SEX-SPECIFIC VARIATION IN THE LOSS OF MASS BY BREEDING ARCTIC SKUAS

Variation in the seasonal pattern of mass changes by adult Arctic Skua s Stercorarius parasiticus on Foula, Shetland, was examined in relatio n to sex-role partitioning of breeding duties. Male body mass remained comparatively uniform throughout incubation and chick-rearing. By con trast, female mass showed an overall seasonal decline, although no sig nificant changes occurred within incubation or chick-rearing. ANOVA co nfirmed a stepwise drop in female mass shortly after hatching, but tha t no further mass was lost during the remainder of the breeding season . We suggest that female Arctic Skuas retain body reserves during incu bation as an insurance against natural fluctuations in food availabili ty and/or to permit earlier relaying if the initial breeding attempt i s unsuccessful. These reserves are then shed shortly after hatching as an adaptive response to reduce wing loading and improve flight effici ency in order to cope with the increasing energetic demands of growing chicks. The reduction after hatching of 4.3-5.3% of the female's incu bation mass was estimated to result in energy savings of 5-10% of BMR in 24 hours, or alternatively, a reduction by 7-9% of the mechanical p ower required during flight.