Seasonal decline of growth and fledging success in snow geese Anser caerulescens: an effect of date or parental quality?

Late-nesting birds frequently have a lower reproductive success than early- nesting ones. This could be a consequence of seasonal variations in environ mental conditions or because Bate-nesting parents an of lower quality than early-nesting ones. We tested the hypothesis that a difference between earl y and late parents: in their ability to raise viable offspring could explai n the seasonal decline in offspring growth and survival in the Greater Snow Goose Anser caerulescens atlanticus, a species with self-feeding precocial young. Over two years, se exchanged complete clutches between early and la te parents, to change the hatching dart: experienced by parents. We exchang ed eggs between 164 experimental nests, whose hatching dates differed by 0 to 9 days ((x) over bar = 3 days). We detected no seasonal effect on surviv al rate in the sample of experimental nests, but there was a seasonal decli ne of growth rate. When delaying or advancing hatching, the fostered goslin gs grew respectively slower or faster compared to the parents' original hat ching date, indicating that variations in environmental conditions are resp onsible for seasonal variation of growth. However, growth of experimental g oslings did not differ from unmanipulated goslings hatched at the same date , indicating that parental duality did not contribute to seasonal variation s of growth rate. We conclude that the seasonal variation in reproductive s uccess is not caused by a difference in parental quality between early and late nesters, but mostly by environmental factors directly related to the d ate df the season.