Distress calls in tufted titmice (Baeolophus bicolor): are conspecifics or predators the target?

Distress calls are produced by a wide variety of small prey species when they are captured by a predator and may serve as signals to warn group members of danger, to attract others that might aid the captured individual, and to confuse the predator. Tufted titmice (Baeolophus bicolor) produce distress calls when captured by a predator or a human. Here, we tested several hypotheses to explain the distress calling in this species. Individual titmice were captured in walk-in treadle traps. We recorded captured individuals in an ordered sequence that modeled approaching threat, capture, and finally escape. Titmice produced more total notes as the level of threat increased (as the observer moved closer to the trap restraining them) and as they were captured and held. Captured titmice produced distress calls comprising many high-frequency components but also many low-frequency broadband components and did so whether other titmice were present or absent. Playbacks of distress calls indicate that the notes of these calls transmit over long distances (>60 m) and do not readily attract larger avian predators to the area. Variation in production of lower frequency and broadband notes by captured titmice was not associated with variation in aggressive behavior. Taken together, our results lend support both to the "protean defense" hypothesis and to a dual "mobbing/alarm" hypothesis. Distress calls in titmice may therefore function either to confuse the predator or to warn conspecifics and facilitate their mobbing behavior.