Where Did The Songbirds Go

Where Did The Songbirds Go

For about the last sixty years there has been a decline in forest songbird populations over much of the eastern United States. The decline has not been uniform for all species; the Acadian Flycatcher and others that migrate long distances to tropical America have suffered more than residents or those like robins and towhees that can overwinter in the southern United States. Nor has the decline been equal in all types of forest; the loss of species from woodlots and small forest tracts exceeds the loss from large stretches of forest such as those of the Great Smoky Mountains National Park.

One suspected cause is quite naturally, the rapid destruction of tropical forests where many migrants overwinter. Perhaps deforestation in Mexico, Costa Rica, and Jamaica, for instance, is responsible for the decline of some species, such as the Worm-eating Warbler. But in the last century about half of the forest breeding habitat of that species in eastern North America was destroyed, while there was relatively much less loss of tropical forests in that period. The result may well have been a surplus of wintering habitat. More recent deforestation has wiped out on the order of half of the tropical forests, and perhaps has just about restored the balance between available breeding and wintering habitat.

Other possible explanations of declines in eastern migratory songbirds have to do with changes within North America. They include increased cowbird parasitism, loss and fragmentation of habitat, and increased nest predation in habitat patches. When Christopher Columbus landed, cowbirds are thought to have been largely confined to open country west of the Mississippi, because the continuous forests of the eastern United States did not provide suitable habitat for their ground feeding or social displays. As the forests were cleared, cowbirds extended their range, occupying most of the East but remaining rare until this last century. Then increased winter food supply, especially the rising abundance of waste grain in southern rice fields, created a cowbird population explosion. The forest-dwelling tropical migrants especially vireos, warblers, tanagers, thrushes, and flycatchers have proven very vulnerable to cowbird parasitism. And that vulnerability is highest for those birds nesting near the edge of wooded habitat and thus closest to the open country preferred by the cowbirds.

This provides one explanation for the much sharper decline of songbirds in forest fragments than in large areas of continuous forest, nest sites in a forest fragment are on average closer to open land than those in continuous forest. In addition, there is evidence that fragmentation with both reduction of total habitat area and increased isolation of habitat remnants, has strong negative effects on forest-dwelling long-range migrants that need forest habitat, while often favoring short-range migrants and residents that do not have such strict habitat requirements.

So it looks as if many factors may be contributing to the decline of songbirds. Thus, sadly, the prognosis is grim. Ornithologists think that cowbirds are likely to continue to increase, and the now-thriving nest predators are unlikely to decline. The loss of songbirds might be halted if conservation depended entirely on temperate zone events, because habitat fragmentation in the United States and Canada could be stopped or its effects controlled. But the inexorable destruction of tropical rain forests shows no sign of abating. If it continues at current rates for another few decades, it seems likely that many of our song bird species will become much rarer or even disappear.

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