Mixed Species Foraging Flocks

By Terry Rich, Golden Eagle Guest Blogger

We have been fortunate this fall to have mixed species foraging flocks in many places along the Boise River. I have repeatedly run into them at Barber Park, Kathryn Albertson Park, and Eagle Island. Most recently, a flock at Barber Park had Downy Woodpeckers, Warbling Vireos, Black-capped Chickadees, Bewick’s Wren, Gray Catbirds, Song Sparrows, Nashville Warblers, Yellow Warblers, Yellow-rumped Warblers, and Western Tanagers. Other flocks have included Western Wood-Pewee, Dusky Flycatcher, Cedar Waxwing, Common Yellowthroat, Black-and-White Warbler, Wilson’s Warbler, and even a Chestnut-sided Warbler.

What to Look For

A mixed species foraging flock is one or more individuals of two or more species moving together through the woods while foraging for food. Another typical feature of these flocks is that they keep moving. One minute you are surrounded by birds, and the next it’s crickets. With a little luck, you can move on and find the flock again. A recent flock at Kathryn Albertson seemed to be moving in a large circle. If you just stay put, they might swing by you again.

At our latitude, we most often find mixed flocks in winter and migration. During the breeding season, most species are defending territories so they don’t form flocks. But once the breeding period is over, many species become more social.

Yellow Warbler by Terry Rich

There are three likely advantages to this flocking behavior. First, members benefit from the knowledge other members have about where the food is. Second, one bird might catch an insect or other prey flushed by another bird. With more birds, you have more disruption in the vegetation. And third, they benefit from the many sets of eyes that might detect a predator more quickly than can a few birds. In fact, I’m surprised I haven’t yet seen a Cooper’s Hawk lurking!

Birds in these flocks forage in their own ways. Woodpeckers work the tree trunks, warblers and vireos glean foliage higher in the trees, flycatchers flycatch, and sparrows and wrens operate closer to the ground. Chickadees forage at all levels in the vegetation.

The presence of two catbirds in the Barber Park group reveals another interesting fact about these flocks. More than two weeks earlier, I found two fledgling catbirds at exactly the same spot. This suggests those two are living there right now and joined the other species as they moved through the area. Studies of large mixed flocks in the tropics have shown that, in some species, birds join a flock while it’s moving through their territory but then remain behind as the flock moves on.

Black-capped Chickadee by Terry Rich

One other common feature of these flocks is that there is typically a species that leads the way. When birding in the tropics, birders are always excited to hear the calls of a lead species because it means there is a mixed flock nearby. It turns out the most common leaders in mixed species flocks north of the tropics are chickadees. There are 15 species in the genus of our Black-capped and Mountain Chickadees (Poecile). Maybe we can give these little guys more credit now than we did yesterday.

Enjoy the Flocks

Mixed-species flocks like this have been found in terrestrial habitats all over the world. Flocks can be composed of a variety of species in different combinations. A flock at Kathryn Albertson had both a Black-and-White Warbler and a Chestnut-sided Warbler. So, when you encounter a flock, be sure to keep looking through them for that rarity. But even more than that, just enjoy this fleeting phenomenon. Soon enough, most of these birds will be gone. You can reach Terry at terryrichbrd@gmail.com.