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In page Eurasian crag martin:

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An Italian study showed that, as with other aerial feeders, the start of breeding was delayed by cold or wet weather, but this had no influence on the clutch size nor on the number of fledged young. Unexpectedly, it was found that once the eggs had hatched there was a negative relationship between temperature and the number of fledged young. The authors suggested that hot weather dried up the small rivers where the parents found food. Colony size did not influence the laying date, the clutch size or the number of successfully fledged young, but this species does not form large colonies anyway.[5]