Population dynamics of the Cattle Egret (Ardea ibis) in south-east Queensland: a 20-year study

Abstract

This study of the Cattle Egret (Ardea ibis) in the Lockyer Valley of south-east Queensland follows changes in the size of the nesting population and breeding success (number of chicks raised per successful nest) for 20 nesting seasons. Annual nest numbers fluctuated somewhat irregularly in this period, from 1421 to 7953. Nest numbers were strongly positively associated with local and regional rainfall in the 12 months prior to breeding, but the randomisation test also indicated a density-dependent relationship between the nest numbers of successive years. Furthermore, an upsurge in nest numbers commenced in the relatively dry year of 1987, after the egrets started feeding on cultivated land. It is proposed that nesting numbers were largely controlled by rainfall in a density-independent way and any density-dependent effect was undercompensatory in its influence on numbers. The change in feeding behaviour was a unique event that probably made substantially more food available to the egrets. Annual mean breeding success ranged from 1.8 to 2.9 fledglings per pair, and was significantly greater following wetter springs. Breeding success was inversely associated with the number nesting that season over the first 14 seasons, but in the ensuing years of prolonged drought both brood sizes and nest numbers were low.