Reproductive response of Great Tits, <i>Parus major</i>, in a naturally base-poor forest habitat to calcium supplementation

Abstract

Recent studies have revealed that some forest passerines have difficulty obtaining sufficient calcium (Ca) for their eggshells in heavily acidified areas. However, the effect of Ca limitation on breeding success in non-acidified but naturally base-poor breeding habitats is not yet clear. The issue itself is important, insofar as the cost of egg formation in a certain habitat depends on the availability not only of energy and proteins but also of shell-formation material. We studied the effect of experimental supplementation with extra Ca on the reproductive output of Great Tits, Parus major, breeding in base-poor pine forests in Estonia. Pairs provided with Ca started egg laying earlier than control pairs, and clutch volume was larger in Ca-supplemented nests. There was also a positive effect of Ca supplementation on fledg- lings' tarsus length and body mass in the year when breeding conditions were the most unfavorable. However, no sig- nificant differences were found between the experimental and control groups in hatching success or number of fledglings. We suggest that Ca deficiency is a less limiting factor in naturally base-poor forests than in highly acidified areas, and that birds are able to adjust their breeding efforts to conditions of Ca deficiency. Great Tits, for instance, may respond to Ca shortage by postponing egg laying until their body condition and rate of food intake reach a critical threshold for producing viable eggs and young.