Troubles Far From Home

Separated by nearly 3,000 kilometers (1,800 miles), our own Green Mountains and the Dominican Republic’s Sierra de Bahoruco would seem to share little in common. However, each provides crucial habitat for Bicknell’s thrush, one of North America’s most rare and vulnerable songbirds. The island of Hispaniola, shared by the Dominican Republic and Haiti, is home to an estimated 90% of the global wintering population of this species, which may number as few as 20,000 individuals. These two countries face unrelenting loss of native habitats from intense human population pressures. Only 1% of Haiti’s original forest cover remains, while 10% of the DR’s forests tenuously persist. As the winter stronghold of Bicknell’s thrush, Hispaniola

is thus key to long-term conservation of this species.

I just returned from Sierra de Bahoruco, where the Vermont Center for Ecostudies has spent the past 14 winters attempting to unravel the elusive winter ecology of Bicknell’s thrush. Although we have found the species inhabiting dense broadleaf forests from sea level to >2,000 meters elevation, most intact habitats occur in mountainous areas, removed from the reach of humans. Our field research has revealed some fascinating and important insights. Foremost among these is that male and female Bicknell’s thrush appear to segregate by habitat type. In undisturbed, primary forests of Sierra de Bahoruco, males outnumber females by a 4:1 ratio. Both sexes maintain discrete territories throughout the winter, and insect food resources are relatively rich in these forests, which we believe represent “optimal” winter habitat for Bicknell’s thrush.

A different situation exists at our study site in the Cordillera Septentrional, a mid-elevation, much wetter area that features more disturbed, secondary broadleaf forest with a relatively open understory. Here females slightly outnumber males, and birds appear to feed more heavily on fruit, a generally lower quality food than insects. We suspect that these forests constitute “suboptimal” habitat for Bicknell’s thrushes, and that, because suitable habitat is limited islandwide on Hispaniola, females may have few options but to inhabit them. The consequences for female survival could be pronounced, with effects on the entire species’ viability. Adding insult to injury is our recent discovery that introduced rats may cause significant mortality during winter, even in the relatively pristine forests of Sierra de Bahoruco.

An intriguing, but troubling, link may exist between these wintering grounds events and the breeding ecology of Bicknell’s Thrush. VCE’s work on Vermont peaks has confirmed a breeding adult sex ratio of 2.2 males per female and a very unusual mating system, in which >75% of nests are attended by two or more males, with highly mixed paternity in broods. What is behind this male-biased sex ratio? We fear that overwintering females may be taking the hardest hit during the annual cycle. The story is complex, with more questions than answers, but the message is simple: effective conservation of a migratory species requires careful attention to both ends of its range.

Birding highlights of the past week included possible early vanguards of the spring migration: a Double-crested Cormorant off Charlotte, pairs of Rusty Blackbirds in both Danby and East Dorset, and two Red-winged Blackbirds in Danby. While these may simply be hardy individuals that overwintered in Vermont, they lend hope that lengthening days will soon spur a steady flow of northward returning birds. Other notable birds last week included single Peregrine Falcons in Middlebury, Weybridge and Burlington, a Belted Kingfisher in Hartland, and six Red Crossbills in Newport. This later species is always rare in Vermont and was an especially nice find in a year when crossbills have been poorly represented in the winter finch invasion that has delighted so many birders.

You can explore all the birds reported last week in Vermont and add your own sightings at Vermont eBird.