Frequency of Extra-Pair Paternity in Birds Estimated from Sex-Differential Heritability of Tarsus Length: Reply to Lifjeld and Slagsvold's Critique

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Abstract
Lifjeld and Slagsvold have recently criticised the use of heritability estimates of tarsus length in relation to the sex of parents as an estimate of the frequency of extra-pair paternity in birds. In this note I refute their critique by (1) reporting heritability of tarsus length from the swallow Hirundo rustica, and (2) commenting on the relationship between the frequency of extra-pair copulations and the frequency of extra-pair paternity in birds. I estimated heritability of tarsus length, a sexually size-monomorphic trait, from swallow male parent-offspring and female parent-offspring regressions. Heritability estimates for males on offspring were 26% (6) (mean (S.E.), N = 5 years) lower than estimates for females. This request suggests that extra-pair paternity is common in swallows. In argue that the frequency of extra-pair paternity may be higher in polygynous than in monogamous species of birds, because the cost of extra-pair paternity is generally higher in monogamous species with extensive paternal care.