Survival of Young Swifts in Relation to Brood-size

Abstract

I DOUBT if it is possible from Dr. Perrins's results to convince the sceptic that swifts are “producing as many surviving young as possible”, and to demonstrate in this way that natural selection has, as he believes, forced their fecundity up to the potential limit of efficiency. A second look at the figures soon puts the conclusion in doubt. I agree that in England clutches of four tend to be inefficient; they constitute a relatively infrequent group, although they seem to have been commoner in 1958–61 than they were in the 1946–56 period^5,6. Setting them aside we cannot be far wrong in assuming (having combined Perrins's samples for 1958–61 with those of Lack and Lack⁶ for 1946–51) that the remaining swifts breeding in the Oxford neighbourhood in recent years have produced clutches of one, two or three chicks in a percentage ratio of about 24 : 61 : 15. Two-chick broods have evidently been by far the most common.