NATURAL SELECTION ON BODY SIZE AND LAYING DATE IN THE TREE SWALLOW.

I measured natural selection on body size and laying date in a population of tree swallows (Tachycineta bicolor) from 1986 to 1988. There was little evidence of selection on body size associated with overwinter survival. Disruptive selection on tarsus length, associated with female reproductive success, was detected in one of three years. Both repeatability and mother-daughter regression suggested that laying date was heritable. I found weak evidence of selection on laying date, associated with both overwinter survival and reproduction in females. The ecological implications of both tarsus length and laying date variation in this population could not be identified. Consequently, although I was able to identify the targets of natural selection, the ecological link between trait variation and selection remains unknown. © 1991 The Society for the Study of Evolution.