INFLUENCE OF NUTRITION ON EGG SIZE IN GRE AT TITS PARUS-MAJOR

During long term studies on the ecology of alpine tit populations we p erformed a supplementary feeding experiment to test for the influence of nutrition on egg size in the Great Tit. All measurements of egg siz e in females that utilised the supplementary food (mixture of curd, mi nced beef heart and dried insects) increased statistically significant , whereas clutch size nas not influenced by the additional food. These results support the hypothesis that females spend surplus energy in t he single eggs of their clutch and not in producing additional eggs. B reeding success is positively correlated with egg size. Therefore incr easing egg size leads to a higher breeding (= fledging) success than e nlarging clutch size The ecological impact of increasing egg size with altitude is discussed in the light of the supplementary feeding exper iment. As genetic adaptations are not to be expected because of the hi gh gene flow, larger eggs at higher altitudes could be the result of s hort, but regular gradations of insect populations and therefore optim al supply with prey. Whether additional food or the quality of parents being able to hold optimal territories is the most important ultimate factor influencing egg size in Great Tits remains to be tested by cro ss-foster experiments.