Identifying songbirds by flocking behavior

It’s early morning and you’re out on a bird walk following your guide under a gray sky. A flock of small songbirds flits over the distant treetops. The birds reveal no field marks (or so you think), so you’re surprised and mystified when the birder leading the group says matter-of-factly, “Eight House Finches.”

“How is that possible?” you wonder. They were just silhouettes flying away, no color or pattern was visible, and there must be hundreds of possibilities.

Well, for starters, there are not hundreds of possibilities. You can be certain that you will never see a flock of towhees or vireos or wrens flying over the treetops. In any given area, only a few species are commonly seen flying long distances in cohesive flocks: finches (redpolls, siskins, goldfinches, crossbills, rosy-finches, etc.), blackbirds (also including grackles and cowbirds), and waxwings. These birds tend to form neat oval flocks.

Other species that often fly in more or less cohesive flocks are larks, pipits, starlings, robins, bluebirds, Yellow-rumped Warbler, a few sparrows (Lark, Vesper, Savannah, Lark Bunting), longspurs, Snow Bunting, and meadowlarks. These species tend to form loose, straggling flocks.

It’s easy to overlook the fact that flocking is a field mark. Maybe it’s not much, but it greatly reduces the number of species you have to consider to make an ID. Expectations are one of the most powerful clues we have for identifying birds. In the same way that we build up mental lists of species that we expect to see in a certain place and time, we can build lists of species based on flying in flocks, flying in lines, diving underwater, and other characteristics.

Once we have a list of expected flocking species for our local area, it takes very little in the way of traditional field marks to identify a flock of House Finches — fairly long square tail, no markings on tail, birds not black.

Besides the tightness of the flock, other important clues to focus on when studying a flock of small birds in flight include:

• Size and shape of the birds – especially wing and tail shapes.
• Landing site – this might reveal habitat preferences: pipits usually don’t land in treetops, and Yellow-rumped Warblers usually don’t land on the ground.
• Flight calls – In most of these species, vocalizations are distinctive and frequent. The lack of calling can also be a field mark. Cowbird flocks are generally silent in flight, unlike blackbirds and grackles.

The keys to learning all of this are simple awareness and experience. If you challenge yourself to identify flocks when you see them, it will soon become second nature.

This is a slightly modified version of a column originally published in Birder’s World (now Birdwatching) in December 2007.