How to count birds in a flock

QUESTION: I've seen a flock of migratory birds heading north. I'd like to know how many are in the flock but there are a lot of them and they're moving rapidly. I can't count them one by one. Experienced bird-watchers seem to know how to do this. What's their technique?

The birds are in motion ... Can you count all of them?

ANSWER: You're right. During migration, a species fly in flocks, sometimes hundreds at a time.

You can try taking a photograph and counting them at your leisure. But even with photography it's not always practical. You'll soon have sore eyes!

E-bird recommends this technique for calculating the size of a flock. E-bird is the online service that enables people to record the kinds and numbers of birds they see.

Identify 10 birds and use them as a template

Count 10 birds individually. This becomes your template. Then calculate how many of these templates will occupy the flock as a whole. For instance, you estimate that there are 12 groups of birds, each occupying the same size or density of the flock. Multiply 12 groups by the 10-bird template. The total number of birds in the flock is 120.

If the size of a flock is enormous—perhaps covering an entire sector of the sky—it will becomes a difficult challenge to come up with an accurate tally. Make a guesstimate of how many birds there might be in a particular sector of the flock. Then extrapolate from that. Again, it will be a rough estimate. The birds are in constant motion and some may be swirling about in three dimensions.

Starlings are notorious for these mesmerizing clouds of dips, swirls, pulsations and congregations. This phenomenon is called murmuration. Not even a photograph will do justice to the calculation. There may be two or three or four birds abreast, but the photo shows an image of just one bird.

While it's best to be as accurate as possible, it's better to err on the conservative side when estimating the size of a flock.