Rare Warbler Eluding Extinction in U.S.
for National Geographic News
|July 26, 2002|
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.
"It was a prescription for extinction," said Michael E. DeCapita, a wildlife biologist in the East Lansing, Michigan field office of the U.S. Fish and Wildlife Service. "We wouldn't be able to maintain the warbler's population without cowbird control."
Since 1972, decoy traps baited with seeds and live cowbirds have been set out across the warbler's breeding grounds. Cowbirds enter the large wire cages through funnel-like openings and usually can't get out. Trappers kill the cowbirds by asphyxiating them with automobile exhaust or by breaking their necks. During the 2001 nesting season, 67 cowbird traps caught 4,399 cowbirds. More than 120,000 cowbirds have been trapped and killed since 1972.
A few years after the control program began, cowbirds were parasitizing only 6 percent of warbler nests, and the number of warbler fledglings had tripled.
To sustain the population, the Michigan Department of Natural Resources and the U.S. Forest Service had set aside land as Kirtland's warbler habitat. Controlled burning and pine plantings kept these areas attractive to the warblers.
Still, the number of males heard in the annual census hovered around 200 well into the 1980s. Wildlife biologists estimate that over 60 percent of the Kirtland's warblers that leave Michigan for their wintering grounds in the Bahamas never return. With cowbird control measures in place, the breeding population had been expected to rise despite this high rate of natural mortality.
A tragic accident in 1980 provided a clue. As part of its habitat management strategy, the U.S. Forest Service started a prescribed burn in the area of Mack Lake. Winds whipped the fire out of control, and what had been intended as a 200-acre (81 hectares) burn turned into a wildfire that consumed nearly 24,000 acres (9,700 hectares), destroyed 40 homes, and killed one Forest Service worker.
"In human terms, the Mack Lake burn was a terrible shame," said DeCapita. "But it was the kind of event Kirtland's warbler depended on before we started putting fires out. It eventually created prime habitat, and the warblers flooded in."
By 1989, jack pines in the Mack Lake burn had grown enough to provide good warbler habitat, and by 1993, the census found 485 singing malesroughly double the numbers of the previous two decades.
Robert A. Askins, professor of zoology at Connecticut College, summed up the Mack Lake burn experience in his book titled Restoring North America's Birds.
"Kirtland's warblers may have declined after 1961 because of the absence of large burns," he wrote. "After cowbirds were removed, the steep decline in warbler numbers ended, but the warbler population did not increase until a large area of new habitat became available as a result of a massive fire."
Creating Breeding Habitat
Quantity is as important as quality, researchers concluded.
"Government agencies got better at habitat management after the Mack Lake burn jumpstarted the warbler population, but the key is the amount of habitat managed each year," said DeCapita. "We need 30,000 to 40,000 acres (12,000 to 16,000 hectares) available at all times to support 1,000 pairs of warblers, and we didn't have that in the early '70s."
Today, 150,000 acres (60,700 hectares) of public land are reserved for Kirtland's warbler management. At least 30,000 acres (12,140 hectares) are always kept in appropriate condition for nesting. To maintain that 30,000-acre base, government foresters and wildlife biologists use a combination of clear-cutting, burning, seeding, and replanting.
At least 1,500 acres (607 hectares) are planted or seeded with jack pines each spring, and up to 1,000 more acres (404 hectares) are planted in the fall. It takes six to ten years of growth for the habitat to be suitable for the warblers.
As required by the Endangered Species Act, a recovery plan was developed for Kirtland's warbler in 1976 and revised in 1985. The plan set a population goal of 1,000 breeding pairs. Although that number has been reached, the warbler won't be removed from the Endangered Species List because the act also requires the population to be self-sustaining, and the warbler's survival hinges on human intervention.
How long will it be before the warbler can make its own way?
"Due to the essentially permanent human-caused changes to the landscape in the warbler's range," said DeCapita, "extensive managementin the form of breeding-habitat creation and cowbird controlprobably will be needed forever."
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