Behavioral Ecology and Socloblology 9 by Springer-Verlag 1978 The Three-Dimensional Structure of Airborne Bird Flocks

Summary. The three-dimensional structure of flocks of dunlin, Calidris alpina, and starlings, Sturnus vulgaris, was studied while birds were in transit between feeding, loafing and roosting sites. A technique was developed that uses standard photogrammetric methods to determine the three-coordinate posi-tion of birds in flocks from stereoscopic pairs of simultaneously exposed photographs. A comparison of nearest neighbour distances indicates that dunlin have a tighter, more compact flock structure than do starlings (Fig. 2; Table 2). Analysis of interbird angles in both the vertical and horizontal planes indicates that each dunlin's nearest neighbour is most likely to be behind and below it. This spatial structure results in areas in which few nearest neighbours occur (e.g., immediately in front and below) (Fig. 3). Flight speeds during transit flights are also presented (Table 4). The spatial structure and behaviour of dunlin and starling flocks appear to be very similar to the structure and behaviour of schools of fish. In~oducfion In order to study the sensory basis of group formation in animals, or to compare such formations between species and environments, it is first necessary to have an adequate description of the structure of the social group itself. Although two-dimensional analyses of certain structural attributes of bird flocks have been attempted using both radar (e.g., Williams et al., 1976) and photographic techniques (e.g., Miller and Stephen, 1966; van Tets, 1966; Nachtigall, 1970; Gould and Heppner, 1974), measurement of the internal three-dimensional struc-ture of airborne flocks has never been accomplished. The purpose of this study was twofold: (1) to develop a stereoscopic camera technique to study the three-dimensional structure of flocks of small birds in the field, and more specifically (2) to characterize the structure and behaviou