Incubation capacity limits clutch size in south polar skuas

Abstract

The incubation-limitation hypothesis suggests that clutch size in some birds is limited by incubation capacity. However, this remains disputed amongst ornithologists. This study aimed to test whether incubation capacity limits the maximum clutch size to two eggs in south polar skuas (Catharacta maccormicki Saunders) by comparing the egg and nest temperatures as well as hatchability between two- egg and three-egg clutches. Although the vast majority of clutches contained one or two eggs, four naturally occurring three-egg clutches were found at Barton Peninsula, King George Island over three breeding seasons (2004-2005, 2005-2006 and 2006-2007). Regardless of clutch size, all incubating parents exhibited two discernible brood patches. The mean egg and nest temperatures of the three-egg clutches were significantly lower than were those of the two-egg clutches. The accumulated time that egg temperature decreased below 308C in three-egg clutches was approximately eight times longer than that in two-egg clutches. The hatchability of natural one-egg (89.5%) and two-egg clutches (95.4%) were significantly higher than that of the three-egg clutches, which was zero. Our results suggest that the maximum clutch size in south polar skuas is probably restricted by incubation capacity.