A method for the detection of non-random associations among flocking birds and its application to sanderlings Calidris alba wintering in N.E. England

Summary Indices of association are frequently used in studies of social behaviour. Observed association indices may be compared with those obtained from computer randomizations of the data in order to test whether any two individuals in a population tend to occur together more than would be expected by chance. Data for individually-identifiable colour-ringed sanderlings Calidris alba are presented to illustrate the importance of the assumptions on which such null models are built. When each individual was given an equal chance of appearing in each flock associations differed from those expected by chance. But when the different numbers of times that each individual occurred at the study site were taken into account associations were not different from those expected by chance. Hypotheses relating association levels to a number of factors were tested. None of the factors examined had significant and appreciable effects on association levels. In particular, there was no evidence for sanderlings moving around in stable groups. Taking into account the different distributions of different individuals, the population of birds present on any day represented a random sample from the population of birds seen during the winter.Correspondence to: G. Roberts