Summer World: A Season of Bounty - Bernd Heinrich (2009)
Chapter 20. Blackbirds
24 September 2005. MOST TREES ARE STILL BRIGHT green, but the forest is now becoming a rich palette where individual trees have definition, as those still green contrast with others—the black-greens of the balsam firs, and here and there some gold and orange and a few points of bright red brilliance from the maples. The colors are most impressive when the sky is leaden, when drifting clouds bring diffuse light that illuminates the colors—bright sun bleaches them out.
Caterpillars of many kinds are common this fall. Now, in late September, is a good time to find some of the big moth caterpillars. Since many of the birds are already gone, perhaps they are now safer from birds—but they are never safe from parasites. I have kept track of a waved sphinx moth (Ceratomia undulosa) caterpillar on an ash sapling as it fed on leaves, trimming them down along the edge instead of leaving conspicuous holes, and then clipping off the remaining uneaten portions. I predicted it would not clip off the last few leaves before it left the plant to pupate. Suddenly it stopped feeding entirely, and for a couple of days it just hung from a leaf in the “sphinx” position. One morning its green skin was covered with ninety-one freshly spun white braconid wasp cocoons. Its skin had little dark puncture wounds where the larvae had poked (chewed?) through to come out.
On sunny days, I hear the constantly chirping crickets, see darting dragonflies, and watch the monarch butterflies heading high and low over the fields and forests, all heading south. The birds are long silent. Or so it seemed until the grackles came through.
I have not seen a grackle or a red-winged blackbird for months. After the geese, they had been the first birds back in our bog in the spring, coming in small groups of a dozen or fewer, but always staying together except during nesting. They built their nests in what looked like a loose community. They raised their young, and then disappeared back into their flocks and left the bog.
Fig. 37. Braconid wasp cocoons spun by larvae freshly emerged from the sphinx moth caterpillar on which they are loosely attached.
I wondered where they went. Today, on my jog through a wooded pass near a cliff where ravens nest, I found out. I came on a swarm of thousands of grackles. Long before I got close to them, or vice versa, I heard their ruckus of squeaks, squeals, screeches, and nasal “ticks,” “tucks,” and “tocks” that together made a roar. A broad black stream of the birds braided through the branches of the maples, oaks, birches, and cherries. I stopped and stood still, mesmerized as a river of them passed over and around me. A crowd would build up in one tree; others would come to join it; more and more would join the crowd; and suddenly they all lifted off in a roar of wings accompanied by momentarily silenced voices. The roar would subside and the chatter resume at another tree. Black streams of them kept flowing by, sounding like wind blowing through trees. In a few moments any one stream would be reduced to a trickle, and then the volume of the flow would pick up again.
The birds of this grackle aggregation were all foraging in the treetops, where many of them were tearing open silked-together maple leaves with little green caterpillars inside. I had seen a similar swarm of grackles several years earlier, but later in the season—perhaps October or early November, after the leaves had fallen. Those birds had foraged on the ground in the forest and they proceeded like a giant wheel, with those on the advancing front flying ahead of those already on the ground, then falling behind as the ones in the rear overtook them, to again fly ahead. And thus they advanced through the woods like a vacuum cleaner, presumably sweeping up food as they went.
Grackles are highly social animals, and as such they have a “personality” that we find appealing; they cozy up to each other, or to us if we become part of their group. Personality shows up through contrasts, and nowhere was the difference more obvious than between a young robin and a young grackle that we raised at the same time. Our pet grackle, named Crackle, followed us—especially when he (or she) was hungry; he would then come instantly if we called him by name. Crackle even followed us into the house; and he banged against the window a couple of times trying to get in, one time hitting it so hard that he got knocked out—we were thankful he revived. He also begged at the door whenever we made some commotion inside. The robin, by contrast, perched on the porch under the branches of our shading cherry tree, and there it stayed and peeped. It seemed to be rooted in place as though wanting us to come to it. The robin gaped in our presence, but it would orient itself only to the food. Crackle looked at us and oriented to the person.
The robin quickly searched for and found worms. Crackle found very little on his own, but he picked up and examined all odd objects we showed him. The robin was hardwired, fixated on worms and uninterested in anything novel. Both birds ate worms, but the robin swallowed them expertly. As soon as it got even just the tip of a worm in its bill—bam, it got the whole worm down the gullet. Crackle had to wrestle with each worm, and as often as not the worm escaped from his bill. Crackle was quick to hide under a tarp in the rain. The robin hunkered down in the rain and got soaked.
Grackles must have some good reason to travel (and sleep) in huge crowds. Aside from the trivial proximal reason that they like company, what is in it for them? What is the selective advantage that makes the grackles want to be associated with others? There are many possible mutually nonexclusive reasons, such as safety in numbers; sharing information so as to find food, identify an enemy, or issue a warning; and better access to food (such as by flushing prey). But I doubt that these late-summer foraging crowds flush much prey for each other; the insects they forage from don’t fly or drop from the leaves. Better access to food, such as raven crowds gain by overpowering strong defenders, is also not a real option. Mutual education—showing each other where to find food—seems possible, but more likely they also compete for food. Only one thing is certain—they already live in a different world from the one they inhabited all summer, even though they have not yet even left the state.