Tawny owls, Strix aluco, laid female-biased clutches on territories with more abundant prey (field voles) in June, the month that chicks fledge. This appeared to enhance the subsequent reproductive success of fledglings, as in 1995 there was a significant correlation between the number of chicks fledged by adult females and the June vole abundance in the territory on which they were reared as chicks. This relationship did not hold for males. Since tawny owls lay eggs in March, these results indicate that owls are able to predict the June vole numbers on their territory, and respond by producing more of the sex most likely to gain a long-term benefit when resources are good.

The incidence of extra‐pair paternity in a Great Tit Parus major population at Wytham Wood, Oxford, in 1985–1987 was determined using two polymorphic allozymes. In 831 nestlings from 94 broods, 27 genetic exclusions were detected in 25 (3%) nestlings from 16 broods. Seven (44%) of these broods contained offspring that excluded the putative male parent from being the genetic parent. The distribution of exclusion types indicated that excluded offspring were the result of fertilizations by extra‐pair males and not of egg‐dumping. The true frequency of extra‐pair paternity was estimated as 14% of offspring. These results suggest a mixed reproductive strategy for males in which they breed mo‐nogamously whilst simultaneously seeking extra‐pair matings with females of other pairs.

We estimated the frequency of cuckoldry (the proportion of offspring resulting from extra‐pair copulations) in Great Tits Parus major using heritability estimates based on the resemblance of offspring tarsus‐length to that of their parents. Our results suggest that cuckoldry occurs and may be relatively common in certain years.

No abstract

The frequency of extra‐pair paternity in a wild colony of swifts Apus apus was determined by multilocus DNA fingerprinting in two successive breeding seasons. The data were used to examine the expectation that extra‐pair paternity is frequent in colonial‐nesting species, either for proximal reasons such as the increased opportunity for extra‐pair matings, or because extra‐pair matings are important in the evolution and maintenance of coloniality. Forty‐two broods containing 88 chicks were analysed. The genetic analysis revealed four cases of extra‐pair paternity (4.5% of chicks) from four (9.5%) nests. Rapid mate‐switching was considered unlikely to be the cause of extra‐pair paternity since three of the cases were in the nests of previously established breeding pairs. Extra‐pair copulations were not observed, but were assumed to be the cause of extra‐pair paternity. The data show that high levels of extra‐pair paternity are not an inevitable feature of high‐density nesting.