CORNELIUS, Jamie M; HAHN, Thomas P; Univ. of California, Davis; Univ. of California, Davis: Physiological Correlates of Nomadic Migration in Red Crossbills

Locating food in a spatially and temporally variable environment is a problem faced by all animals. One tactic used by highly mobile animals such as birds is to move as food supplies change. The challenge is particularly acute for specialists whose preferred food varies unpredictably in space and time. For example, Red Crossbills (Loxia curvirostra) wander widely to locate unpredictable crops of conifer seeds. It is not known whether nomadic migrants such as crossbills make physiological preparations for long distance movements, similar to the seasonal changes in fat deposition, blood physiology, and plumage typical of seasonal migrants. This study searched for evidence of preparation for migratory movements in a nomad by characterizing body condition of free-living Red Crossbills across seasons. Fat deposition and blood hematocrit (indicators of available energy stores and aerobic capacity), showed seasonal peaks in late spring and autumn. Plumage molt (critical for maintaining flight efficiency and insulation) was concentrated in September and October. These data are consistent with typical movement behaviors of Red Crossbills. They move seasonally in June, in search of developing cone crops, and opportunistically in autumn when local seed supplies may decline. Elevated fat depots and hematocrits at these times could facilitate long-distance movements. Fresh plumage in autumn ensures both good insulation and efficient flight. In combination with published studies on captives, these data suggest that Red Crossbills display both seasonal and opportunistic components to their migratory behavior.