Do flocks of great cormorants and goosanders avoid spatial overlap in foraging habitat during the non-breeding season?

Abstract

Species distribution, ecology, and behaviour are shaped, amongst other things, by interspecific, antagonistic interactions, and this phenomenon is particularly noticeable among predators. We studied the spatial co-distribution of two top piscivorous bird species foraging on inland waters outside breeding season. We considered the hypothesis that goosanders, Mergus merganser, and great cormorants, Phalacrocorax carbo, avoid foraging in close proximity to each other. Data collected on five river-reservoir systems in the Western Carpathians (Poland and Slovakia) during two periods (2014-2015 and 2015-2016) showed that goosander numbers reduced significantly and their foraging areas changed when large flocks of cormorants arrived and began foraging. We also found that inter-flock distances were greatest between flocks of goosanders and cormorants, suggesting that the former species avoided the waters occupied by the latter. Distribution of flocks of both species was additionally determined by the location of foraging place in river-reservoir system, weather, and presence of other piscivorous birds (e.g. grebes) and raptors (e.g. eagles). Together with the results of research in adjacent Bohemia, this study suggests that competition between cormorants and goosanders may arise when bodies of water suitable for piscivorous foraging are scattered and limited in number, as in the mountainous areas of Central Europe.