Seasonal pre-migratory fattening and increased activity in a nomadic and irruptive migrant, the Red Crossbill Loxia curvirostra

摘要

It is well documented that irruptive and nomadic migrants move in response to resources that are distributed unpredictably in space, time or both. Increasing evidence, however, suggests that irruptive and nomadic species may use seasonal timing mechanisms to prepare for migrations, despite the more facultative nature of their movements. Here we use data from free-living and captive Red Crossbills Loxia curvirostra, a typical irruptive nomad, to examine three hypotheses regarding control of facultative migration: (1) the facultative migration hypothesis, which states that both preparation and departure decisions are regulated by resource availability; (2) the seasonal preparation hypothesis, which states that preparation is initiated by seasonal factors (i.e. endogenous rhythms and/or photoperiod) but that departure decisions are dependent on local resource availability; and (3) the seasonal migration hypothesis, which states that both preparation and departure decisions are initiated by seasonal mechanisms and are independent of local food resources. Red Crossbills in North America are thought to make temporally consistent spring migrations in anticipation of conifer cone maturation. In this study, fat deposits of free-living Red Crossbills peaked in May and June, exceeding even winter deposits. In agreement with the ?eld data, captive birds on a natural photoperiod with constant food and temperature showed a peak in fat deposition and activity levels in June. These ?ndings are consistent with the seasonal preparation and the seasonal migration hypotheses and contribute to a growing literature that suggests that facultative migrants may prepare for movements in similar ways to seasonal migrants.