Earth Conservation: The Recovery of Kirtland's Warbler

At the end of the late nineteenth century, American loggers began moving westward from heavily-logged virgin forests in the Appalachian Mountains, and instead started to harvest timber from the northern reaches of the midwest, in Michigan, Minnesota, and Wisconsin. In these same forests exists a small, six-inch-long bird species known as Kirtland's Warbler (Dendroica kirtlandii), one of the rarest members of the warbler family and a victim of near-extinction during the past century and a half.

Kirtland's Warbler began to feel the pinch of human habitat modification with the deforestation of their home forests in the northern midwest. As large swaths of jack pine began to disappear, the trees to take their place were mostly of the same species but not of preferable age. Kirtland's Warbler specializes in nesting in stands of eight to twenty-one year old trees, refusing to take residence in either elderly or immature pines. Forest fire suppression further led to uniform stands of incorrect nesting trees, leaving Kirtland's Warbler almost entirely homeless.

The removal of dense pine forests also provided expanded habitat for a species well-known to pester other birds, the Cowbird (genus Molothrus). The cowbird thrives in less-dense forests than those which support Dendroica kirtlandii. The timber-cutting which led to these less-dense forests came at a time when much of the land being cleared was for agricultural purposes, providing grain scraps which allowed the Cowbird to migrate further north and disturb warbler reproduction. When ready to reproduce, a Cowbird female will visit other bird's nests, push out the resident's eggs, and lay her own eggs for the foster parent to raise.

These two mitigating factors became a deadly duo within the first quarter of the twentieth century, and nearly wiped out Kirtland's Warbler during central five decades of that one hundred year time span. Within that time, the entire species was comprised of a only few hundred individuals, dropping to less than two hundred total in the 1970's. This fact only became known after the species was placed on the Endangered Species List in 1967.

As publicity surrounding Kirtland's Warbler began to increase, a growing number of methods were used to induce recovery of the warbler population. These methods included large-scale cowbird trapping, the replanting of previously-forested land tracts, and intense observation of those species members left; by the end of the eighties, nearly eight hundred of the birds were alive, showing that for all intents and purposes, the conservation methods put in place were working.

Today, Kirtland's Warbler lives on, and, though still considered rare and endangered, the species continues to recover and increases in population every year. It is hoped that by the end of this century, it will be as prevalent then as it was a century removed from now.