Determinants of breeding performance of the Eurasian treecreeper

Abstract

The aim of this thesis was to study the determinants of clutch size and reproductive success of the Eurasian treecreepers (Certhia familiaris). The emphasis is on individual foraging behaviour and parental care, and how they relate to reproductive success and food abundance. Clutch size seems to be limited by available food. When food was scarce, parents forced to raise larger brood than their original clutch size changed their foraging behaviour and depleted the food supply in their territory. Parental care was affected by the varying environmental conditions during the first and second broods. Male care was found to be most important in conditions with low food abundance. Females, whose mates were removed during the first broods, compensated for the mate loss by increasing their provisioning rate, but the compensation was incomplete, since the reproductive success of the widowed females was lower than of paired birds. Widowed females also changed their foraging behaviour to be more male-like, which indicates, that the niche separation between the sexes in this species is a result of behavioural plasticity. Experimental manipulation of the number of wood ants on tree trunks in treecreeper territories revealed, that ants are able to deplete the food resources of the treecreepers. Treecreepers also avoided trunks with ants, and foraged for a shorter time on trunks with ants present. Reproductive success of the treecreeper pairs that bred in territories with ants was lower than of pairs breeding in territories without ants; birds in ant-territories produced less fledglings that were also of lower body weight than pairs in territories without ants. Studies of this thesis show, that food abundance is an important determinant of individual foraging behaviour and reproductive success. In addition, the profound effects of ants on treecreepers point out, that competition between organisms in different phyla can be effective in determining individual behaviour and life-history traits.