Experimental manipulation of testosterone and condition during molt affects activity and vocalizations of male blue tits.

Testosterone (T) is a key hormone regulating behavioral trade-offs in male birds, shifting investment towards sexual and competitive behaviors. However, the role of T in regulating male behavior during the molt has received very little attention, although this is a crucial life-history stage. Since the effect of T on behavior may be condition-dependent, particularly during the costly molt period, we studied the effects of T and condition in a two-way design. We manipulated T under two dietary regimes (standard and improved, resulting in an enhanced condition) in captive blue tits (Cyanistes caeruleus) undergoing the first pre-basic molt. T treatment increased song frequency, indicating that song is T-dependent also at this time of year. Males on the improved diet sang less than males in relatively worse condition, providing no support for song as an indicator of male condition. T-treated males exhibited greater locomotor activity than control males, but only when fed the standard diet. Neither T- nor diet-treatment affected plumage maintenance (preening). Although T treatment resulted in a delay in molt progress all birds completed the molt. Taken together our results show that during the molt male birds are sensitive to relatively small fluctuations in T. Similar to its commonly observed effects during the breeding season, T stimulated an increase in song and locomotion. While there might be some benefits associated with such T effects, these must be traded-off against costs associated with conspicuous behavior and increased molt duration.