Selection on Clutch Size in Birds

In bird populations, it is commonly observed that (1) clutch size is positively correlated with the number of surviving young a clutch produces and (2) the most productive clutch size is higher than the average clutch size. We review hypotheses that have been proposed to account for these observations. We then show that the patterns are inevitable if females in different health or nutritional states vary in both the number of eggs they lay and the number of chicks they raise, for which there is good empirical evidence. A persistent selection differential for clutch size is expected at evolutionary equilibrium, even if clutch size is heritable.