Short-tailed shearwaters in Bass Strait, Australia, exhibited extreme synchronicity in laying during 1991. Egg dimensions and masses varied by approximately 40% between females and were related to the date of laying but not the fate of the egg at hatching. The composition of eggs was similar to other petrels, and typical of semiprecocial species. Larger egg-size was attributable to relatively more albumen, but less yolk. Thus, hatching succcss may be proximately constrained by behavioural attributes of the patents.