MORPHOLOGICAL-CHANGES IN A POPULATION OF SAND MARTINS RIPARIN RIPARIAASSOCIATED WITH FLUCTUATIONS IN POPULATION-SIZE

Sand Martins experienced a dramatic population crash in Britain and el sewhere during 1983-84. Numbers breeding in central Scotland in 1984 w ere 27% of those in 1983, probably as a consequence of severe drought in the wintering quarters. Selection for small size during the populat ion crash was demonstrated.(1) This paper shows that by the late 1980s the Sand Martin population in central Scotland had recovered to the l evel 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 fo und before the population crash. The return of severe drought to the w intering grounds in 1990 was associated with a fall in bath numbers an d keel-length at the study site by 1991. We evaluate hypotheses which attempt to explain the positive correlation between body-size and colo ny-size, and conclude that the observed changes in mean keel-length ar e probably caused by selective mortality of different sizes of birds o utside of the breeding season and probably in Africa.