Scientists Track, For the First Time, One of the Rarest Songbirds on Its Yearlong Migration

Kirtland's Warbler

(Wendy Mitman Clarke; March 6, 2017)

At .48 ounces, your average Kirtland’s warbler weighs about as much as a handful of tortilla chips (seven, stacked), or about the same as one baby carrot. And every year, this rare North American songbird travels nearly 4,000 miles round trip, across mountain ranges, the body of a continent, the Gulf Stream and open ocean. Most of this journey has been a mystery, until now.
Using light-level geolocators, Smithsonian scientists have for the first time tracked and mapped the migratory paths of Kirtland’s warblers for an entire year, following them from their breeding grounds in Michigan to their winter homes in the central Bahamas and back. The scientists hope the data will enable conservation managers to better understand how to manage habitat for the warblers, which were close to extinction in the 1970s and have made a significant comeback as an endangered species.

The research, published in the Journal of Avian Biology, also represents a breakthrough for studying other small species’ migrations, which are an elusive but pivotal element of their lives.

“However difficult it may be, it is critical that we understand the full annual cycle of birds, not just what is happening during breeding,” says Nathan Cooper, lead author of the study and postdoctoral fellow at the Smithsonian’s Migratory Bird Center, part of the Smithsonian Conservation Biology Institute. “There is a significant amount of mortality for songbirds that happens during migration, indicating that the conditions birds encounter while migrating might be major factors in a species’ overall success or failure.”

“We know so little about migration for so many species,” says Pete Marra, head of the Migratory Bird Center and co-author on the paper. “This is the rarest songbird in North America, one of the most endangered. The goal is to move toward tracking the same individuals throughout the year to understand where and why birds are dying, and we’re getting closer with this species.”

Kirtland’s warblers are easy to study in one respect; they only nest in dense, young jack pine forests predominately in specific regions in Michigan. But those forests depend upon frequent fires to propagate the jack pines’ seeds, and fire suppression in the mid-century, coupled with nest predation by the brown-headed cowbird, devastated the species. In 1966 the U.S. Fish and Wildlife Service declared the birds endangered; in 1974, researchers identified only 167 singing males.

By planting new young jack pine forest and implementing a cowbird removal program, conservation managers helped the warblers begin to recover their numbers. Today, their population is estimated at about 2,300 males. It’s a success story, but continued management is crucial.

Although scientists know a great deal about the birds on their breeding grounds in Michigan, they know less about their distribution in the Bahamas during the winter, and migration—which kills an estimated 44 percent of Kirtland’s populations—has remained an unknown.

“Given that they’re flying 2,000 miles in two weeks, it makes a lot of sense that there could be a lot of mortality during that period,” Cooper says. “But we don’t know if it’s driven by things that happen during migration, or if it is set up by events that happen during the wintering period.” For instance, a drought in the Bahamas can mean less food, so the birds might be malnourished before they even begin the strenuous, stressful flight of migration. “That’s why things like climate change [contributing to drought in the Bahamas] can affect migration and, in turn, the breeding period.”

The more widely used satellite and GPS tracking devices that work well on larger animals are too bulky and heavy for most birds, but in the 1990s, British researchers developed light-level indicating devices that were small enough to attach to wandering albatrosses. The concept of using light levels to determine location has been used by mariners for centuries. By determining precise sunrise, midday and sunset times, one can calculate a rough position, because the length of a day varies predictably depending upon one’s latitude and longitude.

New light-level geolocators are finally small enough for even diminutive songbirds to carry them, Cooper says.

“They measure the intensity of sunlight every two minutes and save it to the device. It gathers that data over the whole year. We can estimate sunrise and sunset time every day of the year, and from that you can get day length and solar noon,” Cooper says. That data enables researchers to roughly estimate and map the birds’ location.

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