Biology of the Heard Island Shag Phalacrocorax nivalis. 2. Breeding

Publisher Website
Google Scholar
Abstract
The 1992–93 breeding season of the Heard Island Shag Phalacrocorax nivalis was followed through from egg laying to fledging, with measurements made of all eggs, and most chicks, on a five day basis. The start of egg laying of the Heard Island Shag varies widely with the earliest eggs being laid from about mid-September to mid-October. The value in such flexibility might come from the opportunity to take advantage of good weather or food availability. The number of chicks raised per pair varied from one to three, with an increase in foraging effort required for more than one chick. Chicks from clutches of three appeared not to be disadvantaged either in when they fledged or in their condition at fledging. There was no significant difference between the mass of C and A chicks at fledging but B chicks were significantly heavier than A chicks. From 140 eggs laid at Stephenson Roost in 1992–93, 82 chicks fledged; a failure rate of 41.4%. During 1992, two of the 1991–92 cohort were found dead; 133 of 135 survived from May to December. The high success rate in the two years contrasts with earlier poor breeding results and suggests that the population was increasing in this period.