Age-Ratios and Condition of En Route Migrant Blackpoll Warblers in the British Virgin Islands

The en route migration ecology of Blackpoll Warblers (Setophaga striata) is poorly understood, yet intriguing. Blackpoll Warblers undertake the longest open water migration of any wood warbler species, traveling from northeastern North America to South America, with the first potential landfall being the West Indies. This migration requires substantial energy reserves and subjects Blackpoll Warblers to unpredictable weather events, which may influence survival. Few studies have examined age ratios or condition of Blackpoll Warblers while the warblers are en route through the Caribbean region. I captured and banded Blackpoll Warblers in the British Virgin Islands over 10 consecutive autumn migrations. Ratios of hatch-year to adult Blackpoll Warblers. were variable but averaged lower than the ratios reported at continental departure locations. Average mass of Blackpoll Warblers was less than that reported at continental departure locations, with 26% of adults and 40% of hatch-year birds below the estimated fat free mass; hatch-year birds were consistently in poorer condition than adults. Blackpoll Warblers captured in the British Virgin Islands were also in poorer condition than those reported from the Dominican Republic and Barbados; this may be because of the British Virgin Islands being the first landfall after the transatlantic crossing, whereas Blackpoll Warblers arriving at the other Caribbean study locations may have had opportunities for stopover prior to arrival or have departed from farther south on the continent. However, this suggests that the British Virgin Islands likely provide important stopover habitat as a first landfall location for Blackpoll Warblers arriving from the transatlantic migration route.