Seasonal Decline of Growth and Fledging Success in Snow Geese Anser caerulescens: An Effect of Date or Parental Quality?

Abstract

Late-nesting birds frequently have a lower reproductive success than early-nesting ones. This could be a consequence of seasonal variations in environmental conditions or because late-nesting parents are of lower quality than early-nesting ones. We tested the hypothesis that a difference between early and late parents in their ability to raise viable offspring could explain 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, we exchanged complete clutches between early and late parents, to change the hatching date experienced by parents. We exchanged eggs between 164 experimental nests, whose hatching dates differed by 0 to 9 days (R= 3 days). We detected no seasonal effect on survival rate in the sample of experimental nests, but there was a seasonal decline of growth rate. When delaying or advancing hatching, the fostered goslings grew respectively slower or faster compared to the parents' original hatching date, indicating that variations in environmental conditions are responsible for seasonal variation of growth. However, growth of experimental goslings did not differ from unmanipulated goslings hatched at the same date, indicating that parental quality did not contribute to seasonal variations of growth rate. We conclude that the seasonal variation in reproductive success is not caused by a difference in parental quality between early and late nesters, but mostly by environmental factors directly related to the date of the season.