Reproductive response of Great Tits, Parus major, in a naturally base-poorforest habitat to calcium supplementation

Recent studies have revealed that some forest passerines have difficulty ob taining sufficient calcium (Ca) for their eggshells in heavily acidified ar eas. However, the effect of Ca limitation on breeding success in non-acidif ied but naturally base-poor breeding habitats is not yet clear. The issue i tself is important, insofar as the cost of egg formation in a certain habit at depends on the availability not only of energy and proteins but also of shell-formation material. We studied the effect of experimental supplementa tion with extra Ca on the reproductive output of Great Tits, Parus major, b reeding in base-poor pine forests in Estonia. Pairs provided with Ca starte d 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 fledglings' tarsus length and body mass in the year when breeding condi tions were the most unfavorable. However, no significant differences were f ound between the experimental and control groups in hatching success or num ber of fledglings. We suggest that Ca deficiency is a less limiting factor in naturally base-poor forests than in highly acidified areas, and that bir ds are able to adjust their breeding efforts to conditions of Ca deficiency . Great Tits, for instance, may respond to Ca shortage by postponing egg la ying until their body condition and rate of food intake reach a critical th reshold for producing viable eggs and young.