AN EXPERIMENTAL-STUDY OF THE COSTS OF REPRODUCTION IN THE KITTIWAKE RISSA-TRIDACTYLA

The costs of reproduction in Kittiwakes Rissa tridactyla were studied from 1990 to 1993 on Hornoy in northern Norway by enlarging and decreasing brood sizes of two chicks to three and one during the first week after hatching. No parents were able to raise the enlarged broods of three to fledging. Most chick mortality occurred in the 19-22 d age group. Chick body mass and fledging success were lower among enlarged broods than among reduced and control broods. Body mass of females, but not males, was lower at the end of the chick-rearing period in enlarged broods. Only those females rearing enlarged broods experienced a higher mortality during the next breeding season. Our results support Lack's hypothesis of clutch size regulation: Kittiwakes laid the maximal number of eggs they were able to raise successfully to hedging. Furthermore, as female Kittiwake mortality increased when clutches were enlarged, females have a trade-off between investment in current reproduction and investment in future reproduction. Because males and females responded differently to brood enlargement, fitness estimates showed that Kittiwakes may have a sex-specific optimal clutch size.