It has been widely believed that birds timed their breeding seasons so that the young were raised when food was most abundant or easily obtained, but Perrins (1965, 1966) showed for two such different species as the Great Tit Parus major and the Manx Shearwater Puffinus puffinus that the pairs breeding earliest in a season left more progeny than those breeding late. In the cases of some species it...

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