The terrible and abnormal mortality among smaller migrants may be
inferred from the breeding figures. Most pairs of swallows successfully hatch some ten young. One pair this year hatehed twenty. These become mature and them- selves breed at a year old. The natural life of the bird is certainly not less than ten years, during that period they and their children and children's children would be breeding ten chicks a year. Yet the population of swallows, as of most other birds (starlings are perhaps an exception) proves tolerably constant; and after an exceptional year may pitiably diminish. Migration is probably the most dangerous method by which the rigours of winter are avoided ; but an abnormally severe winter proves now and then as mortal to our home-keeping birds as any journey. It may almost wipe out, for example, the long-tailed tits, and may kill great numbers of the thrush tribe. But even at such meteorologiCal crises the worst mortality is generally among migrants from the north. The lorig-tailed tits share their fate with the redwing's.
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