If you know people who bird along the coast, you know they get excited by the potential of tropical storms. Not just for all the usual reasons, but because terrible weather can be terribly good forbirding. Sometimes.

Well, very bad weather can be very bad for birding too, but strong rotating storm systems that gather force crossing the Atlantic a lot nearer the equator than the Vineyard is have the potential to trap southern seabirds within their far-flung spirals, and then deliver them to our neighborhood when the storm passes by, especially when the system weakens and the wind speed drops.

A tropical storm, or a full-blown hurricane, climbing northward off the East Coast in late summer or fall has a pronounced effect on bird movements: it delays the continuing passage of birds toward the south, thus concentrating their numbers by blocking the movement of a weather system out of the north; the heavy precipitation grounds migrants that ordinarily would be overflying or moving through the landscape; the strong winds offset typical migrant flight patterns, including those of species that normally bypass the landmass offshore; it also makes it harder for some birds to find food, because it’s impossible to fly, to see and to forage, or because food items are simply unavailable.

Seabirds may be hundreds of miles north and west of where they were when the gathering storm first absorbed them. When the storm passes or abates, the birds that have been displaced are driven to resume the patterns that sustain them, which the storm interrupted.

On the morning after the passage of Earl, a collection of Vineyard birders scanned from the cliffs of Aquinnah and from Lobsterville and Red Beach, looking for storm-driven birds. Two black terns and two spotted sandpipers out on the oyster rafts on Menemsha Pond were the only finds of note, and neither of those much of asurprise. We then traversed the Island to the Farm Institute, which was way less than spectacular, offering up the usual shorebird suspects, plus a pair of dowitchers and one or two American goldenplovers. Disappointed, we split up and some of us drove the Atlantic leg of the Katama triangle and eventually spotted, far off, a milling swarm ofswallows. 

Thousands of tree swallows, in numbers beyond my ability to estimate, were gathered near Crackatuxet Cove, an area behind the dunes at the southeast corner of Edgartown Great Pond accessible from the right fork at Katama. They flew up together on the breeze, then alighted on the tops of bayberry bushes, then raised up again in unison.

From inside the car and outside on foot we enjoyed the intimate and unusual view of being within a giant flock of swallows. The proximity denied focus on any one individual unless the birds settled on the sandy roadway. Rarely did the small birds seem to move out of unison. Even then, they were likely to be facing the same direction, into the westerly wind. Their movements reminded me of fans doing the “wave” in a stadium. 

Unlike many insectivores, tree swallows may include a fair amount of fruit in their diet, especially at times of year when insect fare is becomes harder to come by. They can digest the waxy outer coatings of bayberries. By late summer the swallows have become markedly social, as very large flocks are formed which eventually become migratory, moving south on routes that favor coastal marshes and river corridors.

There is a spectacle that occurs when masses of these swallows head to roost just after sunset. I have never witnessed it or heard about it happening here on the Vineyard, but birders on Nantucket reported a massive and astonishing roosting event last year. The observers were in kayaks in a marsh as nightfell. All the tree swallows flew up above them until beyondsight. Then, with a surprising loud roar from their myriad wings, they dropped from the heights en masse, thousands and more thousands descending in a funnel cloud to disappear into the t hick of the marsh grasses to spend the night. The entire descent took no more than about thirtyseconds. Videos of this phenomenon show formations like long shoals of fish, or rushing down as a tornado in reverse. Many thousands of birds winging headlong as one fluid entity, then silence.

Finally we managed to break away from watching the swallows. Just before leaving the area we swung the cars into a puddled parking lot where some of us had watched flocking snow buntings last winter, because there was another small horde of swallows on the ground there. As the cars came to a halt, a single pale bird was spotted right in the middle of thegroup. A bird as pale as a discarded tissue in the breeze.

It was almost all white, except for the most subtle pinky brown on the head, nape, back and shoulders. The bill had some light coloration, unlike a typical tree swallow’s all black bill; the feet appeared pinkish instead of black; the eye looked dark, although more of a brown than the others’ beady black; and the underwing linings were a distinct brownish buff. 

Albino and leucistic are by definition two different conditions, referring to a total or to a partial lack of pigment. Since this bird is close to albino, but not completely so, it falls into the category of leucistic. Either way, this is a sweet little avian anomaly, a glowing apparition that we could distinguish far off into the distance when it moved on with its fellows, more like a little white angel amongst the wild birds.

Can I say for sure it was a tree swallow? Well, no, not absolutely, although I would take that bet any day. And what are the chances of finding one in a flock ofthousands? Not very good. One in a million?

Gazette correspondent Lanny McDowell is an avian photograher and freelance writer who lives in West Tisbury. On Monday this week he wrote the following addendum: “On Sept. 13, birders on Nantucket were following up on a sighting of another unusual and very white avian visitor, a white pelican, when they spotted a single white tree swallow within a large flock over the marshes. Yet another observer reported seeing a white swallow on Tuckernuck Island the previous weekend. This little flyer gets around!”