BIRDS OF A FEATHER

In honor of the wintery nip the weather is finally starting to carry, I'm recycling an old post from September 2006.
The temperature in Salt Lake City has yet to drop below 50ºF, but signs of summer's senescence increase daily. Bird migration is well underway, and our least cold-tolerant summer residents, the nighthawks and hummingbirds, are gone. As the sun's increasingly oblique rays approach the horizon, the red and yellow maples and aspens on the surrounding hillsides cast an orange evening glow into the valley. I wasn't struck hard with an awareness of autumn, though, until last night, when I joined a friend for drinks shortly before dusk. As we entered the club, the assiduous squawking of a thousand Common Starlings (Sturnus vulgaris) met us from the interior of an overhead billboard. It's a sound that I know well, and associate strongly with cold weather. For many years, I was obsessed with hawking starlings with Cooper's Hawks (Accipiter cooperi) and, for one season, with a Merlin (Falco columbarius). A major benefit of flocking is evident when watching a raptor chase a group of starlings or other birds. Upon sensing danger, the flock condenses to a nearly solid mass, and appears to move with a singular intention. The skill with which flocking birds can cue each other and fly as one seems supernatural. Unable to snatch an individual from the fluttering swarm, the pursuing predator is reduced to taking swipes at the entity until one bird loses its head and its timing and finds itself alone and vulnerable. The effort needed to take a starling from a flock exceeds that expended on a single bird by a sizable factor.

Starlings aren't alone in their propensity to get close in the winter, in fact, winter flocking is more the rule than the exception. Starlings' fellow immigrants, the English Sparrows (Passer domesticus), are forming similar coalitions in the city, and soon they'll be joined by a host of other flocking species. In summer, Dark-eyed Juncos (Junco hyemalis) are uncommon and inconspicuous solitary birds here, occasionally glimpsed as a single bird flits through high-elevation undergrowth. As soon as cold weather hits, though, flocks of the fat little birds with their executioners' hoods will be ubiquitous throughout the region. In fact, it seems that on the first really cold morning of each year I see my first junco flock, as if winter were dragged right into the yard on their white-edged tails. In my three Pinedale Anticline posts, I described the impressive winter flocking of Sage Grouse (Centrocercus urophasianus) and Horned Larks (Eremophila alpestris). Clearly, flocks are less susceptible to predation, but why do birds concentrate only in the winter, when food tends to be scarce, and competition more of a concern? Since the days of Aristotle, observers have pondered this question. In the thirteenth century, Frederick II of Hohenstaufen asserted that flocks protected birds from predators on their long migrations. Other writers have pointed to birds like robins (Turdus spp.) and waxwings (Bombycilla spp.) that feed largely on berries in the winter. These foods are abundant where they occur, but can be difficult to locate. A large group of birds stands a better chance of discovering a large lode capable of feeding the whole. Flocks have an advantage not only over predators, but over competitors, as well. Chickadees (Poecile spp.) and other small birds show far more aggression to single conspecifics than to groups.
But birds aren't the only creatures that show this behavior. In my Pinedale Anticline posts, I also discussed winter herding of Pronghorns (Antilocapra americana), which is similar to the behavior of cervids. Even tropical deer like the Indonesian Sambar (Cervus timorensis) congregate in small groups during the boreal summer, which is the dry season there—the season when food is most scarce. When I painted The First Phalanx (above), I had read that troops of Mandrills (Mandrillus sphinx) periodically coalesce into large herds led by a coalition of alpha males. I naïvely painted four big males surrounded in a riot of Central African blooms, unaware that these herds form only in the dry season, when such flowering isn't likely. Five years later, I tracked a large herd of the closely related Drill (M. leucophaeus), an easy job, since their fastidious searching of the dry-season forest floor gave the impression that a Zamboni had driven through the jungle. It makes sense that these large monkeys might find it easier to discover populations of mushrooms and small edible animals in large groups, which are also more intimidating to Leopards (Panthera pardus).

All of these benefits, and surely others as well, accrue to the flocks, herds, murders and gaggles, but I think to better understand the problem, the question should be turned on its head. It seems to me that most animals that congregate in winter are better described as gregarious animals that leave the pack to breed. Competition for food may be more severe in the winter or dry season, but even modest competition is too much for inexperienced juveniles. Most creatures are born during the season when foraging is the easiest, but even so, for many species the protection of the flock is less of a benefit than a liability during this crucial period.
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upper: WORKING THE FLOCK--MERLIN & STARLINGS (1988) acrylic 30" x 20"
lower: THE FIRST PHALANX--MANDRILLS (1990) acrylic 20" x 30"