A photographic method of estimating densities of bird flocks in flight

Densities of bird flocks in flight were calculated from photographs for 60 species which ranged in size from 35.3 lb mute swans to 0.046 Ib cliff swallows. An average density of 37 lb per 1000 cu ft of airspace with a standard error of 13 lb per 1000 cu ft was found. The flock densities were found to be inversely related to a "gangliness" factor expressed as the cube of the total length divided by the weight of a bird.