In the absence of warnings from conspecifics, prey have other ways of determining the presence of predators. Some prey use sound cues to detect the approach of predators. For ex, moths respond to the echo-location calls from their bat predators; moths drop to the ground once they hear the sound pulse from the bat. 

Scent can also provide cues to the presence of predators-you may have learned from nature programs that many terrestrial predators approach prey from a direction where the wind will not bring in their scent to alert the prey. Scent can also play a role in aquatic systems where chemical cues from the predator may be perceived by prey. Prey may also be able to detect that others of their species have been wounded or consumed by cueing on blood or body fluids in the water. 

How would being able to detect damaged members be beneficial to individual prey species?and What other kinds of cues might be reliable indicators for predator presence?