Are thyroid hormones integral to migration and breeding?

In seasonally breeding animals, accurate timing of vernal life history stages is critical for both survival and reproductive success. Such timing requires animals to be able to anticipate and prepare for future events and to do so they must rely upon proximate environmental cues. For many temperate zone birds, photoperiod is the initial predictive cue signaling the approach of spring. While photoinduction has been long known, the mechanisms regulating development and expression of vernal migration and breeding remain poorly understood. Strong evidence has emerged in recent years linking thyroid hormone signaling to the photoinduction of breeding. More limited information also suggests a potential role in the regulation of vernal migration and prenuptial moult in photoperiodic songbirds. Here we investigate the development and expression of vernal migration and preparations for breeding in captive Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelii) in a hypothyroidic state. Hypothyroidism was induced by chemical inhibition of thyroid hormone production. To explore differential action by the two thyroid hormones: triiodothyronine and thyroxine, we subsequently performed a thyroid inhibition coupled with replacement of thyroid hormones. We found that chemical inhibition of thyroid hormones resulted in complete abolishment of prenuptial moult, fattening and muscle hypertrophy as well as ultimately captive expression of nocturnal restlessness. Furthermore, chemical inhibition of thyroid hormones caused birds not to undergo gonadal development in response to increasing photoperiod. Replacement of thyroxine rescued all of these elements to near control levels while triiodothyronine replacement displayed variable and delayed effects. Our findings highlight thyroid hormone as being necessary for the development and expression of the vernal migratory life history stage, in addition to the photoinduction of breeding.