Study finds long-distance movements by Kirtland’s Warblers during breeding season

State-of-the-art tracking technology has revealed that during the breeding season, some Kirtland’s Warblers make previously unknown long-distance movements — and the discovery has important conservation implications for many other North American birds.

Scientists from the Smithsonian and Georgetown University report that after the warblers migrated in spring from wintering grounds in the Bahamas to breeding grounds in Michigan, some individuals unexpectedly started moving long distances between distant breeding sites at a time when most remained on their small territories. Their findings appear today in Current Biology.

“Discovering these hidden movements by Kirtland’s Warblers challenges us to reshape how we think about animal movement,” said Nathan Cooper, a research ecologist at the Smithsonian Migratory Bird Center and the paper’s lead author. “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 on breeding grounds in Michigan, all radio-tagged birds initially held one or more small territories. However, 11 percent of breeders and 60 percent of non-breeders abandoned this common space-use strategy and began moving long distances (3 to 48 miles), often at night, between 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. 

Thanks to the Smithsonian’s National Zoo and Conservation Biology Institute for providing this news.

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