Flocking Behavior of Migratory Warblers in Winter in the Virgin Islands

We assessed the flocking behavior of birds on St. John and St. Thomas, U.S. Virgin Islands, with systematic surveys along trails in moist forests. Winter residents (species breeding in North America and that winter in the Virgin Islands), all of which were warblers, comprised 91% of the individuals found in 28 flocks but only 49% of solitary individuals. The average flock size was 4.0 individuals of 3.1 species, and did not differ between St. John and St. Thomas even though the average forest tract on St. John (1,000 ha) was much larger than on St. Thomas (62 ha). Northern Parula (Parula americana) and Black-and-white Warbler (Mniotilta varia), the most common species in flocks, occurred in 76% and 79% of the flocks, respectively. Northern Parula flocked significantly more frequently on St. Thomas than on St. John, but no other species showed a difference in flocking behavior between the two islands. Each flock typically included one individual of each species.