Opportunistic use of foraging resources by heron communities in southern Europe

Neither theoretical nor descriptive approaches to the study of feeding resources have established whether throughout their range species exploit resources with particular characteristics, and whether resource use is opportunistic, selective or determined by competition among consumers I studied 7 southern European communities of 3-6 breeding species of herons Foraging habitats and prey types varied widely among regions for each heron species while prey weights varied less Principal Components Analysis characterized each heron species by its use of a specific key resource, composed of combinations of prey size, foraging habitat and prey type, in order of decreasing importance The largest herons (grey and purple) and the smallest (squacco heron) were characterized by specific prey weights and types, while the intermediate sized herons (little egret and night heron) were characterized by prey types and foraging habitats Similarities among heron species in habitat use. prey types and prey weights were significantly higher than random in only a few species-pairs, and no value was significantly lower than random Aggressions were infrequent both against conspecifics and heterospecifics When compared with explicit predictions of the hypotheses that resource use is shaped by opportunism, independent selectivity, or inter-species competition, the patterns observed indicate that heron species, although partially selective for prey size, tend opportunistically to use similar habitats and prey types, presumably the most profitable and locally abundant Little evidence was found for competitively constrained resource use Herons in southern Europe may not be limited by feeding resources during breeding, although some evidence (significant overlaps, exclusion from colonies) indicates ongoing competition between grey and purple herons