Abstract Deimatic behavior is an antipredator defense such as a sudden movement or sounds performed by prey upon perceiving a threat from a predator. Such behaviors can cause predators to slow or stop their attack, but how different components of deimatic behavior influence predator responses remains untested. We investigated the effect of prey movement, size, and size change on predator attack behavior using a robotic moth and wild Australian magpies (Gymnorhina tibicen) as predators. We first tested birds’ responses to a non-moving control moth and then presented them with a moving moth displaying deimatic behavior. The experiment included three different deimatic behavior treatments with the moving prey: a small moth, a large moth, and a moth that increased in size from small to large. We found that all deimatic behavior treatments were more effective at stopping the first approach compared to the non-moving control moth, and that no treatment was more effective than another. There was no significant difference in attack latency among the treatments, although birds tended to attack the prey more quickly after the display when the moving prey remained small, compared to moving prey that increased in size during the movement. The robotic moth did not include warning colors or chemical defenses. Our results therefore indicate that protective value is conferred by movement alone, supporting the ‘startle-first’ hypothesis that the behavioral component of deimatism can evolve before other defenses.
Hämäläinen et al. (2026) studied this question.