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Fewer migrants are overwintering on Puerto Rico, puzzling researchers

Black-and-white Warbler spends the winter on Puerto Rico.
Black-and-white Warbler in Puerto Rico, by Guillermo J. Plaza.

These are challenging times for Puerto Rico. As if a deep recession and the arrival of the Zika virus weren’t enough, now we’re learning that populations of warblers and other Neotropical migrants that winter on the island have declined.

A team led by ornithologist John Faaborg, of the University of Missouri, and Wayne Arendt, a biologist with the U.S. Forest Service, has been tracking bird populations in the territory’s Guánica Commonwealth Forest for decades.

Guánica, a United Nations International Biosphere Reserve located on the southwestern coast, contains now-rare tracts of subtropical dry forest that once supported a large population of wintering birds. Faaborg and Arendt have been banding birds in its varied microhabitats since 1973, compiling one of the best historical datasets available on wintering birds in the Caribbean.

The trends it reveals are troubling. After years of stability, the winter community declined dramatically after 2001. The researchers say they now capture one-third as many birds as they did 20 years ago. The trend has stabilized recently (through the 2016 banding season), but numbers remain at reduced levels.

Populations of the most common migrants — American Redstart, Black-and-white Warbler, and Ovenbird — have dropped precipitously. Black-and-white Warbler, for example, has declined approximately 50 percent since 1989. Numbers for less numerous but previously regular species — Northern Parula and Worm-eating, Prairie, and Hooded Warblers — have also declined; in some years, none are caught.

Reasons for the declines are not clear. Habitat has not changed significantly. Nor do the declines appear to have been caused by hurricanes or other weather events. Winter survival rates have been constant. And despite declines in Puerto Rico, populations appear to have held steady or declined only slightly on breeding grounds. — Jason Crotty

Learn about the Guánica Avian Monitoring Project.

See photos of Black-and-white Warbler.

See photos of American Redstart.

See photos of Ovenbird.

A version of this article appeared in the August 2016 issue of BirdWatching.


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