Life history correlates of alternative migratory strategies in American Dippers.

Partial migration is thought to be a critical step in the evolution of avian migration, but data on the life history correlates of alternative migratory strategies are extremely limited. We have studied a partially migratory population of American Dippers since 1999. This population is composed of sedentary individuals (residents) that maintain the same territory year round and altitudinal migrants that share winter grounds with residents, but move to higher elevations to breed. We used seven years of data on individually marked birds to (1) determine if individuals consistently use the same migratory strategy, (2) determine if offspring have the same strategy as their parents, and (3) estimate reproductive and survival rates of the two migratory strategies. We evaluate hypotheses for the persistence of partial migration and discuss their implication for the evolution of migration in sedentary populations. Individual American Dippers rarely switched migratory strategy (4/169 monitored more than one year). An individual's strategy, however, was not always that of its parents, indicating that, while migratory behavior may have a genetic component, environmental or social conditions probably influence the migratory strategy that an individual adopts. Sedentary dippers consistently had higher annual productivity (approximately 1.4 more fledglings/year) than migratory dippers, but mark-recapture models suggested that migratory dippers may have slightly higher survival than residents (approximately 3.4%). Migrants were estimated to have lower lifetime reproductive success than residents because their higher survival was insufficient to offset their lower productivity. Our data suggest that alternative migratory strategies in American Dippers are unlikely to be a fixed genetic dimorphism that persists because the two strategies have equal fitness, or because the relative fitness of the two strategies fluctuates over time. Migratory strategies in American Dippers are more likely to be condition dependent, and the two strategies persist because migrants "make the best of a bad job" by moving to higher elevations to breed. Because migrants obtained no fitness benefits by moving to seasonal breeding territories, our data are consistent with the hypothesis that migration could evolve in sedentary populations if competition for limited resources forces some individuals to seek breeding opportunities outside their initial range.