Correlates and consequences of egg size variation in wild Ruddy Ducks (Oxyura jamaicensis)
Pelayo, Jeffrey T.
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Evidence concerning effects of egg size on offspring survival is difficult to interpret because most studies have not controlled confounding influences such as maternal quality which may mask effects of egg size. Thus, I first estimated variation in egg size and then investigated effects of egg size variation on duckling survival in wild Ruddy Ducks (Oxyura jamaicensis). Over two years, I switched 40 complete broods consisting of 244 individually color-marked ducklings of known egg size, body size, and body condition to nests of unrealted foster mothers, and evaluated survival to 31 days of age. Most mortality occurred within the first two weeks after hatching. In 1998, survival probability of experimental ducklings declined with advancing hatch date. In 1999, the body condition index of individuals at hatching was positively related to egg size, and ducklings in better condition had enhanced survival probabilities. In addition, survival increased if ducklings were associated with larger broods. These experimental findings suggested that selective mortality of small egg phenotypes in some years could be related to smaller offspring size and possibly reduced energy reserves upon hatching. Therefore, to determine why ducklings from larger eggs could have enhanced survival probabilities, I examined relationships between size, composition, and quality (indexed by lipid and energy content) of eggs and ducklings. Allometric regressions showed that most egg constituents increased in direct proportion (bs = 1.0) to fresh egg weight, but larger eggs contained proportionately more energy than smaller eggs (b > 1.0). In addition, dry yolk-free duckling weight increased proportionately less (b < 1.0) with fresh egg weight, but larger eggs produced ducklings with greater yolk reserves. Yolk sac weight also increased proportionately more with increasing dry yolk-free duckling weight. Although proportional composition of eggs was similar among large and small eggs, larger eggs contained more total energy than did smaller eggs, and therefore produced ducklings with larger yolk stores. Overall, larger ducklings may be better able to survive adverse conditions because they are in better condition, have larger energy reserves that buffer them against starvation, and likely have better thermoregulatory capabilities than smaller individuals.