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 decrea sing 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 t hree to fledging. Most chick mortality occurred in the 19-22 d age gro up. Chick body mass and fledging success were lower among enlarged bro ods than among reduced and control broods. Body mass of females, but n ot males, was lower at the end of the chick-rearing period in enlarged broods. Only those females rearing enlarged broods experienced a high er mortality during the next breeding season. Our results support Lack 's hypothesis of clutch size regulation: Kittiwakes laid the maximal n umber of eggs they were able to raise successfully to hedging. Further more, as female Kittiwake mortality increased when clutches were enlar ged, females have a trade-off between investment in current reproducti on and investment in future reproduction. Because males and females re sponded differently to brood enlargement, fitness estimates showed tha t Kittiwakes may have a sex-specific optimal clutch size.