BACK TO THE BASICS OF ANTIPREDATORY VIGILANCE - THE GROUP-SIZE EFFECT

A negative relationship between group size and levels of individual vigilance is widespread in socially feeding vertebrates. The main explanation of this 'group-size effect', the many-eyes hypothesis, is based on the simple premise that as group size increases, there are progressively more eyes scanning the environment for predators. Thus an individual forager can devote less time to vigilance (and more time to feeding) as group size increases without any lessening of the group's ability to detect an attack. Basic to this hypothesis is the assumption of collective detection: that all members of the group are alerted to an attack as long as it is detected by at least one individual. In addition, an important presumption associated with the many-eyes hypothesis is that individuals monitor the. vigilance behaviour of their groupmates in determining their own level of vigilance. Neither the idea of collective detection nor behavioural monitoring received strong support in an experimental study of vigilance in mixed flocks of dark-eyed juncos, Junco hyemalis, and American tree sparrows, Spizella arborea. The lack of support for behavioural monitoring was particularly evident; however, some degree of collective detection was apparent. It is possible that anti-predatory rules-of-thumb may explain the group-size effect while keeping intact the basics of the many-eyes hypothesis.