Significance of Clutch-size in Swift and Grouse

Abstract

WYNNE-EDWARDS¹ has so misunderstood the problem of clutch-size in the swift Apus apus at Oxford reported by Perrins² that further clarification seems desirable. First, he confused clutch-size (number of eggs laid) with brood-size (number of young at hatching). This is important because we often manipulated brood-size in the tower, as Perrins stated for broods of four, while most broods of one were due either to our manipulation or to hatching failures. Wynne-Edwards was, therefore, wrong in stating that clutches of four have been commoner in recent years, and that clutches of one, two and three have been in the proportion of 24 : 61 : 15. The swifts at Oxford have normally laid either two or three eggs, and natural clutches of one and four have been extremely rare.