Meeting Abstract
In obligate migrants, increasing day length and the resulting increase in androgen levels are a key mechanism stimulating the transition to a vernal migratory life history stage. Among facultative migrants, the seasonal expression of a migratory life history stage in the spring has been documented in several species. Yet in these species, the roles of increasing photoperiod and increasing androgen levels in triggering this migratory stage remain unclear. Using the pine siskin (Spinus pinus), a nomadic and irruptive migrant, as our model, we investigate (1) whether increasing photoperiod triggers a migratory life history stage in a facultative migrant, and (2) whether an effect of photoperiod might be mediated by increasing testosterone levels as occurs in obligate migrants. Wild-caught pine siskins were held on either winter solstice day length (SD) or a naturally increasing photoperiod (LD) from late-December to June. Behavioral (locomotor activity) and physiological (body mass and fat deposition) indicators of a migratory stage were monitored, and monthly blood samples were collected to measure circulating testosterone levels. LD birds showed increases in locomotor activity, consistent with migratory restlessness, which were not observed in SD birds. LD birds, but not SD birds, also showed increases in circulating testosterone coinciding with the increase in locomotor activity. But, birds in both groups exhibited an increases in body condition consistent with pre-migratory preparations. Thus our results suggest that increasing photoperiod can stimulate the expression of migratory behavior in a facultative migrant, and that this effect may be mediated by an increase in circulating testosterone. However, these mechanisms do not appear to orchestrate initial preparatory changes in body condition such as pre-migratory fattening.
