Reproductive consequences of egg-laying decisions in snow geese

1. Even though feeding conditions typically improve over time during the laying period, clutch size decreases over the course of the nesting period in most bird species. We examined whether seasonal decrease in offspring value could explain the seasonal decline in clutch size in arctic-nesting greater snow geese (Anser caerulescens atlanticus L.). 2. Nesting was synchronized within a year, with more than 90% of the nests being initiated within about 8 days. Despite this high synchrony, there was a steep seasonal decline in clutch size in each of the 7 years of the study, from about five eggs in early clutches to three in late ones (-0.20 egg day(-1)). 3. Late parents performed more poorly than early parents in most components of reproductive success. The relationship between laying date and nesting success was curvilinear, early and late nests having a higher failure rate than those initiated near the median. Prefledging survival decreased by about 50% over the season, although the earliest hatched goslings also tended to have a reduced survival. The postfledging survival showed the strongest seasonal decline, as survival probability of late-hatched birds was about five times lower than in early-hatched ones. 4. Overall reproductive success showed a very steep seasonal decline as the number of young surviving to the first winter was about eight times lower in late-nesting birds than in early-nesting ones. Reproductive success declined slightly in the earliest-nesting birds, suggesting a cost to nesting too early. 5. The observed clutch size generally matched the clutch size that yielded the highest reproductive success for each laying date, except in earliest-nesting birds which should have done better by slightly delaying nesting. Our data suggest that trading off additional eggs for earlier nesting to increase reproductive success is an option in geese. Consequently, the seasonal decline in clutch size may be an adaptive response to seasonally declining survival prospects of offspring.