Looked at in the most general terms, avian migration is a simple phenomenon. Certain bird species avoid winter conditions that they are poorly equipped to handle, fly South in the fall to new quarters, and return North in the spring to breed. Simple, right?
This model is fine up to a point, and Vineyard birders are certainly tuned in to twice-yearly pulses of transient birds. But like most phenomena in the natural world, migration rapidly grows messier as you look at it more closely.
First, it’s useful to consider the notion of relocation, which means very different things to different birds. For some species, migration involves rapid, point-to-point travel, often conducted as fast as a bird can manage it. Many shorebirds famously shuttle between high Northern latitudes and the Southern Hemisphere. And many of our most colorful and popular songbirds rival that journey, even risking lengthy over-water flights as they dash between their breeding and wintering territories.
But for many other birds, migration, especially in the fall when there is no urgency to begin courtship and breeding, is a much more haphazard process. Such “facultative migrants” may move South when cold weather sets in and food supplies dwindle, but without a specific location in mind. They’ll move only until they find better conditions, lingering in an area only until food supplies run low, winter conditions move South to catch them, or the seasons change and it’s time to wander back North again.
Such species may show dramatic year-to-year variation in what percentage of the population chooses to head South. Blue jays, for example, are notorious in this regard, mostly staying put in some years, but staging mass movements in other years when favored foods are hard to find.
These fall movements may not even be exclusively North to South. Birds like snow buntings and pine siskins may move South in response to harsh weather or insufficient food supplies. But once at a more congenial latitude, their movements may be nearly random as they search incessantly for resources.
For any bird species, it’s worth noting that no species ever moves all at once; to do so would be risking catastrophe if the population encounters a severe storm. Individuals within any species leave at different times and move at different paces, in response to their starting latitude, their innate proclivities, or variations in their health or physical condition. Weeks, even months can pass between when the vanguard of a species arrives and when the last stragglers drift through.
Finally, things change over time. Climate change is already altering longstanding patterns of avian migration. For example, older field guides (like the one I learned to bird with in the late 1960s and 1970s) suggest that Baltimore orioles leave the U.S. almost completely for tropical quarters. But this gorgeous species now winters in respectable numbers in the Southeastern U.S., and turns up as a lingering migrant more and more often, even at our latitude.
Migration, then, is an inexhaustibly complicated mess. In early summer, Northbound movements are still occurring even as some individuals of some species are already heading South. And even in the dead of winter, nomadic populations can be moving in any direction, and a spell of mild weather may prompt a little hiccup of Northward movement, readily detectable by the alert observer.
Migration chaos can be observed at any time of year, if you watch closely enough. But it may be easiest to see now, in late autumn. Most birders would say we’re long past the peak of fall migration, and that’s certainly true if you’re talking about the diversity of species on the move. But huge numbers of birds are still in motion. Facultative migrants such as common grackles and American robins may be nearly absent for days or weeks at a time, since our local breeders have departed. But then a wave of birds from farther North may see fit to move, and suddenly we’re inundated.
Flocks of thousands of grackles may be encountered as they stalk through oak woodland, clucking companionably among themselves as they churn the leaf litter in search of seeds or invertebrates. Thousands of robins may aggregate into nocturnal roosts, dispersing by day to forage in flocks of various sizes. Late fall is actually when numbers of these species hit their annual peak on Martha’s Vineyard. Meanwhile, overhead, semihardy species such as Eastern bluebirds continue their untidy migrations, sometimes too high to be seen, and detectable only by their ethereal calls.
From the birder’s perspective, this is all wonderful. While there truly are well-defined peaks in avian migration, the phenomenon as a whole never ceases. Birds of one kind or another are constantly in motion, sometimes in vast numbers when you might expect no movement at all. Surprises are always possible, and to purloin a quip from baseball luminary Yogi Berra, migration ain’t over until it’s over. And that never, ever happens.
