Wild Side : On the move, sadly
It's the start of Departure Season: this weekend, following illumination night, the O.B. fireworks, and the Fair, we can expect the first tick downward in the human crowds. It's departure season for birds, too: the songs of the morning chorus are thinning out, and one by one, the migratory birds we've enjoyed all summer grow scarce and disappear.
It's a melancholy time for me. Songbirds don't move much during the nesting season, and since I pay close attention to them, I grow familiar with (and most unscientifically fond of) the individual birds that live near my home and office. I understand that migration is a central fact of their lives. But I'm saddened when, for example, the local orchard oriole stops singing one day and then can't be found.
Of course there are compensations. Some of our birds don't leave at all. And for the next couple of months, we'll enjoy a host of transient songbirds, warblers, vireos, and others that nest to the north of us and visit just briefly on their way south. We'll even acquire some new species for the winter, birds like white-throated sparrows and juncos that nest in the mountains and boreal woodland and spend the winter in our comparatively mild climate.
But perhaps half of our breeding songbirds leave for the winter, and the Island's bird life will change markedly in the next few weeks. Knowing where these birds go makes one appreciate their traveling abilities, and it highlights the interdependence and biological connections Martha's Vineyard has with the rest of the world.
We host so many bird species, with such varied life histories, that it's hard to give a simple account of our songbirds' travels. But despite the complexities, it's helpful to divide our birds into four rough classes. The first group is our year-rounders: chickadees, blue jays, downy and hairy woodpeckers, Carolina wrens, and dozens of others that are essentially non-migratory. These birds may move seasonally about the Island, following shelter and food sources, and some of them occasionally stage pronounced, if irregular, migrations (many blue jays, for example, head south following summers in which poor acorn production deprives them of an important food source). But in general, these birds rely on hardiness and local knowledge to survive the winter.
A second class of songbirds can be found here all winter, but with one arriving population largely replacing our breeders. Hermit thrushes, for example, nest in modest numbers in pine woods on the Vineyard, and winter (again rather sparsely) in wet, berry-rich thickets. It's hard to prove without a detailed study, but I'm quite convinced the population turns over: There is, for example, a gap in springtime between when the wintering birds disappear in early April and our breeders return in May. Catbirds and towhees may also fall into this class; a few breeders may linger, but mainly I think the entire populations shift south, generally replacing our nesters with a different, smaller wintering population.
A third set of species migrate, but only partially or not very far. Grackles and red-winged blackbirds, for instance, barely leave, lingering here in numbers into December, their northbound vanguard returning in February. There's little evidence indicating where our individual breeders winter; these species are mobile throughout the winter, and "our" birds may well not travel past the mid-Atlantic states. Song sparrows present a slightly different case: many of our breeders head south along the coast, but some remain through the winter, joined by many more birds from northern populations.
And finally, there are the true migrants, birds like the wood thrush, the scarlet tanager, the red-eyed vireo, and the eastern wood-pewee that abandon our region entirely to winter in the tropics. In evolutionary terms, these are not our birds at all: they're best thought of as tropical species that have evolved a northward dispersal for breeding. Some, like the pewee, cross or skirt the Gulf of Mexico and fly all the way to South America. Others, like our prairie and yellow warblers, travel down the coast to winter largely in the West Indies. Amazingly, individual birds often return to nest or winter in the same spots, zeroing in across thousands of miles to find a familiar jungle or woodland.
Migration solves the problem of how a bird that eats mostly insects can survive a northern winter. But it is a taxing strategy, exposing birds to risks of exhaustion, disorientation, predation, and encounters with violent weather. And it demands that a bird have the flexibility to find sufficient resources in two widely separated, dramatically different habitats, as well as at critical rest stops along the way. But migratory pathways stitch together the hemisphere, physically linking the Vineyard to points as distant as Colombia and the Canadian arctic. The Island's biological significance extends far beyond our shoreline.