Some birders focus on “listing” — tallying as many species as they can in a state, a country, or the world. Others drift in the direction of “citizen science,” participating in organized survey projects or simply orienting their leisure birding to close observation and detailed record-keeping. But the one thing that will get virtually any birder excited is finding a rare vagrant — that is, a misdirected bird making an unusual appearance far from where it belongs.
Because birds are so mobile, vagrancy is a routine fact of life in the bird world. And although any particular vagrant may be a rare occurrence in a given location, there are lots of species in the world; so encountering a vagrant of one kind or another isn’t all that rare. But as you gain experience in the field and become more familiar with the vast and ever-growing body of bird records, you begin to realize that vagrancy is less random than you might expect.
Avian species vary widely in how prone they are to wander, and vagrancy for a particular species — or for birds generally — is often associated with seasonality and weather. Very little is truly impossible when it comes to avian vagrants, but experienced observers develop a finely tuned sense of how plausible a particular report is.
One factor shared by most vagrants is a migratory life history. It’s much more likely for a migratory bird, which routinely covers large distances, to turn up out of its normal range than it is for a non-migratory species. And the more strongly migratory a species, in general, the more prone it is to wandering. The point is well illustrated by pairs of closely related species that differ in their migratory habits. Allen’s and rufous hummingbirds, for example, are so closely related as to be nearly identical in appearance, and they are both species of the West Coast of North America. But rufous hummers follow, in general, a much longer migration route than Allen’s, and in consequence, rufous turn up many times more often in the East than does its very close relative.
This is why many so-called Neotropical migrants, breeding in the boreal forest and wintering in Central and South America, are prone to vagrancy. With such a long migration route, all it takes is minor navigational error to take an individual someplace unexpected. Migration also governs the seasonality of vagrancy. Many migratory species spend the vast majority of each year in one of two places — their nesting territory in the north, or their wintering grounds in the south. They have no inclination to wander once they are in either of the locations, until the season prompts them, and they rarely start to move until the correct time. So finding a vagrant during the height of summer or an early arrival in late February is much less likely than finding one during the relatively narrow migration windows in spring and fall.
Moreover, during fall migration, the numbers of any species are augmented by the many “birds of the year” that recently hatched, and these birds, never having migrated before, are especially prone to taking wrong turns. Because of both the inflated numbers and inexperience, vagrants of many species are far more likely in fall than in spring.
Weather is also a major factor in bird vagrancy, with storm systems or high winds capable of driving birds far off course. Tropical storms, for example, are notorious for bringing southern seabirds to northern latitudes or pushing them many hundreds of miles inland. And weather may interact with normal migratory behavior to produce clear patterns in avian vagrancy. The best examples are so-called “overshoot” vagrants — usually Neotropical songbirds that can get caught in storm systems when they’re beginning to move north through the United States. The result can be remarkable waves of southern birds, like hooded or prothonotary warblers that can appear far north of their usual range in early spring, or of more northern species like indigo bunting appearing in the north weeks before their usual May arrival.
There’s still more to it. Some groups, like flycatchers, seem to be especially prone to vagrancy, but for no obvious reason. And some groups, like gulls, seem to show a nearly random pattern of vagrancy. An individual, or even more than one, evidently just decides to go exploring, sometimes all the way to a different hemisphere. And it’s important to remember that our picture of avian vagrancy is imperfect: many, probably most, vagrants are overlooked, and our expectations of what vagrants are likely where may influence how we bird or how we identify what we see.
But finding a bird that has wandered across thousands of miles of ocean, plains, or mountains provides a thrill that doesn’t depend on having a full explanation. The Vineyard’s situation — a last stop for birds wandering east, and a welcome first stop for the occasional vagrant from Europe — ensures that we get far more than our share of vagrants. As the weather warms and bird populations once more go into motion, Island birders will be out in force, looking for the southern, western, or even Eurasian bird that took a wrong turn.