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

We use data from a long-term population study of lesser snow geese (Ch en caerulescens caerulescens) to test an adaptive hypothesis for intra clutch egg-size variation. that females differentially allocate reserv es to eggs within a clutch based on the probability that an egg of a p articular laying sequence will give rise to a surviving offspring. In clutches of 3-6 eggs the first egg was relatively small, the second eg g was the largest, and subsequent eggs declined almost linearly in mas s with laying sequence. Among all clutch sizes the last-laid egg was t he smallest. Mean egg mass did not vary significantly with clutch size . For all clutch sizes, first eggs had the lowest probability of produ cing 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 cl utches of 4-6 eggs, the final egg also had a lower probability of hatc hing successfully compared to middle eggs, due to a higher probability of last eggs being abandoned at hatch. Final eggs were also marginall y less likely to produce a gosling that survived to fledging than were eggs of other laying sequences. After controlling for egg sequence an d clutch size, hatching success was not related to egg mass. The lower probability of success of first and last eggs was therefore due to th eir position in the laying sequence per se, rather than the fact that they were smaller. These results are consistent with the adaptive hypo thesis that nutrient allocation to eggs within a clutch varies accordi ng to the fitness that each egg has by virtue of its position in the l aying sequence per se. However, they do not exclude other proximate (p hysiological) hypotheses. We show, finally, that the most commonly pro posed proximate hypothesis (female nutrient reserve limitation) is unl ikely to be important in determining intraclutch egg-size variation in this species and suggest that other proximate mechanisms warrant furt her study.