Animal distributions are directly and indirectly influenced by predation. Indirect behavioral effects are more difficult to measure, but may have large consequences. For example, the fear of predation can vary spatially and can affect distribution patterns, but quantifying fear can be challenging. This has been described as an organism’s “landscape of fear.” Many studies suggest that animals reduce predation risk by eavesdropping on alarm calls from sentinel species, allowing them to occupy otherwise risky habitats. One unexplored area of study is understanding the extent to which different species alarms vary in quality, and how this variation is distributed in the landscape. We tested this phenomenon in a unique system of avian mixed species flocks in Amazonian rainforests. In this system, flock mates (eavesdropping species) strongly associate with alarm-calling antshrikes (genus Thamnomanes), which act as sentinel species. Up to 70 species are known to join these flocks, presumably following antshrike behavioral cues. Since the flocks are exclusively led by a single antshrike species, this provides a unique natural system to compare differences in sentinel quality between flocks.
We simulated predation threat by flying three species of live trained raptors (predators) towards flocks to compare the ability of sentinels to 1) detect predators, and 2) encode information about magnitude and type of threat. Our field experiments show biologically significant differences in the ability of different sentinel species to detect and distinguish predators. T. schistogynus detected more predators than did T. ardesiacus. Both species used more urgent notes in the presence of smaller raptors which represent a higher level of threat. This variation may have important fitness consequences and shape the “landscape of fear” for eavesdropping species.