Spatial Dynamics of Source-Sink Habitats: Effects on Rare Grassland Birds

Abstract

Fragmentation and edge effects adversely affect passerines in North America, primarily by reducing territory density, reproductive success, and survival. As natural landscapes become increasingly altered and fragmented by human development, understanding the demographic parameters of remaining subpopulations is important. We wanted to determine whether remaining dry prairie fragments in central Florida, USA, were acting as population sources or sinks for 2 rare and declining sparrows: the federally endangered Florida grasshopper sparrow (Ammodramus savannarum floridanus) and the regionally threatened Bachuran's sparrow (Aimophila aestivalis). We obtained sparrow survival estimates from 2 sites in central Florida and combined these with productivity estimates from 3 sites (1996-1998) to determine whether each site was acting as a sourer or sink. We also explored whether subunits within a site consistently functioned as sources. For Florida grasshopper sparrows, we found that core areas >400 m from edge were consistently sources. We think that the only was Florida grasshopper sparrows can persist at these sites is if the core source areas produce enough surplus young to compensate for the sink habitat along the wide borders of these prairie fragments. In contrast to grasshopper sparrows, we found no consistent source areas for Bachman's sparrows. Dry prairie seems to lie sink habitat for Bachman's sparrows, and this species is likely to persist in this habitat only through continued recruitment from adjacent long-leaf pine (Pinus palustris) stands. We think that large prairie fragments, possibly >4,000 ha, are necessary for maintaining source habitat for Florida grasshopper sparrows and possibly other grassland bird species.