for National Geographic News
For the second year running, census takers counted more than 1,000 male Kirtland's warblers in the jack pine forests of northern Michigan. The area is the only place where this endangered speciesthe rarest warbler in continental North Americais known to breed.
The June 2002 census of the warbler's breeding population found 1,050 singing males; a slight drop from the 2001 all-time high of 1,085, but a huge improvement over the 1987 census, which turned up only 167 singing males.
Kirtland's warbler has a loud, distinctive song; the males sing in June at the height of the nesting season to alert other males to their territorial boundaries, and to attract females. Each singing male is presumed to represent a breeding pair. The females do not sing.
Humans, Cowbirds Pose Threat
A relatively large warbler with a gray back, yellow underparts with black-streaked sides, an incomplete white eye-ring, and a habit of pumping its tail, Kirtland's warbler has a breeding range largely confined to the north-central counties of Michigan's Lower Peninsula. Nesting first occurred on the Upper Peninsula in 1995.
The bird is a ground-nesting "habitat specialist" that breeds in young, dense, extensive stands of jack pines. Its breeding success is greater when such stands are interspersed with small, brushy openings.
Historically, wildfires that periodically swept through northern Michigan created good habitat for Kirtland's warbler, though the bird has probably always been uncommon and localized. As the fires consumed old jack pines, they propagated a new generation of trees by opening jack-pine cones, which need heat to release their seeds. The fires also burned off competing ground vegetation.
Human settlement of the warbler's habitat brought with it forest-fire control measures that largely halted the natural regeneration of jack pines, and wide-scale logging around the turn of the 20th century cleared the way for the brown-headed cowbird.
Before Europeans colonized North America, the cowbird was most abundant on the Great Plains, where it followed the vast herds of American bison, feeding on seeds and insects in their wake. With the clearing of forests and the spread of farming, the cowbird's range expanded north and east.
The cowbird builds no nest. It is a "brood parasite"the female finds another bird's nest, often discards one of the eggs, and lays her own. Cowbirds are larger than most of the species they parasitize, so when the eggs in a nest hatch, the cowbird nestling monopolizes the food brought by the parents, and the nestlings of the host species often starve.
To defend their nests, some host species remove the cowbird egg; others cover it with additional nesting material. Kirtland's warbler, a relative newcomer to cowbird parasitism, has evolved no such defense.
The annual census began in 1971, when only 201 singing males were found. Cowbirds were parasitizing about 70 percent of warbler nests.
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