Canary in the Coal Mine

Black-throated green warbler (Setophaga virens).
Photographer: Flickr user Vicki DeLoach

Many colorful migratory songbirds return from their winter homes in Central or South America to the D.C. area each spring to breed. Unfortunately, due to increasing fragmentation of larger blocks of forest in the region as well as deforestation in the tropics, the numbers of these songbirds seen at Rock Creek Park declines year by year. In past decades, some of these migratory birds successfully bred deep in the interior of large, uninterrupted forest blocks that once ringed the District of Columbia. Rock Creek Park served as a "way-station" for birds traveling between these larger forests.

This wood thrush (Hylocichla mustelina) is unknowingly caring for a brown-headed cowbird chick (Molothrus ater) - a common nest parasite species.
Photographer: Flickr user Kelly Colgan Azar
Now, however, as forests in nearby counties are being fragmented by development, not only are these birds losing their habitat, but "edges" between forests and open areas are increasing, and with them, an enemy of migratory songbirds: the brown-headed cowbird. The brown-headed cowbird depends on forest edges, foraging for seeds in open areas but roosting in trees. Many migratory songbirds have open cup nests, in which brown-headed cowbirds, given the chance, will lay their own eggs to be hatched and raised by the host songbird, often at the expense of the songbird’s own young.

Happily, where public and private interests are preserving large blocks of intact forest in the western United States, the decline in numbers of songbirds is not so pronounced. Rock Creek Park – long known as good birding grounds – is serving as an important "canary in the coal mine," to alert people to the negative effects of breaking up the forests in our nation’s capital region and all along the Atlantic flyway for migratory songbirds.

Learn More:

North American migration flyways (map)

Neotropical Migratory Bird Conservation Act (Audubon Society)

Neotropical Migratory Bird Conservation Act (Video)