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Summary.

  • 1
    In northern Europe, the northward movement of the sun in spring has so extensive an influence, that the food of all species of birds reaches its maximum at some time between late spring and early autumn. As a result, and in contrast to the tropics, nearly all species of birds lay their eggs in roughly the same period of the year.
  • 2
    A review of the breeding seasons of British birds reveals some interesting correlations between time of breeding and time of maximum food, and also many apparent puzzles, particularly as regards the marked differences in breeding season sometimes found between related species.
  • 3
    In Lapland, all species start breeding 1–2 months later than in Britain, and the breeding season is much less extended. As compared with Britain, the Corvidae and predatory birds of Lapland lay early relative to the small passerines, but among the small passerines, and also among the predators, the different species tend to breed in the same order relative to each other as they do in Britain.
  • 4
    Quantitative measurement of breeding seasons is much needed, and some examples showing the advantages of this method are given in Tables 1–3 and text-Figs. 1–5.
  • 5
    It is considered that, through the action of natural selection, the breeding season of each species coincides with the time of year when offspring can be raised with greatest success. The major ultimate factor involved is, therefore, the food for the young birds (and hence the factors which determine the abundance and availability of this food). But there may be modifying ultimate factors concerned with the survival of the parents, of the nest and eggs, and of the juvenile birds in the period soon after they leave the nest.
  • 6
    Proximate factors provide a physiological timing mechanism whereby the gonads recrudesce, and eggs are laid, at such a time that, on the average, the young hatch at a season when there is sufficient food to raise them. In European birds, daylength seems the major proximate factor, but there are several modifying factors, perhaps including temperature.