Brood reduction in the Blue-eyed Shag Phalacrocorax atriceps

Brood reduction is common in a population of Blue-eyed Shags on Signy Island, South Orkney Islands. This paper describes possible adaptations which may reduce the brood. In clutches of three, the last egg was smaller, and hatched 2.4 days later than its siblings. Whilst 78–84% of first and second ('A' & 'B') chicks fledged, only 11 % of 'C' chicks did. In a sample of artificially synchronized broods chick survival was as high as in normal asynchronously hatching broods, but there were more cases of total brood loss. The age at which the C chick died was related inversely to the length of the A-C hatching interval. Relative differences in sibling weights were highest during the first 12 days, when most of the C chick deaths occurred. At this age the daily food requirements of each brood of three was one-tenth that of each brood of two just prior to fledging. It is suggested that C chicks were unable to compete effectively for a food supply which was limited by the parents, rather than by the environment. The asymptotic weight attained by A chicks was inversely related to brood size, and was greater than that of B or C chicks. Normal asynchronous broods produced at least one heavy (A) chick and one medium weight (B) chick, whilst in synchronized broods the asymptotic weight attained was similar to that of B chicks in normal broods.