Suchergebnisse

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.