Backpack-toting Bicknell’s Thrushes (BITH) are proving their mettle, and then some. After our exhilarating recovery of 5 GPS tags from male BITH on Mt. Mansfield two weeks ago, VCE was elated to retrieve another 5 units on 14-15 June. And, Quebec has now joined the fray, as our longtime Canadian Wildlife Service colleague Yves Aubry recovered 7 of the 14 tags he affixed to male BITH last June. We’re gaining truly invaluable insights, even if no truly astonishing discoveries have yet surfaced.
In a nutshell, our cursory look at the data contained within these GPS tags (each of which has yielded 40-55 spatially precise “fixes”) shows that 12 of 13 BITH spent the main part of winter in the Dominican Republic (DR), with one male hunkering down in Cuba’s Sierra Maestra. That confirms (for now) what we have long suspected, based on field evidence—that up to 90% of the global BITH population overwinters on Hispaniola, with the great majority of birds confined to the DR. Most of these appear to inhabit cloud forest sites in the island’s most extensive mountain range, Cordillera Central. Surprisingly, we have yet to document a BITH overwintering in Sierra de Bahoruco of the southwest DR, and only one bird in Cordillera Septentrional along the north coast. However, with additional tags surely to come, and a more thorough examination of all data, the full story is far from told.
One Mansfield bird recovered this week showed some intriguing late-season dispersal, moving 43 km northwest on April 28-29 from a cloud forest site in Parque Nacional Valle Nuevo, and another 51 km only 2-3 days later to a high-elevation forest patch in Parque Nacional Armando Bermudez, where its GPS battery died. Given the late dates, it’s tempting to speculate that these were pre-migratory movements towards a northward launching point, but we’ll never know for sure.
One additional highlight for now from the new crop of GPS tag data, which we have only cursorily examined: two BITH made landfall in the Bahamas before continuing to their overwinter destinations in the DR. One bird was on New Providence Island on 15 November, then provided a GPS fix 15 days later during its apparent nocturnal migratory flight southward, as it appeared over open water ~65 km west of Great Exuma (fixes are scheduled at midnight, so it had to have been flying). A second bird was on Acklins on the early date of 15 October. Because GPS battery life limits the number of fixes we can schedule in each backpack, and since our primary research question involves late winter movements of BITH, we scheduled only 3 monthly fixes in September—November, concentrating fixes in March and April. Thus, any data obtained during fall migration are essentially “gravy” for us, though fascinating and informative. We now know what we have long suspected; that the Bahamas are a stopping point for at least some migrant BITH in autumn.On a more pedestrian (but no less important) note, our two days of “regular” banding on Mansfield were productive overall, with the biggest surprise being a female Pine Siskin with a regressing brood patch. The ridgeline was again unusually quiet, especially so under benign weather conditions in mid-June. Our final tally of 67 mist net captures included:
Swainson’s Thrush — 5 new bandings; 4 males, 1 female with full incubation patch
Purple Finch — 3 (1 male, 2 females with brood patches)
Pine Siskin — 1 female with regressing brood patch
Dark-eyed Junco (Slate-colored) — 8 (5 males, 3 females with incubation/brood patches)
White-throated Sparrow — 8 (7 males, 1 female with incubation/brood patch)
Magnolia Warbler — 2 yearling males
Blackpoll Warbler — 15 (11 males, 4 females); 8 new bandings, 4 returns (female from 2019, 3 males from 2021), 4 within-season recaptures; females with incubation patches
Yellow-rumped Warbler (Myrtle) — 7 males: 1 new, 1 return from 2019 and 2 from 2021, 3 within-season recaptures
Black-throated Green Warbler — 1 female with full incubation/brood patch; this may now be a locally-breeding species
We’ll be back at it next week, the following week, and every week through early August, as we continue season #31 of avian monitoring on Mansfield. To say the least, this year’s dimension of BITH GPS tag recovery adds an invigorating element of intrigue and discovery. We have yet to recover a 2021-tagged female, but I’m confident we will. Above all, we are humbled to contemplate that these diminutive 1-ounce songbirds have survived—toting miniaturized backpacks, no less—an arduous annual cycle that encompasses nesting, annual molt, fall migration, overwintering 1500-2000 km away, and a return northward flight. Whew!