Initial Size Hierarchy in Broods of the Shag: Relative Significance of Egg Size and Hatching Asynchrony

Abstract

-We studied the relative importance of hatching asynchrony and intraclutch egg-size variation in the establishment of a size hierarchy in broods of the Shag (Phalacrocorax aristotelis). In 3-egg clutches (87% of all clutches), the second egg averaged 3.0% larger than the first and 2.3% larger than the third. Mean hatching intervals were 11.7 h between the first and second chicks, and 48.3 h between the second and third chicks. In broods of three, the first chick weighed on average 1.9 times more than the last chick at the time the last one hatched. This difference was almost entirely a result of hatching asynchrony. For the last two chicks, hatching asynchrony accounted for 95% of mass difference and the variation in egg size for the remaining 5%. We conclude that variation in egg size has little influence in determining the initial size differences within broods. This applies to several other bird species. Hatching asynchrony and variation in egg size seem to result from selection pressures other than those connected with size differences between nestlings. Received 19 June 1987, accepted 30 December 1987. A PRONOUNCED size hierarchy is found within broods of many bird species (e.g. Bancroft 1984, Shaw 1985, Wiklund 1985). The first nestling hatched is considerably heavier than the last one when the last hatches. Several studies have shown that asynchronous hatching places the last-hatched nestlings at a disadvantage. Last nestlings generally suffer a higher rate of mortality than their older and larger siblings (Davis 1975, Howe 1976, Bryant 1978a, Lundberg and Vaisanen 1979, Horsfall 1984, Shaw 1985), and, in some cases, their growth rate is depressed (Davis 1975, Bryant 1978a, O'Connor 1978, Zach 1982). It has been proposed that a size hierarchy is an adaptation to unpredictable food conditions during the breeding season (Lack 1947, 1954, 1968; Ricklefs 1965; Howe 1976). Within-brood size hierarchies can be brought about by the combined effects of intraclutch variation in egg size, asynchronous hatching, and nestling growth rates. Hatching asynchrony is a result of incubation before egg laying is complete. A hatching span of a day or more for a brood is quite common (e.g. Clark and Wilson 1981), and the chick that hatches first will have received a considerable amount of food, and have grown correspondingly, before the last egg hatches. Hatchling mass is highly correlated with egg size for a large number of bird species (e.g. Parsons 1970, Howe 1976, Ricklefs et al. 1978, Williams and Burger 1979, Furness 1983, Bancroft 1984, Horsfall 1984, Rofstad and Sandvik 1985). The pattern of variation in intraclutch egg size differs between taxonomic groups, however (e.g. Slagsvold et al. 1984), and does not lend itself to a straightforward explanation. To evaluate the relative importance of eggsize variation and hatching asynchrony in the establishment of a size hierarchy in the brood, additional knowledge about nestling growth rates and the relation between egg volume and chick mass at hatching, is needed. We evaluated these factors for the Shag (Phalacrocorax aristo-