Bird ranges are dynamic, and we may be seeing the early stages of expansion by a species in the Caribbean. Two groups of researchers have found evidence of possible colonization of the Virgin Islands by an endemic Puerto Rican warbler. They report their findings in two forthcoming papers in the Journal of Caribbean Ornithology.
Adelaide’s Warbler (Setophaga adelaidae) is a small predominantly gray and yellow insectivore most frequently found in the dry lowlands and scrub of southern Puerto Rico. However, it is a habitat generalist and can be found in other areas. It is less common on the eastern side of Puerto Rico, which is closest to the Virgin Islands.
Adelaide’s Warbler has long been considered non-migratory and endemic to Puerto Rico. In his Guide to the Birds of Puerto Rico and the Virgin Islands in 1989, Herbert Raffaele stated that it is “unknown from the Virgin Islands.” Now, however, it appears that several groups have flown more than 30 miles to St. Thomas and St. John, two of the U.S. Virgin Islands. Both have suitable habitat for Adelaide’s Warbler.
In March 2012, Sean M. Rune and Leann M. Conlon of the University of Maine observed four Adelaide’s Warblers near Santa Maria Bay on St. Thomas, the first known observations outside Puerto Rico. The birds were identified by field marks and song and were later photographed. Additional surveys found at least six birds that appeared to be permanently occupying the location, with at least one probable breeding pair. This location is secluded and infrequently visited by birders, so this group may have been there for some time.
In December 2012 during a Christmas Bird Count, Rune found two birds near Lameshur Bay, on the southern coast of St. John.
Not aware of these sightings, a team of researchers led by Richard R. Veit of the College of Staten Island had been surveying birds near Lameshur Bay since 2011, and they discovered two Adelaide’s Warblers in January 2015. Further surveys that year found three singing males that appeared to be defending territories. They enthusiastically responded to recordings of male birds from Puerto Rico. Researchers also observed two probable females. Photos, vocalizations, and subsequent comparisons with specimens confirmed them as Adelaide’s Warblers. In 2016, they found eight birds with six males defending territories.
It is unlikely that these birds were involuntarily “wind drifted” or caught up in storms, as the prevailing easterly trade winds would have pushed them away from the Virgin Islands. It is more likely that they were pioneering vagrants that arrived via active dispersal, with at least two colonizing groups, one into St. Thomas and one into St. John.
This theory is consistent with research regarding Adelaide’s Warbler. Once considered a single species, the American Ornithologists’ Union split then-existing subspecies into three full species in 2000: Barbuda Warbler (Setophaga subita), St. Lucia Warbler (Setophaga delicata), and Adelaide’s Warbler. Genetic analysis suggests that Adelaide’s Warbler expanded from west to east and two isolated populations ultimately developed into the recently recognized species.
The group on St. John appears to have taken up permanent residence within Virgin Islands National Park. According to Laurel Brannick, a ranger with the National Park Service, they continue to be regularly seen near the Lameshur Bay Trail on the southern part of the island. The current status of the St. Thomas population is unknown.
Unlike the other endemic warbler — the threatened Elfin-woods Warbler — the population of Adelaide’s Warbler on Puerto Rico appears to be stable. The bird is common in its preferred habitat, including Guánica Commonwealth Forest and Cabo Rojo National Wildlife Refuge. — Jason A. Crotty
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