Female bluethroats enhance offspring immunocompetence through extra-pair copulations

Female birds frequently copulate with extra-pair males(1,2), but the adapti ve value of this behaviour is poorly understood(2). Some studies have sugge sted that 'good genes' may be involved, where females seek to have their eg gs fertilized by high-quality males without receiving any material benefits from them(3,4). Nevertheless, it remains to be shown that a genetic benefi t is passed on to offspring(5,6). Here we report that nestling bluethroats, Luscinia svecica, sired by extra-pair males had a higher T-cell-mediated i mmune response than their maternal half-siblings raised in the same nest. T he difference could not be attributed to nestling body mass, sex or hatchin g order, but may be an effect of paternal genotype. Extra-pair young were a lso more immunocompetent than their paternal half-sibs raised in the geneti c father's own nest, which indicates an additional effect of maternal genot ype. Our results are consistent with the idea that females engage in extra- pair copulations to obtain compatible viability genes, rather than 'good ge nes' per se.