Meeting Abstract
Appropriate timing of the transition between breeding and plumage molt is vital to fitness in most birds. Delay of this transition provides increased reproductive opportunities at a cost to future survival, since delayed molt can lead to increased molt rate and decreased feather quality. Within some species, individuals vary in how they mediate this trade-off, as evidenced by wide ranges of molt onset dates. Despite its importance, understanding of the proximate mechanisms underlying molt timing is limited. One important hormonal regulator of this transition is testosterone (T), which maintains reproductive physiology and delays molt. Individual variation in plasma T levels may contribute to variation in molt timing. However, a previously unconsidered alternative mechanism may also play a role: peripheral sensitivity to T (e.g. via variation in receptor density/specificity and/or concentration of enzymes and hormone binding globulins). To test the relative importance of T levels versus T sensitivity, we administered T implants of varying doses prior to molt (late June) in male House Finches and observed molt responses. After removing implants in December, we continued the study through the following year. We found extensive variation in molt delay and suppression, which was not well-explained by T levels delivered by the implants. T treatment during the first year led to a small but significant delay of molt in the second year, compared with controls (empty implants). Greater T sensitivity during the first year significantly predicted greater carry-over of molt delay between years. These results suggest that individual variation in T sensitivity may be more important than variation in T levels for determining timing of the breeding-molt transition as well as long-term effects on molt timing.
