The Kirtland’s warbler is a songbird that grows to an average length of 6 inches (15 centimeters). It has a blue–gray head and upper body with black streaks on its back. Its underside is pale yellow speckled with darker streaks. Males have a black spot on their cheeks while females have a gray one.
Both sexes have incomplete white rings around their eyes. The Kirtland’s warbler has a habit of bobbing its tail as it moves along the ground. The bird has been seen feeding on moths, caterpillars, ants, and numerous other insects.
The Kirtland’s warbler nests and breeds in a very specific habitat. It rejects areas that have dense underbrush or forests that are dominated by deciduous (shedding) trees. It chooses only areas at least 80 acres (32 hectares) in size with large stands of young jack pine on relatively level ground. These areas develop naturally only as a result of intense forest fires.
The warbler moves into the area 9 to 13 years after a fire has swept through and the new jack pines are about 5 feet (1.5 meters) tall. After 6 to 12 years, when the pines have reached 12 to 18 feet (3.6 to 5.5 meters) in height, the warbler abandons the area for a new one.
The warbler builds a nest of grass, bark, and fibers on the ground beneath a jack pine. A female Kirtland’s warbler lays 3 to 5 brown–speckled white eggs between mid–May and mid–July. She then incubates (sits on or broods) them for 10 to 14 days until they hatch.
Habitat and current distribution
The Kirtland’s warbler breeds only in a few counties in the area of north–central Michigan’s lower peninsula where extensive stands of young jack pine are found. Beginning in late September, the bird migrates to the Bahamas and the Turks and Caicos Islands located at the southeast end of the Bahamas. Its winter habitat consists mainly of pine woods. The warbler returns to its summer habitat in early May.
During the summer, a few Kirtland’s warblers may range to Wisconsin, Minnesota, and even Ontario or Quebec, Canada. However, they do not nest in these areas. Biologists (people who study living organisms) estimate that over 1,400 Kirtland’s warblers currently exist in the world.
History and conservation measures
Because of its finicky nesting habits, the Kirtland’s warbler probably never existed in great numbers. At the beginning of the twentieth century, extensive logging in Michigan reduced the bird’s already meager habitat. Then, officials who oversaw the way forests were managed in the state limited the number of forest fires.
As a result, forests grew and developed beyond the specific needs of the warbler. In the 1950s and 1960s, 15,000 acres (6,000 hectares) of suitable warbler nesting habitat existed. Today, only 30 percent, or 4,500 acres (1,800 hectares), exists.
The Kirtland’s warbler has also been threatened by the brown–headed cowbird. This bird normally inhabits farmland and meadowland. As forests have been cleared in Michigan, it has expanded its range into that of the Kirtland’s warbler.
The brown–headed cowbird likes to lay its eggs in the nests of other birds, including the warbler. This behavior is called parasitism (pronounced pair–a–si–TIZ–um). When the cowbird nestlings hatch, they are raised by the new parents.
The parents’ own nestlings often cannot compete with the cowbird nestlings for food, and they starve to death. From the 1930s to the 1970s, as many as 60 percent of warbler nests were believed to be parasitized by the cowbird.
The Kirtland’s warbler made a comeback in the 1990s. Conservationists and public land managers have worked to maintain and develop suitable nesting habitat for the bird in Michigan. They have also tried to control the brown–headed cowbird population within the range of the Kirtland’s warbler.