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Posted by Ken Schneider on 15:13:57 12/23/10
This morning, while returning from our walk into the SW section of the Broward County WCA (the "West Miramar ESL") near our home, Mary Lou and I saw a flock of Tree Swallows that looked like a huge smoke plume. We were standing near the center of the graded and unpaved westernmost section of Miramar Parkway/SW 196th Avenue. The flock spread across about 90 degrees of the horizon (from about 11 o'clock to 2 o'clock, above grass about 4 feet high on levees about 4 feet above our grade and some distant small trees) in front and to our right, about 1/4 mile away. The flock actually faded into the horizon to our right front quarter and also into the distance directly in front of us. Is it possible to estimate the number of birds in this flock?
My camera has a 420 mm telephoto system (300mm f/4 with a 1.4x extender). I did not have a wide angle lens. I estimate that my camers's field of view includes about 10 degrees of horizon and captured about half of the height of the column of birds. My photos included the lowest portion of the flock, where the birds were more densly aggregated.
I took five non-overlapping photos from nearly adjacent sections of the flock that were located mosly in front of me, during a 9 second interval (08:33:10 to 08:33:19). My long lens so limited the field that my photos could not have covered more than 10% of the flock-- possibly as little as 5%.
I selected one photo that was fairly representative of the five shots, taken in about the center point of the flock. This photo was cropped lightly to screen size and sharpened in Picasa. Using Paint.net, I resized my photo to 110 x 87 cm, or 9570 sq cm. I then outlined four 20 x 20 cm squares (400 sq cm each, for a total area of 1600 sq cm) diagonally across a representative sample of the photo, lightly colored each, greatly enlarged the image on the computer screen, and counted the number of birds within each square.
From the upper left, the 4 squares contained 126, 206, 320 and 260 birds, for a total of 912 birds. This works out to 912 birds per 1600 sq cm, or 0.57 birds per square centimeter. Thus, the entire image contained approximately 5,455 birds (9570 sq cm x 0.57 birds/sq cm). If, in fact, my sample included only 10% of the flock, there may have been over 50,000 Tree Swallows in that single flight.
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