Is parental risk taking negatively related to the level of brood reduction? An experiment with pied flycatchers

Predation risk is an important factor affecting investment decisions in wild birds. The parental risk-taking response is assumed to depend on the value of the current brood, but to date empirical studies have yielded equivocal results. We tested the 'reproductive value of the offspring' (RV) hypothesis by assuming that parents perceive the probability of offspring survival as a significant component of RV. We predicted that survival of offspring as perceived by parents would be inversely related to the level of brood reduction. Accordingly, parents with the same number of offspring may take fewer risks if they have lost offspring than if they have not. We experimentally reduced brood size in a small passerine, the pied flycatcher, Ficedula hypoleuca, and exposed parents to a stuffed sparrowhawk, Accipiter nisus, a predator of adults. For a given brood size, the time elapsed until parents first entered the nestbox after removal of the predator was positively correlated with the level of brood reduction. Parental risk-taking behaviour was not affected by brood size per se when adjusted for the level of brood reduction. No sex-related differences in antipredator responses were found. To our knowledge, this is the first experimental evidence that birds are capable of perceiving brood reduction and of adjusting their antipredator behaviour to the changed value of their offspring.