REPEATABILITY OF BREEDING PARAMETERS IN ARCTIC SKUAS

Established pairs of Arctic Skuas Stercorarius parasiticus showed high repeatability in clutch volume (both within and between years), moder ate repeatability in laying dates, but low repeatability in chick grow th. Males that changed partner showed no repeatability in clutch volum e or chick growth but did show repeatability in laying date, whereas f emales that changed partner did not show repeatability in laying date or chick growth but did show repeatability in clutch volume (indicatin g that laying date is determined predominantly by male quality, and cl utch volume by female quality). In addition, pairs with males in bette r body condition laid earlier in the season, and females in better bod y condition laid clutches of greater volume. We suggest that variation in timing of breeding of Arctic Skuas arises from differences between males in their ability to provide sufficient prey to get the partner into reproductive condition, but clutch volume may be constrained by t he female's ability to store the resources for egg production.