The Analysis of the Spacing of Animals, with an Example Based on Oystercatchers During the Tidal Cycle

1. Methods of analysing the spatial patterns of birds are reviewed. Few methods have sought to explain an observed spatial pattern in terms of the behavioural responses between conspecifics. The spacing of oystercatchers, Haematopus ostralegus L., feeding on a mussel bed is investigated using a model of one possible response. 2. The model assumes that each individual avoids all other individuals within a certain distance, the hard-core radius, and has no interaction with other individuals beyond this distance. 3. The positions of the oystercatchers are obtained from a set of photographs. Each photograph is transformed to a plan view, and the resulting set of plan views is assembled to give the spatial pattern of birds on the mussel bed. The observed spatial pattern is summarized by means of the distribution of nearest-neighbour distances. 4. The behaviour of the birds is modelled by placing non-overlapping particles of a given radius within the region of the mussel bed occupied by the birds. The distribution of nearest-neighbour distances for the particles is obtained by simulation. The best-fitting particle radius is that which minimizes the residual error between the observed and simulated distributions of nearest-neighbour distances. 5. Statistical analyses show that avoidance behaviour, as measured by the best-fitting particle radius, decreases during the low-water period. We interpret this result in terms of the changing value of a food item to an individual through time.