Significance of Clutch-size in Swift and Grouse

Abstract

DR. LACK'S communication clarifies some important points, among them the influence of weather at the time of laying in the swift, and the earliness or lateness of the laying date, on clutch-size. But, notwithstanding the errors in the figures I extracted on clutch-size frequencies, he does not challenge my statement that the mean clutch-size in the common swift both at Oxford and in Switzerland is well below the ‘most efficient’ clutch-size, this being the size that on average has yielded the largest number of surviving young. Lack outlines three suggestions as to how this situation could be accounted for on the basis of natural selection acting at the individual level.