Ecological Aspects of Heterospecific Flocks Formation in a Mediterranean Passerine Bird Community

Abstract

Mixed-species bird flocks were studied in a southern Spanish evergreen-oak woodland. Of the eight flock-participating species, four ('flock-positive') tended to take part in flocks more frequently, and four ('flock-negative') less, than expected on a random basis. Flock-positive species tended to broaden their utilization of feeding sites (in height and substrate) when participating in flocks, whereas flock-negative species showed the opposite trend. For all flock-positive species combined, foraging success while in flocks was about twice that for solitarily foraging birds. Among flock-negative species no significant difference in foraging success existed between the solitary and flocking situations. During the flocking period (autumn-winter), if not in flocks, flock-positive species needed more time to find a given number of prey items than during the remainder of the year. When in flocks, they were able to improve significantly their foraging performance. Flock-negative species improved in autumn-winter their foraging success regardless of whether they participated in flocks or not. It is shown that these results can be interpreted as evidence supporting both the predation- and feeding-related hypotheses proposed for the evolution of flocking behaviour. The nature and degree of the advantages gained by participating in flocks