Does available foraging area, location or colony character control the size of multispecies egret colonies?

Abstract

Food supply is commonly regarded as ultimately controlling the size of bird colonies. Most studies examining this problem have been on seabirds, and all in the Northern Hemisphere. To search more widely for evidence of the importance of food as a factor controlling the size of bird colonies, we investigated egret colonies in a Southern Hemisphere region. We examined the relationship between colony size and the area of potential feeding habitat around each colony, compared with variables associated with the location and the physical characteristics of each colony. All colonies (13 in total) along 800 km of coastline in New South Wales, Australia, were studied. Colony size ranged from 7 to more than 2000 nests. There were very few correlations between the number of nests and the areas of different types of feeding habitat within 20 km of colonies. However, the available area of saltmarshes proved to be a significant predictor of colony size for great (Ardea alba), intermediate (A. intermedia) and little egrets (Egretta garzetta). Saltmarshes may be stable, long-term feeding habitats for these three native ‘aquatic feeders’, but not for the terrestrially feeding cattle egret (Ardea ibis). Nest numbers of this latter species were related positively to the area of saltmarshes, and negatively to latitude, suggesting that nest numbers of this exotic species may be influenced by climate, with proximate factors such as colonial nesting with the three native species also being important. Because of the numerical dominance of cattle egrets, the numbers of nests of all species followed the same pattern as that for cattle egrets.