FITNESS CONSEQUENCES OF EGG-SIZE VARIATION IN THE LESSER SNOW GOOSE

We investigated the relationship between egg-size variation and (a) eg g hatching success, (b) chick survival to fledging and recruitment, an d (c) adult female survival, over 12 years in the lesser snow goose (A nser caerulescens caerulescens). By comparing the means and variances of egg size for successful and unsuccessful eggs, our aim was to asses s the relative fitness of eggs of different sizes and to determine the type of selection operating on egg size in this species. As both egg size and reproductive success vary with age in the lesser snow goose w e controlled for the effects of female age. Egg-size variation is very marked in this population, varying by up to 52% for eggs hatching suc cessfully. However, there was no relationship between egg size and pos t-hatching survival of goslings to fledging or recruitment, either wit hin or between broods, pooling across years. Egg size varied significa ntly between successful and unsuccessful clutches in only 2 of 33 indi vidual year comparisons. First-laid eggs surviving to onset of incubat ion, and eggs hatching successfully, were on average larger than unsuc cessful eggs, but this was probably due to the confounding effects of female age-specific and sequence-specific egg survival. Variance of eg g size differed significantly between successful and unsuccessful eggs in only 3 of 24, and 0 of 21, individual year comparisons for pre- an d post-hatching survival respectively. We therefore found little evide nce for a relationship between egg-size variation and offspring fitnes s, or for strong directional, normalising or diversifying selection op erating on egg size, in the lesser snow goose. In addition, there was only weak support for the hypothesis that egg-size variation is mainta ined by temporal variation in selection pressure (sensu Ankney and Bis set 1973). It is likely that egg-size variation represents the pleiotr opic expression of alleles affecting more general physiological or met abolic processes. While this does not rule out the existence of allele s with more direct effects on egg size we suggest that their contribut ion to heritable egg size is small.