Variation in egg-size in the Wedge-tailed Shearwater (Puffinus pacificus)

We assessed variation in the size of eggs of Wedge-tailed Shearwaters (Puffinus pacificus) within and between two colonies in eastern Australia. Eggs at Heron Island were significantly larger than at Lord Howe Island. We also found significant between-season variation in size of eggs at Heron Island, with larger eggs being laid in 2005 than 2006. Mean female size (as indicated by tarsal length) did not differ between the two colonies, or between seasons at Heron Island. However, as with other seabird species, larger females produced larger eggs. Overall, our results imply that facultative changes in female reproductive effort associated with changes in availability of resources are not the main cause of variation in egg-size in this species. The consistently smaller size of eggs for birds at Lord Howe Island compared with those from Heron Island suggests that obligate processes (e.g. natural selection) are more likely to be responsible.