Seasonal variation in pre-fledging survival of lesser scaup Aythya affinis: hatch date effects depend on maternal body mass

Among temperate-breeding birds, offspring survival and reproductive success are often inversely related to timing of breeding. The mechanisms that produce seasonal declines in offspring survival are not fully understood but may be related to temporal changes in parental quality, environmental quality, or both. We analyzed data for lesser scaup Aythya affinis to evaluate hypothesized effects of parental quality and date on pre-fledging survival. Maternal quality, as indexed by body mass, did not have an independent effect on offspring survival in this species. Maternal body mass did not decline seasonally and did not have an independent effect on duckling survival. Although we did not detect an independent effect of hatch date on duckling survival, duckling survival declined seasonally for broods raised by lightweight females, indicating an interactive effect of maternal mass and date. We hypothesize that this interaction may be driven by seasonally declining food resources coupled with the influence of female condition on the ability to monopolize food resources or remain attentive to the brood. We also tested morphological predictions of the date hypothesis by examining physical characteristics of ducklings. When corrected for age and size, late-hatched ducklings tended to have marginally larger digestive systems and smaller leg muscles than did early-hatched birds. Abundances of intestinal parasites acquired through diet decreased marginally in late-hatched ducklings. Results for digestive system and parasite infection patterns suggested that later-hatched broods may shift diets, consistent with a contribution of environmental factors to seasonal variation in offspring survival. Taken together, our results suggest that both female attributes and environmental conditions may influence seasonal patterns of offspring survival in this species.