Statistical approaches to measuring species’ associations in mixed-species bird flocks

Emily Silverman, US Fish & Wildlife Service, Maryland

Measuring species associations in flocks is important for both a theoretical understanding of flock organization and the design of conservation plans for bird communities where flocks are common. Traditional analyses of species associations use tables of co-occurrence, followed by Chi-Square tests. Problems with this approach include a potential lack of independence between flocks and an inability to determine species associations for species that are abundant in flocks. In addition, when multiple habitats are sampled, positive and negative patterns may be the result of habitat affinities, and the spatial structure of the data is ignored. We develop a model to generate flock data in which species associations are known. We compare and contrast traditional analyses of association with a new approach that uses locations, and not flocks, as the unit of analysis. This approach measures the density of birds inside and outside of flocks and the number of flocks by location, thus accounting for the differences between locations in the pool of birds available to flock and the opportunity to enter flocks.