Over the weekend I witnessed interesting behaviour in a flock of swallows along the Speed River. Friday afternoon on the way home from work I stopped in Riverside Park and found a mixed-species flock of swallows feeding over the river just south of Woodlawn Bridge. There were approximately 200 birds of three species, predominantly tree swallows with large comparable proportions of barn and northern rough-winged swallows.

A strong breeze blew almost parallel to the current. Numerous tiny winged insects appeared to be struggling on the surface of the water. Perhaps the breeze had prevented them from escaping it, or perhaps the intense bird activity had driven them down. The swallows would fly swiftly downwind for a distance, then turn and use the headwind to slow them so they could dip close to the surface and pick up bugs on the way back upstream.

I was suprised to see so many birds of different species feeding in one place, and wondered whether it might be a co-operative strategy. Possibly they all happened to be in one place because the food was there. However, something else happened to indicate a social dynamic among the species.

While feeding they seldom vocalized. Suddenly something excited the birds. Perhaps they had detected a bird of prey. All at once they rose above the treetops and began vocalizing noisily. They continued to circle, remaining in a distinct flock. This lasted for 30 seconds, then the swallows returned to the river, resumed feeding, and became quiet again.

An article from Stanford University’s website discusses mixed species flocking. Wikipedia also offers a useful entry on this foraging behaviour. It is fairly common among insectivorous birds such as chickadees, nuthatches, woodpeckers, warblers and vireos in North America, other species in different parts of the world. Scientists have some theories about why they do this, for example the mixed flock may protect less vigilant species from predators so they can concentrate on foraging. Research suggests it is more likely to occur when food is scarce.

Food scarcity probably brought the swallows together. When I asked Chris Earley, interpretive biologist and education direction at University of Guelph Arboretum, about what I had seen, he pointed out that cold weather has not favoured the emergence of insects. The swallows would stand the best chance of finding food close to the river.

The literature scarcely mentions swallows, indicating only that they “rarely join” mixed species foraging. Possibly the swallows appeared together only because they had found the best source of food. However, interaction within the flock suggested something more complex.

I expected it to be an isolated incident, but I was wrong. Two days later, on Sunday afternoon, I went for a walk near the covered bridge, several kilometres downstream from the first location, and again encountered dozens of swallows feeding together over the river. The numbers seemed slightly less, but the proportion of species was similar. Evidently a few individuals had parted ranks, moved to breeding territory or perished, however I believe I stumbled coincidentally on the same group of birds in a different place.

On this occasion I witnessed pairs of birds flying in close formation over the water several times, usually a tree swallow with a northern rough-winged swallow. Tree and barn swallows appeared to be slightly stronger, more agile flyers, however all three are highly competent. The formation flying appeared to be deliberate, however I can only speculate about its purpose.