A new hypothesis is proposed to explain why House Sparrows hatch their broods asynchronously in late season, with the result that last hatched nestlings have reduced survival expectancies. I suggest that late in the season parents hatch a relatively large brood because during the heterothermic stage nestlings will suffer lower heat losses in large than small broods, so that the time devoted to collecting food can be increased. Hatching asynchrony, then, ensures that last hatched young, that cannot be adequately fed, do not compete for food with their older siblings, thus reducing the nest period and predation risks of valuable nestlings. Evidence on seasonal variation of nest predation rates, fledging sizes and nestling feeding rates and attendance is given as a background to, but not as a formal test of, the hypothesis put forward.
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