Does signal deterioration compromise eavesdropping on other species' alarm calls?

Individuals gain valuable information by eavesdropping on others species' signals, but there are potentially greater constraints on eavesdropping than reception of signals from conspecifics. Eavesdroppers rely on signals addressed to others, which may come from unpredictable directions and distances. Furthermore, eavesdroppers might lack familiarity with other species' calls or perceptual adaptations to detect and recognize them, and these difficulties may be exacerbated by signal changes during transmission. We tested whether signal changes differentially affected the response to heterospecific compared with conspecific aerial ('hawk') alarm calls in two sympatric species that respond to each other's alarms, superb fairy-wrens, Malurus cyaneus, and white-browed scrubwrens, Sericornis frontalis. We assessed the effects of signal attenuation (reduced amplitude) and degradation (including reverberation). Attenuation caused a reduction in probability of fleeing to cover, and birds were less likely to flee to heterospecific than conspecific alarms. Signal changes did affect the response to heterospecific compared to conspecific calls, but not in the simple way expected. For conspecifics, degradation had no effect, and attenuation caused a similar reduction in fleeing for degraded and undegraded calls. By contrast, for heterospecifics, attenuation caused a reduction in fleeing for undegraded but not degraded calls, which prompted constant, low fleeing rates. Additional measures of response suggest that a lower probability of fleeing was partly a consequence of poorer detection or recognition of calls, and not merely assessment of reduced danger. Overall, the results are consistent with greater constraints on heterospecific eavesdropping than conspecific communication, perhaps because of lower familiarity or perceptual specialization.