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Bird flocking as a foraging strategy Thompson, William Andrew

Abstract

To assess the survival value of bird flocking, a simulation model was constructed based upon known bird behaviours. The primary source of information for the model was published theory and data on small passerines. By avoiding the confounding factors of predation (on the birds) and breeding, applicability is restricted to winter flocks foraging on low density prey populations. Simulation experiments were performed to examine the effects on feeding success of variation in the behavioural parameters (sensitivity analysis), variation in prey distribution, variation in flock size, differences between birds in behavioural parameters, variation in number of 'prey types' (monomorphic vs. Polymorphic prey populations), and variation in the ratio of abundance for two prey types. Success was measured two ways: 1) as mean capture rate and 2) as the probability of a bird's making no captures in twenty simulated minutes ('risk'). The latter measure may be more appropriate under winter conditions when a single day's inadequate feeding may seriously diminish a bird's likelihood of survival. To examine long-run model behaviour, methods were developed to approximate the simulation with a Markov model. Simulation experiments varying prey distribution indicated that a two parameter characterization of a prey population (e.g. mean density and variance over mean density) is not adequate to estimate the sensitivity analysis revealed the importance of 'giving-up time' (the time from last prey capture until the bird leaves the immediate foraging area) to feeding success. Flocking reduced 'risk' whenever prey were moderately to highly clumped. Flocking also enhanced the mean capture rate when prey were clumped and polymorphic (a polymorphic prey population means two or more major types of prey, not necessarily of the same species). When the prey population was monomorphic, flocking neither increased nor decreased the mean capture rate.

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