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 dynam ics in the distribution of Neotropical migratory birds, we compared ob served numbers of local extinctions and recolonizations to those expec ted under six hypothetical mechanisms of settlement by returning sprin g migrants. These mechanisms were (1) random dispersal among forest fr agments; (2) passive sampling of spring migrants according to fragment area; (3) dispersal among fragments between years according to the di stance to the nearest fragment occupied the Ist yr; (4) resettlement o f spring migrants according to the vertical structure of the vegetatio n in the fragments; (5) site fidelity of experienced breeders; (6) a c ombination of mechanisms 2, 3, 4, and 5. We selected four target speci es: Wood Thrush (Hylocichla mustelina), Black-and-white Warbler (Mniot ilta varia), Ovenbird (Seiurus aurocapillus), and Scarlet Tanager (Pir anga 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 pres ence of any target species and an abundance index in two successive br eeding seasons. Absences were validated using song playbacks. Except f or the random dispersal model, each hypothetical mechanism was simulat ed using logistic regressions on empirical data. Frequencies of popula tion turnovers (local extinctions + recolonizations) were 16-28% betwe en 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-y ear dynamics were constrained by some process(es). Our simulations sug gest that site fidelity might be an important constraint on the distri butional dynamics of these species, even when effects of fragment area and habitat on site fidelity are factored out. This finding has impor tant implications far the response of Neotropical migrant birds to the fragmentation of their habitat.