Clutch size of the kittiwake is influenced by the previous breeding experience of the female and also by the time of laying. Both of these factors play a part in determining the clutch size and the correlation between breeding experience and clutch size is not solely the result of older birds breeding earlier. The clutches laid by females breeding for the first time are, on average, smaller than those laid at the same time by more experienced breeders and are less influenced by the time of breeding. No female breeding for the first time has been found to lay three eggs. There is a correlation between the time of the onset of breeding in kittiwake colonies and the average clutch size. It is suggested that a progressive decrease in the clutch size as the breeding season progresses may be widespread in both passerines and non-passerines and probably occurs only in species which do not normally have two broods in a single breeding season.