From the NYTimes:
Studies in recent years by many researchers, including Dr. Greene, have shown that animals such as birds, mammals and even fish recognize the alarm signals of other species. Some can even eavesdrop on one another across classes. Red-breasted nuthatches listen to chickadees. Dozens of birds listen to tufted titmice, who act like the forest’s crossing guards. Squirrels and chipmunks eavesdrop on birds, sometimes adding their own thoughts. In Africa, vervet monkeys recognize predator alarm calls by superb starlings.
Dr. Greene says he wants to better understand the nuances of these bird alarms. His hunch is that birds are saying much more than we ever suspected, and that species have evolved to decode and understand the signals. He acknowledged the obvious Dr. Dolittle comparison: “We’re trying to understand this sort of ‘language’ of the forest.”...
Dr. Greene, working with a student, has also found that “squirrels understand ‘bird-ese,’ and birds understand ‘squirrel-ese.’ ” When red squirrels hear a call announcing a dangerous raptor in the air, or they see such a raptor, they will give calls that are acoustically “almost identical” to the birds, Dr. Greene said. (Researchers have found that eastern chipmunks are attuned to mobbing calls by the eastern tufted titmouse, a cousin of the chickadee.)
Other researchers study bird calls just as intently. Katie Sieving, a professor of wildlife ecology and conservation at the University of Florida, has found that tufted titmice act like “crossing guards” and that other birds hold back from entering hazardous open areas in a forest if the titmice sound any alarm. Dr. Sieving suspects that the communication in the forest is akin to an early party telephone line, with many animals talking and even more listening in — perhaps not always grasping a lot, but often just enough.