Tale of Two Warblers

A landmark study of American redstarts and black-throated blue warblers offers a key to recent declines in many songbird species

  • Les Line
  • Oct 01, 1994
The trip to a Jamaican mountain village with the improbable name of Rat Trap includes a white-knuckle drive on the wrong side of the road. Guard rails are a wish, cows stand tethered on the narrow verge of hairpin curves, and donkey carts outnumber autos. Much of the traffic this Sunday morning is on foot. Women and children carrying bright parasols descend on a substantial stone church with a breathtaking view of our destination: Copse Mountain and its remnant rain forest.

"Remnant" means just that, 100 acres of wet, old-age forest that ranchers left for timber when they cleared the lower slopes. But on this overpopulated Caribbean island, that small stand of trees is not only a lot of forest, but also critical habitat for neotropical migrant songbirds. This explains why ecologists Richard Holmes of Dartmouth College and Tom Sherry from Tulane University can be found here every November doing curious things with the working bird-man's field kit: nets, cassette player, colored bands, scales, calipers, Day-Glo ribbons and baby socks.

During the spring and summer, Holmes and Sherry direct a long-running and multifaceted study of the population dynamics of American redstarts and black-throated blue warblers that nest in New Hampshire. When leaves and snow fall in the White Mountains, the duo follows these painted birdlets-two of the best-known species in the wood warbler multitude-to their wintering places in Jamaica. While tourists sunbake on the island's resort beaches, the scientists slosh into a steamy mangrove swamp to census redstarts or scramble over a steep mountainside where black-throated blue warblers settle.

This research marks the first attempt ever to analyze the ecological factors that influence populations of neotropical migrants in both summer and winter habitats. The scientists' findings offer new insight into the reasons for a disturbing decline in forest songbird abundance during the past quarter century. Various warblers, vireos, thrushes, tanagers, orioles, flycatchers and whip-poor-wills have become increasingly rare or have disappeared from North American habitats in which they once were abundant.

The probable cause for these declines has been subject to vigorous debate in scientific and conservation circles. For many declining species, Holmes and Sherry believe, events on North American breeding grounds override the impact of deforestation and habitat degradation in wintering areas.

The American redstart is one of the most abundant warblers in North America. It nests across most of the continent in a variety of habitats, from deep woods to streamside thickets. The glossy-black male is a bundle of hyperactivity as he whirls after flying insects, flashing bright-orange patches on wings and tail.

In contrast, the black-throated blue warbler--a denizen of northeastern U.S. hardwood forests--is deliberate in its movements and easy to approach. The bird's name perfectly describes the male. As one handbook notes, "No observer need ever be in any doubt as to its identity."

In autumn, black-throated blue warblers migrate to the Greater Antilles, and American redstarts scatter to Mexico and the Caribbean islands and south to Venezuela. Unlike some warblers, both species are territorial not only during the breeding season in the United States and Canada but also in the tropical lands where they spend two-thirds of their lives. Both sexes of both species vigorously defend wintering sites--about half an acre in size per bird--and they return to the very same places year after year. This behavior makes the two species ideal subjects for a major study of warbler winter ecology.

Calling All Chip-Chips: We park in the pasture below the Copse Mountain forest, clamber over a rock wall that keeps out cattle and in half a minute enter a different world-a cool, mossy, dripping world with parakeets screaming overhead. This is not, however, your nature-calendar rain forest. The place was left a shambles when Hurricane Gilbert raged across Jamaica in 1988, pruning much of the forest's canopy. Trees that once were giants are now branchless trunks shrouded with lianas and bromeliads. The understory, suddenly exposed to the sun, has become an impenetrable tangle.

Holmes wields a machete to clear a trail over potholed limestone hazardous to ankles and shins. In this green mad house, mapping warbler territories on a study plot larger than three football fields truly is an uphill battle.

The slurred "zee-zee-zee" songs of the redstart and black-throated blue are familiar music in the northern spring woods. But wintering warblers are silent except for their sharp "chip-chip" call-notes, and Jamaicans lump the visitors together as "chip-chips" to distinguish them from familiar resident birds. The migrants, nonetheless, respond aggressively to broadcasts of their summer vocalizations, so scientists use recordings and stuffed decoys to lure them into nearly invisible mist nets.

We hear a telltale chip from a wall of philodendrons, Sherry plays his tape of a black-throated blue, and in seconds a natty male appears to confront the electronic intruder. This bird is an unbanded yearling, but it will soon wear its personal combination of two colored plastic bands plus a numbered aluminum band. The biologists have divided the study site into 24 grid plots marked with fluorescent-red ribbons, and a researcher diagramming the boundary of each warbler's territory can identify color-coded individuals by sight.

Despite the hurricane's devastation, the number of black-throated blue warblers wintering on Copse Mountain has not declined. On another Jamaican study plot, redstarts and black-throated blues stayed on their territories after slash-and-burn farmers cleared most of the trees. This suggests, Holmes and Sherry contend, that warblers can tolerate even catastrophic changes to winter habitat, an important clue linking songbird declines to problems up north.

A Bird in Hand: A narrow strip of dry woodland separates the sea from another fragment of priceless habitat, a mangrove swamp at Luana Point on Jamaica's southwest coast where rare American crocodiles lurk in deep holes. At dawn the trees are alive with a rich assortment of neotropical migrants, and the sight of familiar birds from the New England forests doing familiar things in this exotic setting is disorienting. A black-and-white warbler creeps upside down over a sea grape with foliage the shape and size of boxing gloves, while an ovenbird shuffles leaves beneath a thorny logwood tree and its immense termite nest.

Tom Sherry wades from the swamp with a male redstart, banded the previous winter, in hand. The warbler's heart beats rapidly as Sherry blows on its breast feathers to check for orange fat reserves, which should be clearly visible beneath the bird's skin. There are none. "Most birds use up all their fat getting here," Sherry says, slipping the bird into a tiny baby sock for weighing. It tips the scales at 6.9 grams, about a quarter of an ounce. When the warbler left on migration, it would have weighed 10 or 11 grams.

The Tulane ecologist takes various measurements, then plucks one orange-splotched tail feather, which the bird will replace in a few weeks. Like a tree's annual rings, daily growth bars on the feather tell a story: Wide spacing of this redstart's bars means the bird is finding plenty to eat in the Luana swamp. Indeed, overwinter survival of color-banded warblers on the Jamaica study plots is high: 80 percent for redstarts, 66 percent for black-throated blues. These numbers challenge the conventional wisdom that winter is an especially difficult time for neotropical migrants, yielding high mortality.

"Survival is a warbler's only goal in winter," Holmes says, "and the strong attachment of individual birds to specific sites tells us that the quality as well as quantity of habitat is important. Tropical deforestation could one day become a major factor in limiting numbers of long-distance migrants. But our work demonstrates that the size of the breeding population is directly related to productivity the previous summer-the number of young fledged per pair." Habitat fragmentation on the breeding grounds increases both brood parasitism by cowbirds and nest predation, Holmes says. "This could be the major cause of many of the population declines among neotropical migrants."

A Rogues' Gallery: As the warbler flies, the trip from Jamaica to Hubbard Brook Experimental Forest in New Hampshire covers about 1,800 miles. The June woods are sodden after a night of marching thunderstorms, and fog hugs the ground between boles of 80-year-old yellow birch, beech and sugar maple. "This is about as close as you can get to the forest primeval in New England," says Richard Holmes, leading the way to a birch sapling where a female American redstart sits tight on her nest several feet above our heads.

In the dim light the tiny cup, camouflaged with shredded birch bark, is hard to spot. The female, with no help from her mate, made hundreds of trips over a week's time with grass, rootlets, plant down, lichens, spider silk and other building materials before the nest was ready to receive its consignment of four eggs. All that work may go for naught."There's a 50 percent probability that a predator will take the eggs, and an 8 percent probability that the female will be killed during the 20 days from the start of incubation to fledging," says Tom Sherry.

Hubbard Brook is a 7,800-acre bowl of unbroken hardwood forest set aside by the U.S. Forest Service in the 1950s as a natural laboratory. Habitat fragmentation is not a factor here, and predator numbers are influenced by natural events, such as the size of the fall nut crop, rather than by disruptive human activities, such as clearcuts or second-home developments. Yet the loss of redstart eggs and nestlings to predation has exceeded 70 percent in some years.

Redstart nests invariably are wedged in a fork close to a tree's main stem,where they are vulnerable to tree-climbing scavengers-red squirrels, flying squirrels, fishers and chipmunks. In one study, 8 of 11 nests protected by squirrel baffles fledged young, while only 10 of 42 nests without baffles were successful. Black-throated blue warblers, nesting close to the ground under a concealing umbrella of hobblebush or striped maple leaves, fare somewhat better, but still 40 percent of their nests were taken during one summer of heavy predation.

The list of known and suspected nest predators at Hubbard Brook is long, including blue jays, deer mice, raccoons, ravens, sharp-shinned hawks, barred owls, saw-whet owls, weasels and red foxes. Researchers filled fake birds' nests with quail eggs, rigged trip wires to flash cameras, and by summer's end had assembled a rogues' gallery of egg-robbers. One mug shot showed a black bear with its snout in the nest.

"If we're getting this level of predation in a pristine ecosystem," says Holmes, "imagine what can happen to nesting success in a fragmented landscape where songbirds have to cope not only with increased pressure from predators but cowbirds as well." The problem is especially acute in rural developments with roaming house cats and with bird-feeders that boost numbers of blue jays, squirrels and cowbirds.

Other factors besides predation affect nesting success: long stretches of wet weather, for instance, and food shortages. Students on the New Hampshire team watch videotapes from remote cameras to chart the kind and quantity of insects that the parent birds bring to their nestlings.

There are no simple answers, the New Hampshire researchers say, for the neotropical migrant dilemma. "So many factors can affect bird populations that it is hard to pin down a cause," says Sherry. Even after years of study, the American redstart and black-throated blue warbler still have secrets to tell.

Les Line, former editor of Audubon, followed redstarts and black-throated blue warblers from New Hampshire to Jamaica for this article.

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