The performance of 144 pairs of European kestrels breeding in upland young conifer plantations was studied from 1976 to 1979. Laying started between 15 April and 1 June, and was 14 days later, on average, in the poor vole year of 1977 than in the other years. Some 38% of pairs failed to fledge young, mainly because they deserted clutches during incubation. The proportion of pairs that failed was positively correlated with total rainfall in May, but apparently not with estimated spring vole numbers. There was a marked seasonal decline in both clutch size and the probability of breeding successfully, so that early laying pairs fledged 2.7 times as many young per attempt as did late pairs. Within years, there was considerable variation in the laying date between pairs. On the available data, environmental factors, such as the habitat or vole numbers around the nest, appeared to have less influence on this variation than did the age of the pair members, their pairing date or their previous breeding experience.