In birds, large egg size often enhances subsequent offspring survival, but most previous studies have been unable to separate effects of egg size from other maternal influences. Therefore, we first evaluated variance components of egg size both within and among individual female Ruddy Ducks (Oxyura jamaicensis), and then tested for egg-size-dependent survival of ducklings in the wild by switching complete broods among females. Forty broods consisting of 244 individually color-marked, day-old ducklings of known egg size were given to foster mothers, and survival was monitored to one month. Analysis of mark–resighting data showed that offspring survival was best modeled to include effects of egg size and hatching date; survival probability increased with egg size, but declined with advancing hatching date. Duckling body mass, body size, and body condition measured at hatching were positively correlated with egg size. Unlike most other duck species, and for reasons that are speculative, egg sizes varied within clutches nearly as much as they did among clutches. Selective mortality of small egg phenotypes during the first weeks after hatching likely is the result of smaller duckling size and reduced energy reserves, characteristics that must be particularly unfavorable in adverse environments.
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Vol. 120 • No. 2