Mixed Flocks

by Carl Strang


Yesterday at Mayslake I saw 15 juncos, 2 song sparrows, 3 downy woodpeckers, a goldfinch and a white-breasted nuthatch apparently moving together as a mixed flock. I’m more accustomed to seeing such behavior in the fall, so today I want to provide some background for this phenomenon.




At the September peak of the fall songbird migration, I have found that the quickest way to the greatest diversity of warblers and other migrants is to listen for the chickadees.




Black-capped chickadees are on the verge of forming their winter flocks and are very vocal at that time of year. From the standpoint of a naïve migrant, tired and hungry from the night’s migratory jump south, the chickadees represent a familiar bit of normalcy in an unfamiliar location.


Yes, I know, I’m projecting human emotions onto distinctly non-human animals that in fact are closer to dinosaurs than to us. But hear me out. I’ve gone north in the late summer and early fall, and I have seen how the black-capped chickadees up in Canada attract other birds to them like dung beetles to, well, dung. Chickadees are active and vocal, easy to find in the quiet autumn forests. Other birds easily can keep track of where the flock is going just by following the chickadees. When the migrants drop down out of the sky at dawn they are entering unfamiliar territory. Where’s the food? Where do the local predators hang out? The chickadees know. They are in the area year ‘round. They are active, vocal, and easy to follow.




There is evidence in the scientific literature supporting both predator avoidance (lots of eyes and ears on the watch, also the ability to hide in the flock) and improvement in individual foraging efficiency (for instance, one bird may catch an insect that is flushed by another) as reasons why mixed flocks form. I see no reason why both advantages can’t play a role, perhaps with different importance at different times.


I have thought that it would be an interesting inquiry to see if there are consistent patterns in the structure of mixed flocks. Patterns could be spatial, with different species or individuals holding consistent places within the arrangement of the individuals of the moving flock. There could be rules governing the number of species, number of individuals, or distribution of individuals among species. But so far, I just have some partial counts of the members of a few mixed flocks.


How to collect the data? In September the trees still are fully in leaf, and it’s a challenge just to find all the members of a flock let alone keep track of them all. But it’s fun to find the flocks and tally their members, keeping alert to possibilities, hoping a hint of a pattern might show itself.


Here are just a few examples of flocks I’ve seen:


  1. Algonquin Park, Ontario, Mizzy Lake Trail, 29 August 2001, at an edge between mixed forest and a lake. Black-capped chickadees, several black-throated green warblers (appear to be sticking together to form their own group within the flock), at least 1 blue-headed vireo, 1 female or young blackburnian warbler, 1 chestnut-sided warbler, and 1 Tennessee warbler.
  2. Algonquin Park, Bat Lake Trail, 30 August 2001. Black-capped chickadees, red-breasted nuthatches, golden-crowned kinglets, 1 or 2 black-throated blue warblers, at least 1 Tennessee warbler, yellow-rumped warbler. The first three species are the vocal ones. These flocks are distinctive: you go for hundreds of yards seeing or hearing no small birds, then suddenly there is one of these diverse groups in a small area.


Golden-crowned Kinglet

Golden-crowned Kinglet




  1. West DuPage Woods Forest Preserve, West Chicago, Illinois, 7 May 2000. Several blackpolls in a mixed flock with yellow-rumps, chestnut-sideds, a golden-winged, a blue-winged, some Tennessees, a couple black-and-white warblers, several scarlet tanagers and rose-breasted grosbeaks of both genders, a Swainson’s thrush or two, a couple indigo buntings. No chickadees in this flock, but in spring migration many birds are singing so flock cohesion is easier.
  2. First mixed flock of the 2008 fall migration observed at Fullersburg Woods, Oak Brook, Illinois, on 25 August has 2 chickadees, a downy woodpecker, a Tennessee warbler and a Canada warbler.
  3. Fullersburg, 29 August 2008, a mixed flock with 3 chickadees, 3 Tennessee warblers, a white-breasted nuthatch, a magnolia warbler, a parula. Chickadees were doing a lot of hanging upside down, Tennessees less acrobatic running along tops of branches and reaching, magnolia and parula more rapid movements, hopping between branches, nuthatch on bark, all in top half of canopy.
  4.  Kettle Lakes Provincial Park, Ontario, 13 September 2008. Large mixed flock in an area around 75 yards in diameter: at least 2 black-capped chickadees, 5 golden-crowned and 4 ruby-crowned kinglets, 4 yellow-rumped warblers, 2 red-eyed vireos, downy woodpecker, black-and-white warbler, black-throated green warbler, redstart, red-breasted nuthatch. I’m hearing white-throated sparrows, but they seem all near the ground rather than up in the trees with the others. Weak songs from ruby-crowneds, the black-throated green and the black-and-white. This is mainly an area of aspens with some jack pines.


Clearly there are many species that can be found in these mixed flocks, and resident birds like downy woodpeckers and white-breasted nuthatches are common members. Descriptions like these are the only data I have so far, but it’s fun just to find the flocks and try to account for all their members. Maybe some day a pattern will show itself.




For now, I’ve got yesterday’s mixed flock to ponder. No chickadees were present, but with the vegetation open, now that leaves have dropped, maintaining visual contact is easier especially with so many individual birds involved. Yet another observation for my growing files on the subject.

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