A distance-dependent estimation of foraging ranges of neighbouring bird colonies

A reliable estimation of foraging ranges is often an indispensable prerequisite of research in animal ecology and in species conservation. In colonial species, home ranges of members of one colony are frequently overlapping and for practical and theoretical reasons, it can be necessary to assess the foraging areas of whole colonies. Here, we show a method to calculate foraging areas for 13 neighbouring colonies of great white egrets (Ardea alba). Observations of flying individuals and aerial survey data of foraging egrets were combined to in three calculation steps. Firstly, we examined flying paths of egrets from six different colonies to the foraging areas and found an exponential decrease of bird density with increasing distance from the colony. Secondly, we used this distance relationship to calculate the probability of colony membership of 2885 foraging egrets, which were counted by nine aerial surveys. Third, the spatial distribution of these memberships was then basis for calculating the final colony foraging areas. We suggest that distant-dependent foraging patterns could be used in other colonial organisms to assess colony membership in larger sets of survey data. Furthermore, we discuss probable causes for the exponential shape of the distance function in great white egrets.