Males of socially monogamous bird species may be faced with providing costly care for unrelated offspring when nests have extra-pair young (EPY). Theoretical models predict that cuckolded males should lower their parental investment as the likelihood of paternity decreases. However, empirical data are not always in support of this prediction.Here, we explore parental behaviours within the context of extra-pair patenity (EPP) in a population of the varied tit Parus varius in China. The results showed that 39.5% of nests were cuckolded and 16.4% of chicks were sired by extra-pair males. We found no evidence that male and female varied tits reduced their feeding rates or relative feeding effort to EPP broods, or that they decreased provisioning for EPY in mixed broods. There was also no direct effect of EPP on the reproductive success of breeding adults and the body condition nestlings near fledging. The lack of reliable cues of EP copulations (EPC)s by social mates available for the males, and/or the absence of strictly environmental pressure on males that would favor discrimination may account for a lack of an adjustment in feeding effort. The absence of discrimination between own and EPP chicks in parental care suggests that females pay no fitness cost as a result of EPCs, which may explain the high frequency of EPY in nests.
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