Research Communications

Biologists track the impact of forest habitat change on warbler bird populations 

By Jessica Salerno

Photo credit: Courtesy of Kelly Williams

Researchers measure the wing of an adult hooded warbler under study. Photo credit: Courtesy of Kelly Williams.

Bird watching began as a hobby for Kelly Williams, but it's now a full-fledged career. She's spent the last few years studying the hooded warbler and whether changes in its habitat have impacted the bird's reproduction rates.

In forests such as Tar Hollow State Forest in Chillicothe, the site of her study, various practices are used to reduce the incidence of wildfires. Forest management professionals may selectively log trees or set controlled fires. But these changes also may create stress for the birds, which come to Ohio each spring.

"They usually start arriving here around the last week of April to breed, and by the end of July or August they head back to Central and South America for the winter, so they're here for a very short time," says Williams, who earned a doctoral degree in biological sciences at Ohio University and now is a visiting professor.

During the warblers' Ohio season, Williams walks the forest terrain almost every day to collect data on the animals. A grant from the Ohio University Student Enhancement Award supports the travel of undergraduate research assistants who must drive an hour from Athens to reach the site. The funding has been a huge help, she says, as the site is too large for her to cover on her own.

Williams studies three stands that each span 75-80 acres. She's been researching these areas since 2009, which has allowed her to see some of the same birds return to the site.

To monitor the birds, Williams and her team capture them in a mist net (nylon mesh suspended between two poles), and then attach small, government-issued aluminum bands to their legs. She weighs the animals, draws blood for hormone testing, and collects fecal samples.

As the bird is common in Ohio and nests close to the ground, it's easy to observe. The relatively small birds (adults weigh about 11 grams) are olive green, bright yellow, and black in color.

Williams' study will explore whether the birds exhibit increased levels of corticosterone, a hormone emitted under stress and energy demands. It can impact reproduction, growth, and immune function.

So far, she's found that the hematocrit (the percentage of the volume of whole blood that is made up of red blood cells) of nestlings in the selective harvest stand was lower than those in the unaltered forest. This may impact survival, she notes.

The birds already face a high mortality rate, she adds, as 50 percent of the nestlings die shortly after they've hatched. Predators such as snakes, deer, and even other birds find an easy target in the hooded warblers because they nest so close to the ground.

"We need to give these things a chance; they're amazing creatures," Williams says. "If we're doing things to the habitat so they're not able to breed, we need to know."

This article appears in the Autumn/Winter 2013 issue of Ohio University's Perspectives magazine.