Reproductive Trade-Off between First and Second Clutches in the Great Tit Parus major: An Experimental Study

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Abstract
The size of first broods was manipulated in great tits (Parus major) to create 5 groups of nesters with brood sizes deviating from the original brood size by -4, -2, 0, +2 and +4 young. Pairs with enlarged first broods were less likely to have a second clutch than control pairs or pairs with reduced first broods. Thus, an allocation trade-off between investments in first and second breeding was evident. Adult condition and adult weight in the late nestling period were not correlated with the experimental treatment, and seemed not to importantly influence the trade-off. Rather the currency of investment involved in the trade-offs was time. Enlarged broods has small and underdeveloped fledglings, and suggestedly demanded a prolonged period of adult attendance, which made a new nesting unfeasible. Several cases where such underdeveloped young grew considerably after fledging were documented.