"Approaching predators" refers to animals or individuals who are getting closer to others with the intention of causing harm, attacking or hunting them. Full definition
Not only do their alarm calls tell others about the type and size of approaching predators, but it seems they can also warn of the hue of an imminent threat. (newscientist.com)
But it may be sensitive to changes in brightness or wavelength, such as moving shadows cast by approaching predators, or light fluctuations associated with different times of day. (sciencedaily.com)
Is their hearing sensitive enough to hear approaching predators? (treehugger.com)