Few studies have considered the effects of interspecific competition between distantly related taxa on the reproductive success of individuals. We compared the food supply, laying date, clutch size, and breeding success of a small double-brooded passerine bird, the Eurasian Treecreeper (Certhia familiaris) between territories with or without colonies of red wood ants (Formica rufa group) during four years. Both the wood ants and Eurasian Treecreepers 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 this difference was more pronounced during the second clutches. In territories without ants, treecreepers started breeding earlier and increased the size of second clutches, whereas birds breeding in territories with ants decreased clutch size in second breeding attempts. In addition to hatching later, nestlings in territories with ants achieved lower body mass near fledging and suffered higher mortality than nestlings in territories without ants.
Consequently, double-brooded treecreeper pairs produced an average of 2.3 more fledglings, also of higher quality, in territories without ants than in territories with ants. There were no differences between the territory types in any measure of habitat quality other than food abundance. Our results suggest that wood ants reduce territory quality of Eurasian Treecreepers by 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 success of individuals.
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