DYNAMICS IN SUBDIVIDED POPULATIONS OF NEOTROPICAL MIGRATORY BIRDS IN A FRAGMENTED TEMPERATE FOREST

To study the hierarchy of variables involved in the year-to-year dynamics in the distribution of Neotropical migratory birds, we compared observed numbers of local extinctions and recolonizations to those expected under six hypothetical mechanisms of settlement by returning spring migrants. These mechanisms were (1) random dispersal among forest fragments; (2) passive sampling of spring migrants according to fragment area; (3) dispersal among fragments between years according to the distance to the nearest fragment occupied the Ist yr; (4) resettlement of spring migrants according to the vertical structure of the vegetation in the fragments; (5) site fidelity of experienced breeders; (6) a combination of mechanisms 2, 3, 4, and 5. We selected four target species: Wood Thrush (Hylocichla mustelina), Black-and-white Warbler (Mniotilta varia), Ovenbird (Seiurus aurocapillus), and Scarlet Tanager (Piranga olivacea). In 50 fragments of mature deciduous forest (3.0-129.8 ha, median = 11.8 ha) within a 10 X 10 km square, we recorded the presence of any target species and an abundance index in two successive breeding seasons. Absences were validated using song playbacks. Except for the random dispersal model, each hypothetical mechanism was simulated using logistic regressions on empirical data. Frequencies of population turnovers (local extinctions + recolonizations) were 16-28% between the two breeding seasons, varying with species. Observed numbers of population turnovers were best approximated by the combination model (model 6). Expected numbers of population turnovers were consistently higher than observed numbers for models 1-4, suggesting that between-year dynamics were constrained by some process(es). Our simulations suggest that site fidelity might be an important constraint on the distributional dynamics of these species, even when effects of fragment area and habitat on site fidelity are factored out. This finding has important implications far the response of Neotropical migrant birds to the fragmentation of their habitat.