EXTRA-PAIR copulations, in which a female copulates with a male other than her mate, are known to occur in many bird species(1). Here we study a wild population of reed buntings, Emberiza schoeniclus, using single-locus DNA fingerprinting(2,3) and find an exceptionally high proportion of extra-pair paternity that accounts for 55% (118/216) of young and 86% (50/58) of nests. Twelve pairs each raised two broods in a single season in which the proportion of extra-pair young differed between the two broods. A highly significant relationship between a male’s parental investment and his degree of paternity was revealed by a comparison between the first and second brood of each pair, with more paternal care usually being provided at the nest that contained a lower proportion of extra-pair young. We propose that males can assess their likelihood of paternity and adjust their nestling provisioning rates accordingly.