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

Abstract

The 1992—93 breeding season of the Heard Island ShagPhalacrocorax nivalis was followed through from egglaying to fledging, with measurements made of all eggs, and most chicks, on afive day basis. The start of egg laying of the Heard Island Shag varies widelywith the earliest eggs being laid from about mid-September to mid-October. Thevalue in such flexibility might come from the opportunity to take advantage ofgood weather or food availability. The number of chicks raised per pair variedfrom one to three, with an increase in foraging effort required for more thanone chick. Chicks from clutches of three appeared not to be disadvantagedeither in when they fledged or in their condition at fledging. There was nosignificant difference between the mass of C and A chicks at fledging but Bchicks were significantly heavier than A chicks. From 140 eggs laid atStephenson Roost in 1992—93, 82 chicks fledged; a failure rate of41.4%. During 1992, two of the 1991—92 cohort were founddead; 133 of 135 survived from May to December. The high success rate in thetwo years contrasts with earlier poor breeding results and suggests that thepopulation was increasing in this period.