Why do male birds not discriminate between their own and extra-pair offspring?

A number of recent models of optimal paternal investment predict that males should alter their investment in offspring in response to changes in paternity or certainty of paternity. One way in which it has been suggested that male birds might do this would be to recognize their own offspring and to discriminate in their favour. Despite frequent statements that nothing is known about whether birds possess this ability or whether they exercise it, there is a considerable body of evidence suggesting that males do not discriminate against non-related offspring. This evidence is reviewed and an attempt made to explain the absence of kin discrimination in males under these circumstances by considering potential mechanisms of kin discrimination. Although there is selection for males to discriminate in favour of their own offspring it is argued that they are unable to do so because of conflicts between the male, female and offspring over signalling identity, and because the circumstances associated with extra-pair paternity disrupt the operation of some mechanisms. Male birds might possess behavioural 'rules of thumb' which lead to behaviour that appears similar to offspring recognition, but the sophistication of such rules is likely to be limited by the stochasticity inherent in fertilization.