Female collared flycatchers adjust yolk testosterone to male age, but not to attractiveness

The differential allocation hypothesis predicts that females invest more resources into reproduction when mating with attractive males. In oviparous animals this can include prefertilization decisions such as the production of larger eggs and the deposition of hormones, such as the steroid testosterone, into yolks. On the other hand, a compensatory hypothesis posits that females allocate more resources into the eggs when mated with males of inferior quality. In the present study, we show that free-living females of the collared flycatcher (Ficedula albicollis), a small passerine bird, do not produce larger eggs or deposit more testosterone into eggs when mating with attractive males reflected by a large forehead patch size, which is contrary to the prediction of the differential allocation hypothesis. However, we found higher yolk testosterone concentrations in eggs laid for young than older males. Because in young males genetic quality, parental experience, or willingness to invest into paternal care is likely to be low, high yolk testosterone level in their clutches may indicate that their females follow a compensatory tactic. They may elicit more paternal care from young, inexperienced males by hormonally increasing nestling begging. Laying date was also correlated with yolk testosterone level; however, when we controlled for it, male age still remained a strong determinant of testosterone allocation.