USA. spect to female quality in Wood Ducks (Aix sponsa) as indexed by post-laying body mass (Hepp et al. 1987). Egg composition, however, is not known to vary greatly due to other factors (e.g. clutch size; Rohwer 1986, Arnold et al. 1991; but see Owen and West 1988). Intraclutch variation in egg composition has received less attention (but see Nisbet 1978, Alisauskas and Ankney 1985, Rohwer 1986, Meathrel and Ryder 1987). Nest attentiveness in precocial waterfowl commonly increases through the laying period with persistent incubation beginning after the clutch has been completed (Caldwell and Cornwell 1975, Cooper 1978, Afton 1980, Kennamer et al. 1990). Despite up to four days of total asynchrony among eggs in experimentally manipulated Lesser Snow Goose (Chen caerulescens) clutches, hatching synchrony can still be achieved (Davies and Cooke 1983). Kennamer et al. (1990) documented 100% hatching success in a Wood Duck clutch with up to four days of natural cumulative asynchrony. Apparently, embryos communicate with one another to synchronize their hatching by adjusting developmental rates (Vince 1964, Ockleford and Vince 1985). Reasons for such a complicated incubation strategy in species that hatch synchronously may relate to a combination of egg-viability and nest-predation effects (Arnold et al. 1987, Flint et al. 1994). In any case, egg com-