IS INTRACLUTCH EGG-SIZE VARIATION ADAPTIVE IN THE LESSER SNOW GOOSE

We use data from a long-term population study of lesser snow geese (Chen caerulescens caerulescens) to test an adaptive hypothesis for intraclutch egg-size variation. that females differentially allocate reserves to eggs within a clutch based on the probability that an egg of a particular laying sequence will give rise to a surviving offspring. In clutches of 3-6 eggs the first egg was relatively small, the second egg was the largest, and subsequent eggs declined almost linearly in mass with laying sequence. Among all clutch sizes the last-laid egg was the smallest. Mean egg mass did not vary significantly with clutch size. For all clutch sizes, first eggs had the lowest probability of producing a gosling that successfully left the nest (47-64% compared to 74-85% for all middle eggs). This was mainly due to higher pre-incubation failure of first eggs (27-31%) compared to other eggs (0-13%). For clutches of 4-6 eggs, the final egg also had a lower probability of hatching successfully compared to middle eggs, due to a higher probability of last eggs being abandoned at hatch. Final eggs were also marginally less likely to produce a gosling that survived to fledging than were eggs of other laying sequences. After controlling for egg sequence and clutch size, hatching success was not related to egg mass. The lower probability of success of first and last eggs was therefore due to their position in the laying sequence per se, rather than the fact that they were smaller. These results are consistent with the adaptive hypothesis that nutrient allocation to eggs within a clutch varies according to the fitness that each egg has by virtue of its position in the laying sequence per se. However, they do not exclude other proximate (physiological) hypotheses. We show, finally, that the most commonly proposed proximate hypothesis (female nutrient reserve limitation) is unlikely to be important in determining intraclutch egg-size variation in this species and suggest that other proximate mechanisms warrant further study.