Foraging behavior, physiological state and site fidelity of resident (sedentary) and migratory American Dippers (Cinclus mexicanus) were compared to assess whether differences in over-wintering behavior or physiology explain the lower annual survival rate of resident individuals. Residents spent more time resting (23 vs. 14% of the time) and less time foraging (71 vs. 81% of the time) than migrants. However, there were no detectable differences in energetic intake (kJ/hr), foraging success (kJ/hr spent foraging), or physiological state (measured as size-corrected mass, hematocrit, leucocrit, total white blood cell count, and heterophil to lymphocyte ratio; immunoglobulin, triglyceride, and free glycerol levels; and total antioxidant capacity and total oxidative status of plasma). Foraging success, size-corrected mass and plasma triglyceride levels increased with date for both migrants and residents, indicating that both groups built up fat reserves as winter progressed. Resident individuals exhibited higher winter site fidelity than migrants (sighted on the same section of the river on 70 vs. 53% of weekly censuses). The lower site fidelity and, hence, greater movement of migrants may increase survival by allowing them to seek more suitable microhabitats during adverse conditions. Further work on the relationship between mortality rates, movement and winter home range size of migrant and resident dippers is needed to test the latter hypothesis.
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Vol. 33 • No. 4