Wild Side : Flocks
Though the shoulder season has taken on a serious slouch, visitors continue to pour into the Vineyard. They just aren't human ones. Noted by birders and non-birders alike, multitudes of sea ducks (scoters and eiders) have been rafting up or chugging past the south shore. Wasque and Squibnocket are, as always, the best points for duck viewing. And over up-Island woodlands and farms, clouds of blackbirds (red-wings, cowbirds, and especially grackles) have been seen rolling through the sky or landing to forage noisily on the ground.
The arrival of these avian migrants is a predictable, annual event, though the birds may be particularly plentiful this year. Simply put, the Vineyard is a migratory crossroads for many things - birds, bats, fish, insects - but ducks and blackbirds rank among the largest, most numerous, and most obvious.
The flocking instinct is highly developed in these birds, and flocking confers major advantages in terms of survival. Flocking lets inexperienced birds benefit from the navigational expertise and skill at finding resources of older birds. Flocking is an efficient way to exploit patchily abundant food supplies, like shellfish beds or seed-rich agricultural fields. For the ducks, flocking provides efficiency in their epic travels: flying in linear skeins, each bird rides the slipstream of the bird in front of it, reducing the energy needed flight.
Blackbirds, in contrast, knot up into dense flocks with little apparent structure; while just a dozen or two may team up for migration, flocks of tens of thousands sometimes visit Martha's Vineyard. Large or small, a flock seems to roll along, constantly changing shape like a cloud of smoke. Membership in a flock provides blackbirds with a measure of safety from predators. For a diving falcon to plunge headlong into a flock would be suicidal; unless the hawk can cut a single individual out and attack it, the members of a tight flock are almost unassailable.
The influx of these birds represents a major ecological event. Let's use a conservative estimate of 20,000 ducks in our waters in the past week, and perhaps half as many blackbirds on land. Huge quantities of prey items - mostly mollusks for the ducks, anything small and edible for the blackbirds - will be consumed. Many of the nutrients contained in those prey will be released back into the environment as droppings, available for other living things to tap. The annual arrival of these birds simultaneously taxes and fuels our local ecosystem.
To help grasp the magnitude of such a phenomenon, consider what it would be like if biomass equal to these flocks arrived in the form of wolves instead of birds. The ducks, of course, are not eating the mammalian prey wolves prefer. But predators are predators; despite any differences, the "gray wolf equivalence" model probably does roughly approximate the regional impact of these birds in terms of energy needs, volume of food consumed, and concentrated nutrients freed up for reuse.
In any event, eiders average about six pounds apiece, scoters around three; let's assume an equal mix of species and use an average weight of 4.5 pounds for our ducks. Our 20,000 birds add up to 90,000 pounds duck meat on the hoof. Meanwhile, an average gray wolf of the subspecies that formerly inhabited our region averages about 75 pounds; it would take 1,200 of them to equal the weight of our duck flocks. Imagine 1,200 wolves descending on the Vineyard; over the course of a winter, they'd eat everything in sight, probably starting with free-roaming cats, then polishing off the deer, then settling for rodents, and finally biting the bullet and mopping up the skunks. After that, I'd recommend staying indoors.
Averaging about a quarter pound, our 10,000 grackles add up to about 33 wolves in size - nothing compared to the ducks, but still, a major factor on the ground. Flocks of hundreds or thousands of grackles can sometimes be observed in up-Island woodlands, advancing in skirmishing order, flipping fallen leaves to find invertebrates, seeds, salamanders, or other edibles. I can't believe they miss much, and the annual influx of these birds is undoubtedly an influential force in regulating populations of these small prey species.
The implications of migration extend beyond our own region. The ducks, come spring, head north to their tundra breeding grounds, where they continue to play a powerful role in shaping the ecological balance. The blackbirds, likewise, eat almost constantly as they move farther south to winter and then disperse back north to breed.
So in a very real sense, what happens to migratory species in our region affects the ecology of the whole hemisphere: the survival or failure of these birds around the Vineyard reverberates across eastern North America. We're famous for our rarities. But our role supporting migrants may be our most important ecological function.
