When a male changes his ways: sex differences in feeding behavior in the pied flycatcher

Although sexual dimorphism is a well-known phenomenon, several aspects of it remain unresolved. For example, it is not clear how environmental conditions influence sexual dimorphism, especially in behavioral traits. In a study of pied flycatchers, we tested whether there are differences in the composition of food brought to young by the different parents and whether such sex differences are dependent on the hunger level of nestlings. We found that in normal feeding conditions, female parents provisioned nestlings with relatively more food collected from the tree canopy (caterpillars), whereas males brought more food caught in flight (adult lepidopterans). To imitate a temporary worsening of environmental conditions, we experimentally increased the hunger level of nestlings. Male parents responded to this manipulation by changing their foraging behavior such that the sex difference in provisioning behavior diminished. Possible explanations for the observed sex differences are discussed. This result is in accordance with the general pattern previously found for sex differences in animal size and certain other traits—namely that sex differences tend to diminish in harsh environmental conditions. It is possible that a decrease in sex differences in harsh conditions represents a more general pattern than previously assumed.