How many black-capped chickadees do you have coming to your bird feeder? Two or three, some of you may say. Others who watch diligently may have a higher estimate—perhaps 5 to 10? But when ornithologists have started banding chickadees at feeders, they typically discover the number is much higher than any of us imagine.

Herb Wilson, an ornithologist at Colby College, conducted an interesting experiment in 1995 that sheds light on this question. Herb placed feeders along a remote stretch of road in Flagstaff, Maine, where there were no homes or other buildings. He counted the chickadees coming in to the feeders throughout the winter and, not surprisingly tended to tally between two and eight each time.

But for us humans it is almost impossible to tell one individual chickadee from another. Herb was aware that it was possible that even though we may see only a few chickadees at our feeders at one time, there may be more than that number that are coming and going. To find out if this was true, Herb banded as many chickadees as he could at each feeding station in November. Each chickadee received an individually unique combination of color bands so he could tell them apart by sight in the future. As the season progressed, he surveyed the birds on a weekly basis and noted how many birds coming to the feeders were banded and how many were not. He also kept track of which individual banded birds he saw.

Herb then used the data that he collected to carry out something called a mark-recapture analysis. The idea behind a mark-recapture analysis is fairly straightforward. Imagine if you had a pocket full of pennies. You reach in and take out 10 of them, color them bright orange, and put them back in your pocket. You mix all the pennies in your pocket with your hand and then pull out (recapture) 10. If one of those 10 pennies is one of your marked orange ones then that tells you that about one in ten of all the pennies in your pocket is marked. And if your 10 orange-marked pennies make up one-tenth of all the pennies in your pocket, then you must have about 100 pennies in your pocket. If, on the other hand, you found 5 orange pennies out of 10, that would tell you that there were only about 20 total pennies in your pocket.

Using this same idea with marked chickadees, Herb discovered that his feeders were being visited by a remarkable number of birds through the season—20 to 110 individual birds! As you can see from this example, figuring out the total number of birds or other animals in a given area or population is not always as easy as it may seem. Add to the complexity of population estimation the issue of animals that are difficult to see or hear and that move around a lot and you can quickly begin to appreciate the difficulties. Fortunately, biologists, statisticians, engineers, and other types of scientists have developed an increasingly sophisticated set of tools and techniques that provide better population estimates. And those population estimates are often critical in determining the best course of action for managing habitats, harvests, and what species may need conservation actions.

Jeffrey V. Wells, Ph.D., is a Fellow of the Cornell Lab of Ornithology. Dr. Wells is one of the nation's leading bird experts and conservation biologists and author of “Birder’s Conservation Handbook”. His grandfather, the late John Chase, was a columnist for the Boothbay Register for many years. Allison Childs Wells, formerly of the Cornell Lab of Ornithology, is a senior director at the Natural Resources Council of Maine, a nonprofit membership organization working statewide to protect the nature of Maine. Both are widely published natural history writers and are the authors of the book, “Maine’s Favorite Birds” and “Birds of Aruba, Bonaire, and Curaçao: A Site and Field Guide” from Cornell Press.