With spring migration building up steam, Vineyard birders likewise ramp up their birding. Migrations offer a major increase in the number of birds out there to watch. And they offer our only opportunity to see many more species that stop to refuel here on their way to distant breeding grounds.
Migrations also put birders on alert for the unexpected: Species that don’t belong here but turn up anyway, in the wrong region or sometimes even on the wrong continent. Seeing such birds is fun; finding one yourself is even better. It would be easy to imagine that such avian vagrants would simply be accidents, random particles of distant bird life. Indeed, chance plays a big role in the vagrancy of birds. But perhaps surprisingly, very clear patterns exist of what birds tend to wind up far from home, and what species don’t.
Understanding bird vagrancy is handy as a field tool, helping a birder know what rarities to keep in mind in certain places or conditions. And it helps one sort out the numerous reports of rarities, some well-documented and some just hearsay, that circulate among Vineyard birders in a typical year. Is a report plausible enough to make it worth going to try for a look? Is it plausible enough to consider part of the Island’s avian record? Such questions matter to birders. And their answers depend partly on the infinitely varied life histories of birds.
Consider the Carolina chickadee and its northern relative, the boreal chickadee (both close relatives of our own black-capped chickadee). Carolinas occur in the southeastern U.S., as far up the coast as central New Jersey. And the boreal chickadee nests, at least at higher elevations, as far south as central New England.
Neither, as far as I know, has ever been recorded on the Vineyard, and you might think that they’d be about equally likely (or unlikely) as potential vagrants. In fact, I’d be bluntly skeptical of any Carolina chickadee report on the Vineyard, though they may get here eventually as their range shifts north in response to global warming. But a report of a boreal chickadee, at least under some conditions, wouldn’t surprise me at all.
The Carolina chickadee, though abundant and widespread, has very little migratory tendency. Since they don’t wander, they’ll never get here. But the boreal chickadee is prone to “irruptions” — wintertime movements to the south, in response to failing food supplies. In a year when this species is moving in big numbers, it’d be easy to imagine one showing up at an Island feeder. I’d still be skeptical of a Vineyard report that didn’t come in the context of other regional reports, however.
Bird species vary widely in how migratory they are – how frequently and how strongly they engage in seasonal movements. Among closely related species (as is the case with the chickadees), the more mobile species is usually much more likely to wander. And as you’d expect, the direction a species naturally migrates in affects where that species is likely to turn up as a vagrant: a migratory species that breeds in the southwestern U.S. will likely be less probable here than a close relative breeding in the Canadian Rockies.
Some groups of birds are also much more prone to vagrancy than others. Flycatchers, for example, are notorious for botching their migration. And the migratory habits of some birds makes them much more plausible here at one season than at others. For instance, yellow-throated and prothonotary warblers, breeding birds of the Southeast, turn up every few springs here, as northbound migrants that overshoot their destination, making April reports quite routine. An Island sighting of one of these species in October, though, would really get my attention; fall records of these species are truly rare events.
Vagrancy does not just reflect blunders or bad luck for birds: from the perspective of a species as a whole, the tendency serves a vital purpose. On the one hand, getting lost can be lethal for individual birds, or at least their genes, which are unlikely to make it back to where potential mates are found. But on the other hand, wandering is one way to discover new regions suitable for breeding or wintering. So the few vagrants that do survive may contribute a lot to the resilience of a species.
In fact, every bird species has to be programmed for at least some degree of mobility, or the species could never survive. Ultimately, there is little that is totally impossible with birds: birders learn to take any report of a rarity seriously at least to start with, because strange things happen. But the details of avian biology make some things much, much stranger than others.