Morphological changes in a population of Sand MartinsRiparia ripariaassociated with fluctuations in population size

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Abstract
Sand Martins experienced a dramatic population crash in Britain and elsewhere during 1983–84. Numbers breeding in central Scotland in 1984 were 27% of those in 1983, probably as a consequence of severe drought in the wintering quarters. Selection for small size during the population crash was demonstrated.1¸ This paper shows that by the late 1980s the Sand Martin population in central Scotland had recovered to the level typical of the early years of the decade. Moreover, keel-length, used as a measure of body-size, had assumed a similar value to that found before the population crash. The return of severe drought to the wintering grounds in 1990 was associated with a fall in both numbers and keel-length at the study site by 1991. We evaluate hypotheses which attempt to explain the positive correlation between body-size and colonysize, and conclude that the observed changes in mean keel-length are probably 'caused by selective mortality of different sizes of birds outside of the breeding season and probably in Africa.