For the Birds Radio Program: Forest Fragmentation
(Recording of a Whip-poor-will)
Come May, we may not be facing a “Silent Spring,” but we are losing a tragic number of birds throughout the United States. Hardly anyone sees Scarlet Tanagers anymore, and some neighborhoods in Duluth have completely lost their Warbling Vireos, Wood Thrushes, and other songbirds. Most Northland children have never heard a Whip-poor-will.
The cause of these delines is complex, but current research indicates that one major factor is probably forest fragmentation.
Forest fragmentation is the process of breaking up a large forest with roads, power lines, and development until in place of an untouched and unbroken forest we have a mosaic of smaller woodlots. One might think that x number of acres of woodland would support the same number of species whether it is unbroken or not, and that because of the edge effect there might even be more species in a forest fractured by roads, but the opposite is true. The edge does open things up for a few opportunists and game species, but its greatest benefits seem to be to predators and to cowbirds, which lay their eggs in the nests of other birds. These nest parasites were part of the American scene long before European man came, but they were largely restricted to the Great Plains, following American Bison for the insects and seeds they stirred up. Cowbirds apparently pretty much limit their nesting efforts in forests to within about 300 feet of the edge, and so historically birds deep within forests did not encounter them. But the extermination of the bison coincided with the introduction of cattle farming and the opening up of the eastern forest, and the cowbird quickly expanded its range. A few species were immediately and dangerously affected–most notably Kirtland’s Warbler, which nests only in jack pine stands in the northern part of Michigan’s lower peninsula. But the effects on most birds were more insidious and subtle. Margaret Morse Nice wrote a paper about the limited effect cowbirds have on the overall nesting success rate of Song Sparrows, but these adaptable birds, which winter in the southern United States, can nest two or even three times a season, unlike migrants from the tropics, which arrive in late May and head south again too early to nest more than once. If a cowbird lays its egg in a warbler’s nest, that pair of warblers will most likely not reproduce themselves that entire season.
So slowly, almost imperceptibly, migrants like warblers, vireos, tanagers, and some flycatchers have disappeared. Whip-poor-wills and Broad-winged Hawks have also vanished from many small woods. The trends are clear when you examine data from breeding bird surveys, which were begun by Chandler Robbins, the author of the Golden field guide, Birds of North America, and research biologist for the U.S. Fish and Wildlife Service at Patuxent, Maryland. Dr. Robbins was in Duluth last week to talk about forest fragmentation, and how land use patterns in the United Sates exacerbate the loss of the tropical rainforest for these vulnerable migrants, whose world is shrinking at both ends.
Landowners and people involved with land use planning for townships and cities need to be informed about the serious problems involved with fragmentation. Some may ask why it is so important to save a handful of songbirds when we still have robins and Blue Jays and plenty of other birds. But no species is insignificant. Birds are more easily detected than other wildife and plants in an ecosystem, and so they serve as indicators of the health of the entire system. Just as robins acted as mine canaries to warn us of the dangers of DDT, so too the loss of warblers and tanagers in a forest warns us of the decline of countless other intertwined life forms. And it is essential to realize that once a species is gone, it is gone forever. If we squander our natural resources in the name of profits for a handful of aquisitive developers, how will our children and grandchildren judge us? And how will we judge ourselves? (Recording of a Whip-poor-will)
This is Laura Erickson, and this program has been “For the Birds.”