Theoretical models on parental care predict that males should decrease their parental effort when paternity is in doubt. Males may use some cues to assess their certainty of paternity, and try to avoid rearing offspring sired by extra-pair males. We have previously reported in a socially monogamous passerine, the blue tit (Cyanistes caeruleus), that males decorate their nests with feathers, and that when this ornament is manipulated, males appear to have suspicions about the presence of an intruder male. Here, we decrease the male's certainty of paternity through experimental feather supplementation to analyse whether the outcome of our experiment supports the assumptions of the parental care theory. Male C. caeruleus responded to the feather supplementation experiment by reducing their parental investment (feeding frequency and nest defence) in comparison with control males. The occurrence of extra-pair offspring in experimental nests was double than that in controls. This suggests that the manipulation was successful not only in altering males' perceived paternity, but also, indirectly, the actual paternity. Furthermore, males that gained extra-pair young also had a higher than average probability to lose paternity in their nest, which may imply that male C. caeruleus faced a trade-off between obtaining extra-pair fertilizations and maintaining paternity in their own nest. Overall, this study supports the idea that males are prone to decrease their parental effort when they perceive that the risk of losing paternity is high. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109, 552–561.