On Sunday, January 20, I enjoyed watching grasshopper nymphs in my yard as they frolicked in sunny, 50-degree weather. And by this past Sunday, the weather had warmed up to a tolerable 25 in our yard, with warming sunshine. But in between, the Vineyard got a taste of a good, old-fashioned winter, with nighttime lows in the single digits, a brutal north wind, and a couple of modest snowfalls to cover the ground.
While hardly unprecedented on the Vineyard, such sustained cold snaps have grown rare in recent years. But they still happen often enough to exert a significant effect on wildlife populations. Prolonged cold, especially when paired with enough snow to cover much of the available food, represents a sort of ecological bottleneck, killing many individuals that, for whatever reason, are not equipped to survive until conditions moderate.
The effects of this cold wave were vividly illustrated on the very active Vineyard Bird Alert Facebook page (www.facebook.com/groups/173039372709139/). “I found a catbird…in the snow,” posted Adrian Wright of Oak Bluffs; “you could see his tracks circling around and then he must have just [lay] down, exhausted, hungry and given up.”
Lanny McDowell posted about an exhausted chickadee he found, “operating in slo-mo. I picked him up off the side of a tube feeder. ‘He’ can just barely cope — has the will to get food, but his capacity to process it is way below par.”
Elsewhere on the Island, two avian oddities gave up the ghost. A ruby-throated hummingbird that had been relying on a hummingbird feeder at Seven Gates finally disappeared last week. And one of the Island’s best birds of 2012, an Allen’s hummingbird that had been visiting a feeder off of Lambert’s Cove Road since early December, failed to report for duty last Friday morning and is presumably dead. Any hummingbird, of course, is a rarity here in winter; the Allen’s was especially notable, since this is a species that breeds in coastal California and winters in Mexico. By rights, it should have been in subtropical habitat thousands of miles away, instead of freezing to death in a bush on Martha’s Vineyard.
One feels sorry for these birds, especially ones like the Allen’s hummer that have achieved celebrity status (at least among birders). At the same time, most observers also accept that this is part of the grand scheme of nature: a complex competition, with survival and reproduction as the prizes and an early death as the penalty for bad luck or bad choices. Yet there is a way in which even the losers play a key role in the survival of their species.
Consider that heroic Allen’s humming bird, a first-year male with, obviously, a migratory navigation system that had gone askew. But if every Allen’s hummingbird had precisely the same migratory impulse, the entire population of this species would travel the same path at the same time in the fall. This would work fine as long as conditions for migrating, and on the wintering grounds, were good. But the species as a whole would be vulnerable: with, so to speak, all their eggs in one basket, most of the population could be wiped out in single bout of bad weather, or a large-scale alteration of some crucial habitat.
So having some measure of variation and imprecision programmed in helps a migratory species hedge its bets, spreading the population out in time and space so that no single catastrophe can kill every individual. The Vineyard Allen’s, unfortunately for him, was an extreme case, leading him into unworkable winter conditions. But some of his colleagues, not as far out on the spectrum of unusual instincts, are serving as pioneers, possibly discovering new areas to winter or breed that will help their species survive over the long haul. And who knows: as our climate changes over the coming decades, that fatal urge to head east instead of south may prove to be the salvation of this species.
Likewise, Adrian’s catbird may have lost the game, and its choice of wintering this far north proved to be a lethal one. But a bad outcome doesn’t necessarily mean that a choice was a bad one: had this winter been as mild as last one was, this bird would have had a big advantage come spring. He’d be among the first to arrive on his breeding grounds, giving him a head start in establishing a territory or locating resources. It’s a gamble: sometimes you win, sometimes you lose.
It isn’t just wintertime cold that represents such an ecological bottleneck. Severe storms, especially if they occur during a period when birds are migrating, can put enormous pressure on populations. Likewise, summer heat and drought can cause extensive mortality or breeding failure among wildlife of all types. But because wild animals are individuals, varying in their instincts, skills, and behavior, such bottlenecks actually work to strengthen the long-term prospects of species. Many individuals die. But the ones that don’t carry the flexibility the species needs to adapt to change.