Data show that when small birds are exposed to a model of a predator, their body mass may either increase or decrease. Although attempts have been made to explain the data using previous models, these models are based on a constant level of predation and hence are not appropriate for making predictions about the response of a bird to the sight of a predator. We have developed a novel model that includes encounters between a bird and potential predators. We show that, depending on the biology of the predator, optimal body mass may either increase or decrease. The model also makes predictions about the foraging behaviour of the bird after it has seen a predator.