It’s late April, and the shoreline teems with sanderlings fueling up for their push toward Arctic breeding grounds. Hundreds of these little birds scurry up and down the saturated sand: chasing, scolding, fluttering, resting, stretching, preening, probing for mole crabs. Their constant chatter seems to confirm, “We’re busy, we’re together, we’re safe!”
Not far offshore, skeins of saltwater birds—Pacific loons, brants, surf scoters—stream northward past the coast. In the season to come this seascape may hold a vortex of seabirds frothing over a ball of northern anchovies. Cormorants and grebes will dive below the surface, corralling the baitfish from below, while brown pelicans crash into the feast from the air. Agile gulls will work every available angle.
All these avian activities bring the adaptive value of flocks into focus. Within a collective, an individual can be more certain of success in its primary goal: eat and don’t get eaten.
Consider the many small songbirds in an alder grove on an early spring morning. Sunlight has just warmed the mid-canopy, and a chorus of call notes means the little mob of insectivores is eager to forage. Some 20 individuals of several species—their chips and trills and yelps all laid upon a matrix of chickadee chatter—begin moving together. As the birds flit and feed, they constantly inform one another of location, direction, and well-being: “Going this way now; staying here for a bit; over here looks good; OK, the coast is clear!”
Multiple species can thrive in such close association because each exploits a particular niche. A Wilson’s warbler gleans minute spider-egg clusters; red-breasted nuthatches pry up flakes of bark; a brown creeper lifts and looks under mats of lichen; various flycatchers and showy warblers make aerial forays to snatch insects mid-flight. Each participant in this movable feast benefits from the combined vigilance of many eyes alert to the gliding sharp-shinned hawk (“Dive! Hide!”) or roosting great horned owl (“Mob it! Call in the jays!”).
But studies suggest there’s a trade-off in these behaviors. For songbirds in a mixed-species feeding flock, the chickadees or their kin are “nuclearspecies,” numerous and vocal. “Follower species,” including the downy woodpecker, stay near these sentinels and relax their own vigilance, despite the additional competition. And sanderlings may squabble over little feeding territories—but when food is either abundant or sparse, competing isn’t worth the effort.
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Science has also plumbed the ways that enormous flocks shape-shift almost magically in flight. Frame-by-frame visual analyses of dunlins on Puget Sound yielded the “chorus-line hypothesis,” which likens their 3D undulations to the wave that sports fans generate in a stadium. Similarly, photographic probes of huge starling murmurations show that each individual member of those rippling shapes reacts to just six or so of its nearest neighbors.
Evading flocks. Traveling flocks. Feeding flocks. Might there be sleeping flocks—a committee of turkey vultures roosting communally at night? Or nursery flocks—the covey of California quail, five pairs together herding their crowd of chicks? Indeed, the very word flock seems inadequate to embrace all these energies. Its etymology is surprisingly spare: 800 years ago, a “flock” was a tuft of wool or bunch of animals. A meager 19th-century linguistic expansion included a group of birds.This may explain the many eccentric English nouns for birds plural: an exaltation, a raft, a muster, a siege.
What might we call the kaleidoscope of sanderlings on the shoreline just now? A collective peep-shriek erupts, and the whole scattered flock explodes into the air. A throng of small bodies coalesces into a tight ball, a ribbon, a zigzag, confounding a hungry merlin’s sharp eyesight. The falcon cannot punch through the swirling mass without risking injury. When she cannot pick off a slower individual, she zips away.
The sandpipers’ fright maneuvers gradually relax. Their “safe now” consensus returns. Settling back along the tide line, they take up where they left off—finding food morsels, snatching someone else’s, scratching an underwing, napping with one eye open—all the while constantly chattering together.