Dead heterospecifics as cues of risk in the environment: does size affect response?

Organisms can minimize their exposure to risk of death or injury by assessing their environment and modifying their behavior accordingly. There is evidence that the current or recent presence of a predator introduces cues to the environment that organisms may use in risk assessment. However, we know little about whether terrestrial organisms use the remains of victims of predation as one such cue of elevated predation risk. A previous study showed that western scrub-jays (Aphelocoma californica) respond both to dead conspecifics and to encounters with a predator with alarm calling and aggregation (cacophonous aggregations), suggesting that they use dead conspecifics as indirect evidence of predation risk. Here we examine whether western scrub-jays also use dead heterospecifics as an indicator of risk. We find that jays respond with cacophonous aggregations to dead sympatric and allopatric jay-size heterospecifics but react weakly if at all to smaller heterospecifics. This suggests that size may be an important factor in determining whether a dead heterospecific is a relevant cue of risk. To our knowledge this is the first controlled experiment showing an animal using the visual cue provided by a dead heterospecific as an indicator of risk and communicating this risk to other conspecifics.