Environmental determinants of distribution and foraging behaviour of cormorants (Phalacrocorax spp.) in temperate estuarine habitats

Thr distribution and behaviour of cormorants in estuarine environments was examined on the central coast of New South Wales, Australia, with respect t o habitat associations at different spatial scales. No consistent variation in abundance was found for four species of cormorants (great Phalacrocorax carbo, pied P. varius, little black P. sulcirostris, and little pied P. me lanoleucos) with state of tide (high and low) and time of day (early, middl e, and late) at five estuarine locations. Differences in abundance were fou nd among locations that were not confounded by short-term temporal variatio n (i.e. time of day and tide). Differences in abundance were detected among habitats (e.g. bays, creeks, and headlands) separated by hundreds of metre s to kilometres in different estuaries. Cormorants of all species were rare on the open coast and near the entrance of estuaries. Abundances of cormor ants varied greatly within and among cracks, bays, and river channels. Pres ence of seagrass beds explained much of this variation and most of the corm orants swimming and feeding were found near seagrass. Mapping of seagrass b eds and the positions of cormorants at scales of metres to hundreds of metr es showed a close relationship between the presence of swimming and roostin g beds and the presence of seagrass beds for P. melanoleucos and for P. car bo. We argue that cormorants make decisions to visit particular estuarine h abitats, especially those with seagrass, where many types of prey (e.g. fis h and crustaceans) are probably most abundant. These choices must be interp reted in the contest of decisions that cormorants make on scales of hundred s to thousands of kilometres during periodic excursions to the interior of Australia. Further, environmental threats to seagrass beds could impinge on these mobile visitors to the same extent as on more permanent residents.