Egg size, clutch size and breeding success of the Glossy Ibis Plegadis falcinellus

Abstract

Glossy Ibis Plegadis falcinellus were studied at a freshwater marsh near Balranald, New South Wales, during fortnightly visits from September to November 1981. Glossy Ibis bred amongst Straw-necked Ibis Threskiornis spinicollis and Sacred Ibis T. aethiopica on top of knocked-down lignum Meuhlenbeckia cunninghamii. Egg-laying by Glossy Ibis extended for over five weeks and began after all the Straw-necked Ibis and Sacred Ibis had completed their clutches. Two to six eggs were observed per nest but the most frequent clutch was three eggs and the mean clutch size was 3.06 eggs. Of the clutches that were completed, 88% hatched some young and it was estimated that the average fledging success was nearly 1.0 young per nest. There was little evidence of predation of eggs or young and it was concluded that starvation of young caused the low breeding success. The timing and synchrony of breeding by the Glossy Ibis are considered in terms of predation and food availability.