Source‐Sink Dynamics of a Declining Eastern Kingbird Population and the Value of Sink Habitats

I developed a demographic model for species inhabiting a mosaic of habitats of varying quality and among which dispersal was limited only by territorial behavior. I used the model to describe an ongoing decline of an Eastern Kingbird ( Tyrannus tyrannus) population breeding in the Charlotte Valley of central New York and to explore the possibility that the population utilized a network of source‐sink habitats. Population size declined in upland and floodplain habitats between 1989 and 1999, but remained stable along a riparian zone (the “creek”). Population size after 10 years was predicted accurately when I used the empirically observed, habitat‐specific rates of adult survival, productivity, and adult dispersal among habitats, and assumed a 5% rate of immigration into the upland. Simulation of the effect of density‐dependent variation in reproductive success or a small increase in adult survival in all habitats failed to improve the model's predictive ability. The model indicated that none of the subpopulations was self‐sustaining over 50 years and that population decline was driven by declining adult survival. Further analyses, however, showed that small increases in adult survival along the creek changed the creek habitat into a source and reversed population trends. Simulation of realistic rates of habitat loss as a result of forest maturation did not affect predicted population size at 50 years, but, assuming that the area of pasture and grazed lands continues to decline at current rates, the kingbird population is predicted to become extinct locally within 40 years. Riparian zones appear to be source habitats for kingbirds and should be targeted for protection. Sink habitats were also important because in every year most of the kingbird population bred on the floodplain or in the upland and many floodplain birds later dispersed to the creek. Preservation of open space is thus critical for maintenance of kingbird populations in eastern North America.