Fall migration always has some surprises in store. One of the most delightful vagrants this fall was a Fork-tailed Flycatcher. This species normally occurs from Mexico all the way south to Argentina. A few vagrants occur along the eastern seaboard every year, usually between September and November.
The Maine bird was found at Gilsland Farm in Falmouth on September 16 by a local birder, Angus King. The electronic word got out and dozens of birders got to see the impressive bird. Alas, it was last seen on September 19.
Fork-tailed Flycatchers are white underneath with a gray back and a bold, black cap. The hallmark of the species is the fantastic tail. The outer tail feathers are ridiculously elongated. In flight, the tail feathers spread to make the distinctive fork. The tail of a Fork-tailed Flycatcher is like the tail of a Barn Swallow on steroids! Relative to body size, the tail of a Fork-tailed Flycatcher is the longest of any bird, two to three times the body length. Females and young birds have less ostentatious forked tails.
Like other members of its family, Fork-tailed Flycatchers sit on perches and make sallies to capture flying insects. When insects are not flying, the birds make do with berries and other fruit. The bird at Gilsland Farm had to resort to fruit-eating some of the time because of the morning fog that kept insects from flying.
Ornithologists recognize four subspecies of Fork-tailed Flycatchers. The three northern-most subspecies (from Mexico to northern parts of South America) tend to be sedentary birds. The fourth subspecies is migratory, wintering around the equator and moving south as far as Argentina to nest. They nest in the austral summer, between October and January.
Details from photographs of the Maine Fork-tail indicate it belonged to this southern-most subspecies. I think the timing of Fork-tailed Flycatcher vagrancy offers some insight into the reasons for the appearance of these birds in such far-flung places.
We know that birds have remarkable abilities to navigate. Different species use different sets of clues to guide them on their way: celestial maps, the earth’s magnetic field, the position of the sun and geographic features. Remarkably, these navigational abilities are frequently genetically encoded; no training by the parents is required.
For instance, most breeding shorebirds on the Arctic tundra depart on their fall migration two to four weeks before their young are capable of long-distance flights. That is why we see an initial pulse of adult shorebirds in fall migration, followed a month or so later by a pulse of juveniles. I marvel at the ability of these juvenile shorebirds to migrate long distances to a wintering area they have never known.
Sometimes, birds get confused and undertake a reverse migration. Essentially, they confuse north for south. It is reasonable to think that the Fork-tailed Flycatchers seen in New England are such reverse migrants. In September, it would normally be time for them to migrate south into southern South America to breed. If one of these austral migrants migrates in the diametrically opposite direction, it would find itself in North America.
Reverse migration has been used to explain one of the most remarkable vagrants in Maine. In November of 1977, a streaked flycatcher showed up at Biddeford Pool. It was initially identified as a Sulphur-bellied Flycatcher, a tropical species that gets as far north as southern Arizona. A more careful look revealed that the bird was a Variegated Flycatcher, a South American species.
Some populations are not migratory and others are austral migrants. An austral migrant should be moving south in November to breed. Reversal of its navigational cues could explain its appearance in Maine. The Maine bird was the first record of the species in North America. There are five subsequent records of the species from Tennessee, Ontario, Washington state and two from Florida.
Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at firstname.lastname@example.org
[First published on October 1, 2017]