Tracking Technology Exposes Secretive Long-distance Movement of Kirtland’s Warblers

State-of-the-art tracking technology reveals previously unknown long-distance movements of Kirtland’s warblers during the mating season that have important conservation implications for North American birds.

Published today, Aug. 20, in Current Biology, Smithsonian and Georgetown University scientists found that after completing their annual migration—more than 1,700 miles from wintering grounds in the Bahamas to breeding grounds in Michigan—some of these songbirds unexpectedly started moving long distances between distant breeding sites at a time when most individuals remained on their small territories. 

“Discovering these hidden movements by Kirtland’s warblers challenges us to reshape how we think about animal movement,” said Nathan Cooper, lead author and research ecologist, Smithsonian Migratory Bird Center. “As technology continues to improve, scientists will almost certainly detect similar long-distance movements in other bird species. If birds are moving farther than previously understood, we may not be conserving and protecting all the land area and habitats they require.”

Cooper attached tiny 0.3-gram radio tags to more than 100 Kirtland’s warblers on the wintering grounds in the Bahamas and then used a continent-wide network of automated telemetry receivers called the Motus Wildlife Tracking System to discover these surprisingly long-distance movements during the breeding season. After arriving to breeding grounds in Michigan, all radio-tagged birds initially held one or more small territories. However, 11% of breeders and 60% of non-breeders abandoned this common space-use strategy and began moving long distances (3 to 48 miles), often at night, between spatially isolated breeding areas.

Traditionally, birds have been thought to rarely leave their well-defended territories during the breeding season. However, birds that fail to attract a mate, or those whose nests fail early in the season, sometimes abandon their territories and begin moving more widely than the rest of the population. These individuals, known as “floaters,” typically move secretly through the territories of other birds, presumably to find open territories and available mates. However, only one of the Kirtland’s warblers that moved long distances in this study successfully bred later that season, suggesting either that mid-season mating opportunities are rare or that individuals had another purpose in mind. Interestingly, the frequency of long-distance movements peaked while other birds were feeding their loud nestlings and fledglings, suggesting that the transient birds may have moved in order to identify locations where other warblers successfully bred. Previous work has shown that many animals “prospect” for information about where to breed, but this behavior has mostly been documented at short distances.

Scientists have known about the existence of “floaters” in bird populations for decades, but their secretive behavior and unpredictable patterns of movement make them challenging to capture and even more difficult to track. To determine how rare these long-distance movements might be, researchers combed through hundreds of previous studies and found that although movements outside of the territory appear to be common, Kirtland’s warblers moved significantly farther than other species. The farthest-flying Kirtland’s warbler moved 48 miles, or more than 500 times the radius of an average territory, and nearly four times farther (relative to territory size) than any other species reviewed.

“This is a game changer,” said Peter Marra, co-author on the paper and director of the Georgetown Environment Initiative, professor of biology at Georgetown University and scientist emeritus at the Smithsonian Migratory Bird Center. “The process of dispersal, or moving from one breeding site to another, has profound effects on a species’ ability to respond quickly to habitat loss and climate change. If long-distance dispersal is commonly informed through prospecting behaviors, scientists will need to rethink how they make predictions about how populations will adapt in the face of large-scale environmental change.”

The U.S. and Canada have lost nearly 3 billion birds since 1970, according to a study published in Science last year by the Smithsonian Migratory Bird Center and several leading bird organizations. Understanding how, when, where and why birds move is one piece of the complex puzzle of understanding why birds are declining and how to conserve them for the future. 

The Smithsonian Migratory Bird Center (SMBC) is dedicated to understanding, conserving and championing the grand phenomenon of bird migration. Founded in 1991, and part of the Smithsonian Conservation Biology Institute (SCBI), SMBC scientists work to conserve migratory species through research and public education that foster a better understanding of migratory birds and the need to protect diverse habitats across the Western Hemisphere. SCBI plays a leading role in the Smithsonian’s global efforts to save wildlife species from extinction and train future generations of conservationists, spearhead research programs at its headquarters in Front Royal, Virginia, the Smithsonian’s National Zoo in Washington, D.C., and at field research stations and training sites worldwide.

Advancing Georgetown University’s commitment to the environment, sustainability and equitability, the Georgetown Environment Initiative brings together students, faculty and staff from across disciplines—from the natural sciences, social sciences, humanities, public policy, law, medicine and business—to contribute to global efforts to deepen understanding of our world and to transform the Earth’s stewardship. 

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