Reproductive success of Eurasian Treecreepers, Certhia familiaris, lower in territories with wood ants

Few studies have considered the effects of interspecific competition betwee n distantly related taxa on the reproductive success of individuals. We com pared the food supply, laying date, clutch size, and breeding success of a small double-brooded passerine bird, the Eurasian Treecreeper (Certhia fami liaris) between territories with or without colonies of red wood ants (Form ica rufa group) during four years. Both the wood ants and Eurasian Treecree pers forage on tree trunks and utilize the same food resources. It has been shown that the wood ants are able to depress the available food supply for the treecreepers and interfere with their foraging behavior. We found that food abundance was lower in territories colonized by wood ants, and that t his difference was more pronounced during the second clutches. In territori es without ants, treecreepers started breeding earlier and increased the si ze of second clutches, whereas birds breeding in territories with ants decr eased clutch size in second breeding attempts. In addition to hatching late r, nestlings in territories with ants achieved lower body mass near hedging and suffered higher mortality than nestlings in territories without ants. Consequently, double-brooded treecreeper pairs produced an average of 2.3 m ore fledglings, also of higher quality, in territories without ants than in territories with ants. There were no differences between the territory typ es in any measure of habitat quality other than food abundance. Our results suggest that wood ants reduce territory quality of Eurasian Treecreepers b y means of food depletion and have negative effects on the breeding success of individual birds. These results show that competition between organisms in different phyla may be effective in determining the reproductive succes s of individuals.