Habitat fragmentation, especially in the rapidly urbanizing eastern United States, has contributed to one of the most studied and publicized conservation problems in North America: the decline of forest-dwelling, migratory songbirds.
Wood warblers are typically the dominant avian insectivores in eastern U.S. forests, but they are often negatively affected by fragmentation and tend to be absent from small or isolated forest patches.
Surprisingly, we know little about the distribution and abundance of warblers in the ever-expanding matrix of urban and suburban habitats that is replacing natural forest cover.
The Great Falls and Turkey Run parks along the George Washington Memorial Parkway afford a unique opportunity to study the effects of urbanization and habitat fragmentation on warbler populations. These 2 parks provide intact, mature forest sites embedded in one of the most developed landscapes in the Mid-Atlantic.
In May of 2006, the Smithsonian Migratory Bird Center initiated a 2-year study of breeding wood warbler species at Great Falls and Turkey Run. Our objectives were:
if the rare and declining cerulean warbler (Dendroica cerulea) bred in the parks
the distribution and abundance of other warblers
the relationship between warbler abundance and landscape features
The study was designed to establish baseline data for natural resource managers of the National Park Service. We combined field surveys with a GIS analysis to map warbler density within the park boundaries. Here we report the results of our warbler survey for the National Park Service (NPS).
Great Falls Park
We found 5 species of breeding wood warblers in the 325-hectare Great Falls Park:
- Northern Parula
- Worm-eating Warbler
Prothonotary warblers were limited to a single pair detected at an isolated flooded swamp community. We detected no cerulean warblers.
The abundance of some wood warbler species was related to landscape features at Great Falls:
- Worm-eating warblers and ovenbirds were significantly more abundant on southeast-facing slopes
- Northern parula was most abundant at sites near the Potomac River
- Louisiana waterthrush abundance was not significantly associated with distance-to-water, but we only considered major watercourses in our analysis
- Contrary to our expectations, no species' abundances were significantly associated with forest canopy cover, although worm-eating warblers were marginally more abundant at interior forest points
Turkey Run Park
We detected 2 species of wood warbler, Louisiana waterthrush and northern parula, breeding within Turkey Run Park. We detected no cerulean warblers in Turkey Run.
Abundance of both Louisiana waterthrush and northern parula was associated with landscape features at Turkey Run.
- Louisiana waterthrushes were significantly more abundant at points close to the Potomac River and to Turkey Run itself
- Northern parula was also more abundant along the Potomac River, but this relationship was only marginally significant
- Similar to Great Falls, neither species was significantly associated with forest canopy cover.
In summary, our data indicate that the Great Falls and Turkey Run parks support sizeable breeding populations of at least 4 warbler species, 2 of which, prothonotary and worm-eating warblers, are PIF "Species of Continental Importance."
These parks encompass some of the largest remaining blocks of intact deciduous forest in the National Capital region. Our results suggest that current forest canopy cover within Great Falls and Turkey Run does not negatively affect wood warbler abundance.
Therefore, the unfragmented forests along the George Washington Memorial Parkway likely represent some of the best remaining breeding habitat for wood warblers and other Neotropical migratory songbirds in the Washington, D.C. metropolitan area.