Wednesday, October 01, 2008

Kirtland's Warbler Population Continues to Grow

A census of breeding Kirtland's warblers turned up the highest count since 1951, with 1,791 singing males. It is not clear from the article whether that number includes the pairs in Wisconsin and Ontario, or just counts birds in Michigan. Despite the high numbers, the warbler species remains vulnerable because of its specialized habitat needs.

The singing male population has exceeded 1,000 for seven consecutive years. Scientists previously established five years as a key threshold for declaring the Kirtland's warbler recovered.

Even so, it's widely agreed the warbler cannot be removed from the federal endangered list for the foreseeable future because its peculiar habitat requirements make it unable to survive without human assistance.

It nests and breeds almost entirely in young jack pine stands of the type found in Michigan's northeastern Lower Peninsula, while spending winters in the Bahamas.

State and federal agencies use clearcutting and burning to remove older trees in the Michigan habitat zone. They plant or seed about 3,000 acres of jack pines each year on state and federal lands.
Habitat specialists like Kirtland's warbler are increasingly vulnerable as our landscape becomes more homogenized and humans encroach on formerly wild areas. Kirtland's warblers depend on jack pine trees between 16 and 20 years old for nesting. The trouble is that jack pine cones will only release seeds after fires, which also kill the older trees. Under wild conditions, fires will occur often enough (thanks to lightning strikes and the like) to maintain sufficient habitat for breeding warblers. However, when fire suppression is common, to protect either houses or potential lumber, available habitat will shrink rapidly, as it did in the 20th century. Thus we need constant intervention, in order to mitigate the side effects of other forms of intervention.