Sunday, March 9, 2014
I am going to delay the second and third columns on the results of Maine’s Christmas Bird Counts. Ice storms and snowstorms have forced the postponement of many Maine CBCs. So I have little to report since the first column.
Today let’s think about chickadees. These endearing birds are regulars at any feeding station. Sometimes 10 or more may be seen at a feeder at once. However, the total number of individuals visiting your feeder is far more than that. I’ll describe some of my own research to demonstrate that point.
During the winter, black-capped chickadees form season-long flocks. The flock is made up of a local pair of adults joined by usually between eight and 12 juvenile birds (none of whom are the kids of the adult pair). Often this cohesive flock is joined by one or two white-breasted nuthatches, red-breasted nuthatches, tufted titmice, golden-crowned kinglets or downy woodpeckers.
The flock defends a large territory (10-25 acres) against incursions by neighboring flocks. A flock makes sure it has exclusive access to the food in its winter territory.
But what happens when we put out well-stocked bird feeders through the winter? Ecologists know that territorial behavior is only seen when the benefit of having sole access to resources is greater than the cost of defending those resources. If food is essentially unlimited, what is the benefit of defending a territory? Shouldn’t there be enough food for all?
To explore these questions, I set up a number of feeding stations in the spruce-fir forest on the eastern bank of Flagstaff Lake. Few people live along the upper part of Long Falls Dam Road in the winter so I knew that the chickadees there would not have access to other feeding stations.
At each feeding station, I captured chickadees with a mist-net. Each bird was fitted with a numbered, aluminum band and two plastic color bands. I used a number of different colors of bands and each bird was given a unique combination of bands. Therefore I could recognize individuals at the feeders without having to recapture them to read the band number.
Over the course of the winter, I conducted four 30-minute observations each week at each of the four feeding stations. For each half-hour, I recorded each visit to the feeders by chickadees, identifying the color-band combination of banded birds and others as unbanded.
These censuses got manic at times. Once I had 374 visits in a half-hour. I could only keep up by recording the data into a hand-held recorder with my other hand holding by binoculars to determine the color-combinations of visitors.
At the end of each weekly census, I refilled the feeders. The feeders had sunflower seeds continuously from late October until the end of March.
At the end of the study, I used a technique called mark-recapture analysis to determine the total number of chickadees visiting each feeder. I knew how many banded birds I had at each station (15-20) but I had to use the mark-recapture technique to estimate how many unbanded chickadees were taking advantage of my sunflower handouts.
Mark-recapture is a straightforward technique. One marks some individuals of a population (the color-banded chickadees in my case). Future sampling should yield a mix of marked and unmarked individuals. If most of the individuals subsequently sampled are marked, one can infer that the unmarked individuals are relatively few in number.
In my case, feeder visits by unmarked birds greatly exceeded visits by banded birds. The mark-recapture software I used indicated that between 80 and 120 chickadees were visiting my feeders each week. But the birds still maintained their flock integrity so I rarely would have more than a dozen birds at any time. Territories broke down when food became unlimited.
Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at: