Blackpoll Warbler––An epic migration brings a tiny songbird to us from the Amazon basin
A recent Birder’s Notebook article featured the astonishing 11-day migration of bar-tailed godwit “B6,” and the Nome-based research project that documented the longest known nonstop migration in the animal kingdom.
Many other birds breeding on the Seward Peninsula make epic journeys to get here, too. The journey of “B6” was not the first record-breaking feat of endurance by a migratory bird that was revealed by research in the Nome area.
In 2016, a team of Canadian biologists from the University of Guelph in Ontario arrived in our Banner Creek neighborhood equipped with mist nets and tiny geolocators with their sights set on catching and tagging blackpoll warblers to track their migration.
Blackpoll warblers breed across the northern boreal forests of Alaska and Canada, extending into tall shrub habitats beyond the forests. Banner Creek offers good breeding habitat, and the tallest willows provide the handsome black-and-white male blackpolls with song perches from which they declare their territories by day and night.
The Canadian research team was successful. They outfitted the little warblers with geolocators in Nome, which is near the westernmost extent of the blackpoll’s breeding range, and in three other locations across their breeding range: in Churchill, Manitoba; Whitehorse, Yukon Territory; and in Denali National Park.
Blackpolls are too small to carry devices that transmit location data. These warblers, weighing just under half an ounce, were fitted with tiny geolocator backpacks weighing .018 ounce with 12 months of battery life.
Geolocators collect and store light-level data at regular intervals, which allows the bird’s location to be determined later from the recorded day length and the time of solar noon. A challenge is that the bird has to be recaptured the following year to collect the data.
The biologists were remarkably successful and were able to retrieve geolocators at all four locations the following year.
The data showed that when blackpolls leave the Nome area in August, they set out on the longest known migration of any North American songbird. They make an annual round trip to and from their Nome area breeding grounds of up to 12,400 miles. And the blackpoll’s journey includes the longest known nonstop, overwater flight of any songbird.
Over an 18-day period, the warblers’ travels took them across the entire North American continent, over the boreal forests of Alaska and Canada, to the Atlantic shores of the Carolinas. There, Nome’s blackpolls spent nearly a month feeding and fattening until they doubled their body weight.
Once fueled up for the flight, the long-haul travelers waited for a night with a tailwind, then launched out over the open Atlantic on a nonstop flight to the northern shores of South America.
The blackpolls’ 1,400 to 2,113-mile transoceanic flights took up to three days of continuous flapping with no food, water or rest before landfall. Eventually, at the end of journeys of up to 6,200 miles the warblers reached wintering areas in the Amazon basin in northern Colombia, Venezuela and Brazil.
Long wings provide speed and efficiency in flight. Blackpolls migrate farther than other warbler species and have longer wings. And the blackpolls from the Nome area, whose migration is longer than that of the other blackpolls in the study, had the longest wings.
Blackpolls are a fairly common breeder in the forests and tall shrub thickets of the Seward Peninsula. They generally arrive at the end of May. The males come first to establish their territories, often reclaiming their previous territory, singing from high perches to stake their claim.
These small songbirds sing at such a high frequency that some people cannot hear their metallic, high-pitched song. Thus, the blackpoll’s song has been referred to as nature’s hearing test.
Females arrive a few days after the males and soon pairs form for the season. Some pairs reform if both the male and female return to their previous territory. Most pairs are monogamous for the season, but occasionally males breed with more than one female.
The female selects a nest site within her mate’s territory. She builds the nest, often on a branch close to the main stem of a shrub or tree. In mid-June she lays four to five eggs, which she alone incubates while her mate sings and guards the territory.
Both parents feed the chicks as they develop, in the nest and for about 25 days after the nestlings fledge. By mid-August, most blackpolls have set out on their marathon journey.
The blackpoll’s diet is primarily insects, supplemented with berries and seeds. They move along the branches and trunks of trees and shrubs, picking insects off the surface of leaves and branches as they go. They forage more slowly and methodically than most other warblers, the males often singing as they feed.
Blackpolls are listed as a common bird in steep decline. Though still numerous, they are one of North America’s most rapidly declining songbirds, estimated to have lost 88 percent of the population in the last 40 years.
Research such as the migration study are attempts to learn more about the species in order to understand the challenges they face in our rapidly changing world.
While the blackpoll’s northern breeding range is still relatively intact, habitat loss on their South American wintering grounds is occurring at a rapid rate. The 12,400-mile annual journey to get here and back is fraught with risks: storms, cat predation, collisions with buildings and vehicles, and the search for diminishing islands of habitat where they can feed and rest in our human-altered landscapes.
Declining insect numbers and changes in timing of insect abundance relative to migration and brood rearing may be significant factors.
However, many Nome-bound blackpolls have weathered their epic journeys and this summer they are abundant in the tall shrub thickets around Nome. And I’m pleased to have once again passed “nature’s hearing test.”