For a couple of minutes, chickadees are the only birds I see—cute for sure, but also common and not that exciting. Then, a movement catches my eye that registers as different. I look through binoculars, and yes, a bright yellow face streaked with black: a Townsend's warbler.
The longer I stand there, the more movement I see all around. I spot a brown creeper, working its way up the trunk of a large oak, perfectly camouflaged with the bark.
Towhees kick up the leaf litter. Bushtits mingle with the chickadees. A pacific-slope flycatcher flutters out from a branch and back to its perch.
Forest birds often travel in mixed flocks like these. I first learned about this phenomenon in the rain forest in Costa Rica, but it's common here in the Bay Area as well. Mixed flocks allow birds the benefits of foraging in a group—more eyes and ears to spot danger—with less competition for food than there would be in a flock of just one species. Chickadees gather bugs and seeds mostly from small branches and leaves. Flycatchers grab flying insects out of the air. Towhees find their food on the ground.
This is an especially good time of year for watching mixed flocks because our resident birds, such as chickadees, towhees, and creepers, are joined by migrants on their way south, such as Townsend's warblers. Townsend's warblers nest in conifer forests of western Canada and the Pacific Northwest. Some winter in a narrow band along the coast from Washington to Baja, but most travel on to Mexico or Central America. Their lifestyle changes quite a bit in different parts of their year: in nesting season they live high in old-growth conifers forests, eating mostly insects. During migration they come lower in branches of all sorts of their trees, and eat a lot of flower nectar. In winter in Mexico and Central America, many guard trees infested with scale insects that secrete a sugary liquid that the birds eat.
I wonder why the mixed flock behavior evolved for some species but not others. When I notice chickadees, I always scan for other species, but dark-eyed juncos—another of our most common forest species— almost never seem to join mixed flocks. I wonder how a mixed flocks joins together. Do the birds happen upon each other by chance and just stick together? Do they seek each other out using calls? Or start the day in the same area each morning?
Each time I happen upon a mixed flock, the mix of species is different. While chickadees are almost always a main ingredient, sometimes oak titmice, bushtits, or ruby-crowned kinglets form the nucleus. The mix may include woodpeckers, nuthatches, or vireos. A mixed flock is a dynamic and eye-catching example of the interweaving—through competition, cooperation, and micro-habitats—of different species in any ecosystem: the mix of live oaks, bay trees, and ferns along the seasonal creek bed in the canyon outside my work; the spring collage of lupine, poppies, and checkermallow by Volmer Peak in Tilden Park.
LINKS:
Washington Post article about mixed flocks on the east coast
Cornell Lab of Ornithology Information about:
Tropical Nature: Life and Death in the Rain Forests of Central and South America, a wonderful book where I first read about mixed flocks
I really appreciate the links you added! I love thinking about mixed flocks helping provide look outs for everyone in the flock without competing for food. This is so lovely and informative.
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