Mixed Species Flocks in the Boreal Forest

Black-capped Chickadees are one of the core species of mixed-species flocks in the Boreal Forest.

Black-capped Chickadees are one of the core species of mixed-species flocks in the Boreal Forest.

This morning, just after rising from bed, I peered out the front window of our cabin to see a mess of songbirds flitting around in the spruces and birches out the window. Through my bleary eyes, and the drizzle outside, I was able to see at least four Black-capped Chickadees, a Boreal Chickadee, half a dozen Dark-eyed Juncos, two Orange-crowned Warblers, and a Yellow Warbler. They were moving together through the trees, the juncos low, landing on the ground to pull a morsel from the grass and leaf-litter. The warblers were higher up, probing the birch leaves and spruce needles, and the Chickadees split the difference, foraging in a tight cluster in the middle and lower branches.

I find mixed-species flocks utterly fascinating. Multiple species of birds (even hundreds of species in the bird-rich neotropical forests of upper Amazonia) will forage together as they move through the woods. I first worked on this phenomenon as an undergraduate at the Evergreen State College. As part of a natural history class, I spent many hours watching overwintering flocks at the nearby Nisqually National Wildlife Refuge. Later, in my final year at Evergreen, I travelled with a group of students to the remote Cocha Cashu Biological Station in Manu National Park, Peru. There, I studied a fascinating group of birds that make their entire living by following raiding swarms of Army Ants and eating the insects the ants flush in front of them. These birds flock together, taking advantage of a rich, but difficult resource.

Dark-eyed Juncos frequently occur in mixed-flocks during fall.

Dark-eyed Juncos frequently occur in mixed-flocks during fall.

Here in Alaska, this time of year, the flocks are common. Post-fledging juveniles and adults are preparing for migration and trying to eat as much as possible. When I was working as a bird-bander a number of years ago, I could always tell when it was going to be a good day based on the amount of flocking activity we saw on the walk out to the station.

Birds gather in these groups for a few reasons. First, more watchful eyes mean a better chance of detecting predators. Watch enough flocks for long enough and you are bound to see a Sharp-shinned Hawk, a Goshawk, or even a ground-dwelling Ermine make an attack. With all those vigilant birds around, it’s a lot harder for a predator to be successful than if they were stalking an individual foraging alone.

Second, birds flock together because it helps in the search for food. Just as more eyes means better detection of predators, it also means more eyes searching for food. Food in a forest is never evenly distributed, it’s patchy. Insects attack a stressed tree, berries ripen in choice areas, certain ponds or lakes produce more aquatic insects than others. When one of these areas of abundant food is found, all the birds in the flock will benefit.

Warbler too, like this male Yellow Warbler are often found in big mixed species flocks during fall, as well as on their wintering grounds in Central America.

Warbler too, like this male Yellow Warbler are often found in big mixed species flocks during fall, as well as on their wintering grounds in Central America.

It gets more interesting. Birds foraging in resource rich areas, that is places with lots of food, tend to flock less than those in more impoverished habitats. Where there is lots to eat, they don’t need the help searching and so they tend to work alone. But where food is scarce, it makes sense to take advantage of a community.

Migrant and resident birds both occur in mixed flocks but it’s the residents that serve as the core species. In the case of my flock this morning, the chickadees were the centerpiece. Chickadees forage in family groups. And since they breed earlier in the season than most of the migrants, too they are out and about with the youngsters long before the seasonal species. As the migrant young begin their independence, the chickadee flocks are already active and the migrants simply follow along. So important are the residents to the flock structure that it’s rare to see a mixed species flock that doesn’t include resident species. As the migrants join, they expand the size and diversity of the flocks. Here in interior Alaska, by late August and early September, flocks can number hundreds of individuals and over a dozen species.

With that number of birds, why isn’t there stiff competition? Interesting question, and one I had when my curiosity was first piqued by mixed flocks. If you spend enough time watching, it’s a question that answers itself: they don’t forage in the same places or even for the same food sources. Sparrows, for example, tend to spend the bulk of their time on the ground or in low branches searching the forest floor for seeds and berries. Warblers are often up higher, in the periphery of the vegetation, gleaning bugs and insect larvae from the leaves and twigs. Woodpeckers, nuthatches, and creepers stick to the trunks of trees. Flycatchers will use the outer branches as places from which to sally out and opportunistically grab flying insects. Chickadees, generalists that they are, will work a number of niches, searching out the most productive spots. Even in my brief encounter this morning, I was able to see how the species distributed themselves through my yard-trees.

I photographed this Alder Flycatcher as it moved with an early-season mixed flock in July, a couple of years back.

I photographed this Alder Flycatcher as it moved with an early-season mixed flock in July, a couple of years back.

Earlier, I noted that mixed flocks are more common in poor foraging areas. This made me wonder whether the prevalence of mixed flocks can be seen as an ecological indicator? In one study I found, the authors determined that in areas with lots of spruce bud-worms, a common and locally abundant food source in the boreal forest, flocks were not as frequent or as large and diverse as in other areas.

Bud worms are good for several species of birds, and some warblers even specialize in them, but the worms are none-too-good for the trees themselves. Spruce bud-worms are responsible for the death of entire swaths of forest across the north and mountain west. In the past, bud-worms were limited by cold winters, but as the climate warms, winters are not as cold as they once were; the bud-worms are booming, killing off trees by the millions. Eventually, perhaps not too many years from now, the budworms are going to eat themselves out of trees and will suffer a population crash. The birds will have less to eat when this happens. Will we see an increase in flocking behavior? I suspect so.

It’s a great irony that one of the most enjoyable aspects of birding: watching mixed foraging flocks, could actually be a symptom of poor foraging habitat.

There is a lot more to find out about mixed flocks, and I still have a ton of questions. But in the mean time I think I’m going to step outside and see if I can’t find another passing flock. I recommend you do the same.

Information in this post came primarily from the following articles:
LaGory, Kirk E. , Mary Katherine LaGory , Dennis M. Meyers and Steven G. Herman. 1984.
Niche Relationships in Wintering Mixed-Species Flocks in Western Washington. Wilson Bulletin Vol. 96(1) pp. 108-116.
Hobson, Keith A., and Steve Van Wilgenburg. 2006. Composition and timing of post breeding multi species feeding flocks of boreal forest passerines in western Canada. Wilson Journal of Ornithology I I8(2):164-172.