Selection against large size in the Sand Martin Riparia riparia during a dramatic population crash

In 1984 breeding Sand Martins declined to 27% of their 1983 numbers at a large colony in Scotland. A population crash of similar magnitude occurred over much of central Scotland and over Britain as a whole. The decline in numbers was probably attributable to severe drought conditions experienced overwinter in the Sahel zone of Africa. There was a significant reduction in the mean keel‐length of Sand Martins captured at the main study colony in 1984 compared with individuals measured in both 1982 and 1983, suggesting that selection for small body‐size had occurred during the period of high overwinter mortality.