Consequences of Egg Size for Offspring Survival: A Cross-Fostering Experiment in Ruddy Ducks (Oxyura Jamaicensis)

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.

Low Frequency of “Double Molt” of Remiges in Ruddy Ducks Revealed by Stable Isotopes: Implications for Tracking Migratory Waterfowl

Ratios of the stable carbon (δ13C) and hydrogen (δD) isotopes in newly grown remiges of Ruddy Ducks (Oxyura jamaicensis) are influenced by the isotopic character of food types and ambient water. Systematic isotopic foodweb and water differences between coastal wintering grounds and prairie breeding grounds of Ruddy Ducks provide the basis for using isotopic analyses of feathers to trace the location and timing of molt. Anecdotal evidence, based largely on captive birds, suggests that Ruddy Ducks replace their remiges twice each year (once each on the breeding and wintering grounds), but a recent literature analysis indicates that this phenomenon is rare. Thus, we investigated the extent to which a biannual molt of the remiges occurs in the wild and at the population level. We analyzed the stable isotopes of carbon (n = 57 birds) and hydrogen (n = 50 birds) in flight feathers to estimate the prevalence of the so-called “double molt” of remiges in free-living Ruddy Ducks. Our data showed that natural populations of Ruddy Ducks express an overwhelming unimodal distribution of isotope ratios in their remiges, suggesting that they undergo a single molt at or near the breeding grounds. Only 3 to 6 of 50 birds from Manitoba showed isotopic evidence consistent with growing remiges on the wintering grounds. Feathers from Ruddy Ducks harvested during the fall in the Mississippi Flyway had isotopic profiles consistent with growth on northern freshwater breeding sites. Thus, our results confirm that the replacement of remiges twice each year by Ruddy Ducks is rare, and they suggest that this dual stable-isotope technique can be used to infer general molting origins of North American waterfowl.