Vigilance (scanning) and other behaviour patterns were studied in free-ranging evening grosbeaks (Coccothraustes vespertinus) at feeders to assess how flock size and flock geometry influenced the behaviour of individual birds. The present results indicate that the way in which individual grosbeaks are positioned with respect to one another affects many aspects of their behaviour, especially when a flock contains four or more birds. Birds in a linear array which have difficulty seeing one another, when compared with individuals organized in a circle which can easily see one another: 1. are more vigilant; 2. change their head and body positions more often; 3. react to changes in group size more slowly; 4. show less coordination in head movements; and 5. show more variability in all measures. These differences in behaviour can be explained from a cognitive ethological perspective that favours intentional or representational explanations. Specifically, the data suggest that individual grosbeaks, when scanning and moving about, are visually monitoring the flock in which they are feeding and gathering information about a number of variables including flock size, what others are doing, where others are, which individuals are present, phenotypic features of flock members, food resources, or the location of potential predators. Individuals likely use visual records of the behaviour and perhaps the phenotypic features of others, and this information influences various aspects of their behaviour.