Last week I flew from Pittsburgh to North Carolina via New York City. I took the scenic route and got to see lots of Pennsylvania and southern Appalachia from the air. It's a vantage point that reveals much about man's impact upon the land, especially large tracts of forest.
From 30,000 feet towns, farms, schools, malls, parking lots, and highways dot and bisect the forested landscape. Even in remote areas, power line rights of way and small country roads are visible. It's part of the price we pay to live the way we do.
For more than 30 years, ecologists have warned that fragmenting large chunks of forest was ecologically perilous. Many species of plants and animals require large tracts of unbroken forest.
Scarlet tanagers, wood thrushes, pileated woodpeckers, cerulean warblers, and worm-eating warblers are just a few of the deep forest birds that decline or even disappear when tracts of forest are broken into many small pieces. Biologists call this "forest fragmentation."
Forests fragmented by farmland and community development are lost until these areas are abandoned; forest-dwelling birds are unlikely to ever be seen in those areas again. Fragments created by timber harvest are relatively temporary. It may take 100 years, but if left untouched, these areas will revert to a forested condition.
Though forest-interior birds require large tracts of unbroken woodlands, fragmented forests also create conditions that lead to lower nesting success. Predators such as raccoons, coyotes, cats, blue jays and crows frequently raid bird nests, but they are not abundant deep inside large forests. Fragmentation caused by roads and other developments creates "edge habitat," discrete lines along the borders of forest and fields. The aforementioned predators thrive in edge habitat.
Brown-headed cowbirds also prefer edge habitat. Though not literally a predator, cowbirds are brood parasites. During the nesting season, female cowbirds watch from treetops to find nests of other species. Then each morning female cowbirds lay an egg in the nest of another species. Cowbird hens can lay up to 40 eggs during a single nesting season. Warblers, vireos and indigo buntings are among cowbirds' favorite hosts.
Cowbirds are members of the blackbird family, so their eggs and hatchlings are larger than the eggs and hatchlings of smaller host species. Furthermore, cowbird eggs usually hatch a day or two before host eggs.
Young cowbirds get a tremendous head start on their nest mates. They monopolize the food parents bring to the nest. The bottom line is that cowbird chicks in a host nest usually fledge successfully while the host chicks starve. Cowbirds may not be predators, but their presence reduces the population and nesting success of host species.