A spatial model of antipredator vigilance

The authors are also members of the Centre for the Study of Evolution at the University of Sussex (Email: C.J.Proctor{at}sussex.ac.uk and M.Broom{at}sussex.ac.uk) Many species of animals have to perform two contradictozy tasks:feeding, and avoiding becoming food for others. A large numberof theoretical and empirical studies have investigated the trade-offbetween feeding and antipredator vigilance, especially in birds.An important factor which has been neglected in these studiesis that of the area occupied by the flock. If individuals feedclose together, competition increases and feeding rates decrease.However, if individuals space themselves widely, then vigilanceefficiency goes down and there is an increased predation risk.We develop a vigilance model which allows birds to control thearea the flock occupies as well as their vigilance rate. Theoptimal strategy is found for the birds under a variety of environmentalconditions. In particular the effect of each environmental parameteron this optimum is considered in turn. How the model can beadapted for different bird species is also investigated.