Issue 6

Where Have All the Birds Gone?

by Zoë Hoyle

You might say Paul Hamel is drawn to working with difficult birds. First, it was the cerulean warbler, now the ivory-billed woodpecker—one a tiny piece of blue sky, the other so large and spectacular that it’s been called the “lord god” bird. Or maybe he’s trying to help understand what has made both large and small birds disappear from the forests of the Lower Mississippi Alluvial Valley (LMAV)—and what sort of forest management practices could help bring them back.

Hamel, research wildlife biologist with the SRS Center for Bottomland Hardwoods Research has a long history of studying the impact of landscape change and forest management on neotropical migratory birds, which typically breed in North America and migrate south in the winter. Changes to both the breeding areas and winter habitats of these birds have impacted their numbers over the last half century. Hamel’s research on one neotropical migrant, the cerulean warbler, has taken him to the lower slopes of the Andes in South America, as well as into the forests of the LMAV.

The cerulean warbler gets its name from the male’s color—bright sky-blue above and white below, with a black collar and narrow black streaks along his flanks. The female is dull turquoise above with a pale blue crown, and yellowish white below.

The cerulean warbler, once very common in the forests of the Eastern United States, has steadily vanished from sight, its numbers declining by 70 percent since the mid-1960s. When present, the small (4-inch) bird mostly stays in the upper canopy of mature deciduous forests. The bird spends most of the year in Western South America, but its breeding range used to cover much of the Eastern United States and Southern Canada.

“The Lower Mississippi Valley, which is in the traditional breeding range of the cerulean warbler, is an area where the bird is much less abundant than before,” says Hamel. “We really don’t know why specifically. There is a lot less forest, and it is arranged differently, which presents many more problems for the bird.”

A Steady, Puzzling Decline

In the heyday of the cerulean warbler, the forests of the LMAV were connected and compact—meaning there was a lot of interior forest in relation to the perimeter. Today, the forests that remain tend to be in low areas, often along rivers. The fragments may seem large—sometimes 300,000 acres—but if you look closely at a map you can see that they run in long strips that are as little as 5 miles wide. Again, there’s no real way to know exactly what aspect of forest fragmentation has led to the steady (3 percent per year) decline in cerulean warbler populations and a shift in the bird’s range towards the Northeast, away from the Lower Mississippi Valley. “We can’t really tell you what kind of forest structure is ideal or why the warblers choose to nest where they do,” says Hamel. “The way we approach it is to look at where we find cerulean warblers and then make inferences about why that habitat attracts them.”

In 1992, Hamel and Bob Cooper (then at the University of Memphis and now at the University of Georgia), led by Winston Smith (formerly with SRS and now with the USDA Forest Service Pacific Northwest Station), started a long-term study of neotropical migratory birds on three sites in the LMAV known to harbor cerulean warblers. The sites, all located in the flood plain of the Mississippi River, are under three different ownerships: one on private timber company land in Arkansas; the second on State park land in Shelby County, TN; and the third on the Chickasaw National Wildlife Refuge in Lauderdale County, TN. “We’ve gone back every year to monitor the territories of not only cerulean warblers, but also American redstarts, northern parulas, yellow-throated warblers, and other neotropical migrants,” says Hamel. “This gives us a snapshot of what is happening with birds that depend on forest habitat to breed.”

The timber company site located in Desha County, AR, is the only site where experiments have been done on managing forests specifically for the cerulean warbler. “We have learned from observations that male ceruleans prefer tall trees with spreading crowns as song trees, while females tend to favor positions lower in the canopy,” says Hamel. Researchers worked with the owners of the site, the Anderson- Tully Company, to reserve a 135-acre plot from any forest management for 10 years. At the end of that time, the plot, which lies within a surrounding area of 320 acres, was divided in half. The timber company applied their normal partial-cutting prescription to one half. This involved cutting in the overstory to reduce mortality, improve species composition and spacing, and increase growth of the residual stand. It also involved removing many stems from the midstory to encourage the regeneration of desirable species. On the other half, they left a larger proportion of the midstory trees, with the intent of providing habitat for cerulean warblers. “So far, we have cerulean warblers using the side treated for them, and not using the other side,” says Hamel. “Anderson- Tully Company, now a part of the Forest Land Group TIMO, continues to be a highly valued cooperator in the cerulean warbler work.”

Other research findings confirm habitat loss as the main reason for the decline of cerulean warbler populations in North America. Studies also confirm a growth in populations where forests are regenerating. “We have some evidence that we can regenerate and manage forests to create or improve habitat for the cerulean warbler,” says Hamel. “But we need to act quickly and throughout the bird’s range to address its decline.”

Why Did the “Lord God” Bird Disappear?

Since the April 2005 report of sightings of the ivory-billed woodpecker, believed extinct for 50 years, teams of volunteers have been combing the Big Woods of the Cache River and lower White River in Arkansas, hoping for another glimpse of the bird known as “Elvis” or the “lord god” bird, this last from the exclamations of those seeing the large, dramatic bird for the first time.

Unlike most of the LMAV, the Big Woods of Arkansas still contain areas of oldgrowth forest, including cypress trees estimated at 800 and 1,000 years old. The area surrounding the Big Woods is actually a patchwork of bottomland forest and agricultural fields—many in rice. The reported sighting of the ivorybilled woodpecker has spurred efforts to increase the size of protected lands and to afforest portions of the area.

In 2005, an Ivory-Billed Woodpecker Recovery Team was formed to prepare a comprehensive recovery plan for the species. Pete Roussopoulos, SRS Director, was named to the executive committee, and Hamel was named to the biology working group. The bird was known to have inhabited large areas of bottomland forest with patches of dead and dying trees that harbored the insects it prefers. The exact reason for its disappearance from a range that once spanned the South is still unknown.

“Tasked with writing the biology part of the recovery plan, we started reviewing the records of the bird in this region, reading James Tanner’s 1942 monograph of studies done on the Singer Tract in Louisiana,” says Hamel. (Renowned for his studies of the ivorybilled woodpecker, Tanner was one of the last to see the bird before the recent sightings.) “It was Tanner’s opinion that the viability of the ivory-billed woodpecker was limited by the supply of the insects it feeds on. We decided to do a food provision study to explore this idea.”

The cerulean warbler depends on the upper canopy of mature deciduous forests to breed. (photo © Mike S. Nichols)

Tanner based his idea on the insects he found in nests, by examining trees ivorybills had foraged on, and by analyzing the stomach contents of ivory-billed woodpeckers in collections. Along with a number of specific insect foods still available in the area, Tanner identified three primary forage trees: sweetgum, sugarberry (a type of hackberry), and Nuttall oak. All are still common in the Mississippi Alluvial Valley; Nuttall oak is a very popular species for afforestation efforts.

Building a Mystery

Tanner described ivory-billed woodpeckers as scaling the bark off trees to locate the wood-boring beetle larvae they preferred. When Hamel brought in SRS entomologist Nathan Schiff to work on the study, a mystery developed. “When I showed Nathan the list of insects Tanner had identified from stomach and nest leavings, he pointed out that all these insects burrow deep into the tree. There’s no way the bird would find them by stripping off the bark.”

This apparent contradiction led Hamel and Schiff to start a study to quantify the volume of insect food available to the ivory-billed woodpecker in relation to tree species and conditions. This spring they chose four sites, two in the Big Woods of Arkansas and two across the river in Mississippi. Choosing individual trees from Tanner’s three preferred species, Hamel and Schiff subjected them to four levels of insult, from no wounding up to girdling to kill. At the end of this growing season, when insects have had plenty of time to lay their eggs, the researchers will go back and harvest half of the research trees, then place them in isolation chambers to measure insects as they emerge.

“We’ll be looking at how many insects emerge in relation to the level of wounding,” says Hamel. “I don’t think anyone’s done this type of ecological assessment of the insects attracted to dead and dying trees. We hope it will give us some answers about food availability for ivory-billed woodpeckers, but it will also yield information about insect predation that forest managers can use to increase the yield of their stands.”

More trees on the land may lead to more warblers and neotropical migrants nesting high in the canopy—and maybe to more sightings of the “lord god” bird.

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For more information:

Paul Hamel at 662–686–3167 or phamel@fs.fed.us.