Meeting Abstract
The photoinduction of breeding and migratory behavior in songbirds has been a subject of intense research focus since Rowan’s discovery in (1926). The transition from a wintering life history stage through pre-alternate molt into migration and then breeding requires coordination of numerous changes in physiology, morphology and behavior. Previously we found that chemical thyroidectomy with the anti-thyroid agent, Methimazole, led to suppression of circulating thyroxine levels, pre-migratory fattening, flight muscle hypertrophy, and onset of nocturnal restlessness (Perez in prep). Additionally, Methimazole treatment was found to arrest pre-alternate molt and gonadal growth. In order to confirm that the observed effects of Methimazole induced thyroid knockdown are indeed due to suppression of circulating thyroid hormone levels and not direct drug effects we conducted a thyroid rescue experiment. White-crowned sparrows were held on natural photoperiod and divided into a control and three treatment groups. All treatment groups received Methimazole both orally and via silastic implant. One of the three groups was maintained as a Methimazole only control while the other two received triiodothyronine and thyroxine respectively via silastic implant. As expected Methimazole treatment suppressed events associated with pre-alternate molt, migration and initiation of gonadal recrudescence. Thyroxine replacement rescued fattening, mass gain, vernal molt, and testicular growth to control or near control levels. Curiously, the theoretically more potent triiodothyronine treatment, displayed only partial rescue. These findings confirm that observed changes in behavior and physiology are due to thyroid suppression and not direct drug effects.
