Morphological changes in a population of sand martins riparia riparia associated with fluctuations in population size

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 colony- size, 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. © Taylor & Francis Group, LLC.