Nestling testosterone controls begging behaviour in the pied flycatcher, Ficedula hypoleuca

Begging signals and endogenous testosterone (T) levels of young birds have been shown to be positively correlated. If T is causally involved in controlling the level of begging effort, an endocrine control mechanism could explain the evolution of begging as a costly signal reflecting need. We tested experimentally whether elevated circulating T levels enhanced begging behaviour in nestling pied flycatchers, Ficedula hypoleuca. A pilot study confirmed that nestling T levels could be elevated within a natural physiological range using an oral dose of T. After T-dosing, nestling begging behaviour was measured as: i) the duration of begging displays and ii) the maximum height of begging stretches. Our results show that nestling T levels were elevated at 90 min post dosing and that at this time point both measures of begging behaviour were performed more intensely by T-dosed nestlings than controls. Nestling begging displays in response to dosing varied between individuals, which in part was explained either by the date in the breeding season or nestling mass. The results of this study confirm the causal nature of T in controlling nestling begging signals and suggest that it may be part of the mechanism that controls begging behaviour in nestling birds.

Nestling testosterone is associated with begging behaviour and fledging success in the pied flycatcher, Ficedula hypoleuca

Animal signals are hypothesized to be costly in order to honestly reflect individual quality. Offspring solicitation signals given by nestling birds are thought to have evolved to advertise either need or individual quality. We tested the potential role of testosterone (T) in controlling the intensity of these signals by measuring begging behaviour as: (i) duration of the begging display and (ii) maximum height of the begging stretch, and by sampling endogenous T levels in nestling blood. We tested nestling pied flycatchers (Ficedula hypoleuca) using well-established experimental paradigm involving transient food deprivation to encourage begging behaviour and then blood-sampled nestlings at the end of these tests for T levels. Our results show that individual nestlings with the most intense begging displays had the highest circulating levels of T immediately after testing. In addition, we found substantial differences between broods in terms of circulating T. Finally, we found evidence that broods with higher levels of T showed increased fledging success, indicating a benefit for increased T production in nestlings. The potential trade-offs involved in T-mediated begging behaviour are discussed.