Behavioural responses of Eurasian treecreepers, Certhia familiaris, to competition with ants.

Competition for a specific resource that is essential for the survival of both the competitors may be intense even between very dissimilar taxa. However, the importance of the effects caused by such interspecific competition has seldom been emphasized. These effects can appear as differences in individual foraging behaviour during the breeding season, which can result in critical variation in fitness. In this study we examined the effects of wood ants (Formica rufa group) on the abundance of other invertebrates on tree trunks and on the foraging site selection of breeding Eurasian treecreepers, which use the same habitat as wood ants. Arthropods were scarcer on the trunks with ants present; the treecreepers avoided these trunks and foraged for a shorter time on trunks with ants than on trunks without ants. We also tested experimentally the existence of competition between ants and treecreepers by comparing the foraging behaviour of breeding treecreepers on spruce trunks with ants, without ants and with experimentally reduced numbers of ants. On average arthropods were scarcest on trunks with ants present. Male treecreepers also foraged for a shorter time on spruce trunks with ants. The reduction in ant numbers allowed food resources on trunks to recover over a week and led to longer foraging times of the treecreepers on these trunks than on trunks with ants present. The longest treecreeper visits were on trunks without ants. Our results suggest that competition between two very different taxa may be effective in determining the behaviour of foraging individuals.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour