Birds' lives are so filled with danger that they're on alert at all times, just to stay alive. Luckily, they have several tactics to avoid becoming the next meal for a cat or hawk. One, of course, is to fly away, a good choice, but not always possible. Another is to call in a troop of other birds to set up such a clamor that the predator is driven from the neighborhood. But in cases where the threat is too close and too dangerous, birds freeze in hiding while making soft, high-pitched sounds that serve as a warning to other birds.

In our own backyards, the birds likeliest to call out high-danger warnings are robins and chickadees, and many wild creatures recognize such calls. Catbirds, sparrows, finches and nuthatches dash for safety when they hear a robin's barely-audible-to-humans "seet-seet" sound, because they know a bird-eating hawk is hunting nearby. Even chipmunks, red squirrels and woodchucks heed these warnings, running on their short legs for cover.

'Watch out!'

If all the backyard birds suddenly fly frantically away, it's generally because one bird has called out the signal to flee, having spied a bird-eating hawk on the wing. Woodpeckers will often freeze in place at a suet or peanut feeder, hoping to escape the hawk's notice.

The robin utters its calls about a high-risk danger from deep within a shrub or vine tangle, counting on the hawk being unable to locate their source. The intent is probably to warn the robin's mate and offspring, but other species have learned to heed them, too. Researchers have long been fascinated by this kind of avian communication, and are studying it by using props such as stuffed owls and robotic hawks to gauge birds' reactions.

Some bird scientists were surprised to learn that some kinds of birds' danger calls convey not only the presence of a predator, but also what kind of animal it is and how dangerous.

Chickadee chatter

Chickadees, so adept at so many things, are aces at warning other chickadees of danger, at the same time alerting all other birds within hearing. If a chickadee spots a red-tailed hawk, the small bird may call out "dee-dee," which might translate to something like "Big bird, but not the kind that eats chickadees." However, it ups the ante considerably if it spies a sharp-shinned hawk, a species known to hunt smaller birds. Then the chickadee will issue a string of "dees" from a hiding place, conveying both the fact that danger lurks nearby and that it's a bird-killing hawk.

Cornell Lab of Ornithology researchers have found that while individual species have their own alarm calls, these may be understood by many other kinds of birds because they share characteristics, such as a similar frequency range.

"It's the universal language of 'look out!' " says Janelle Morano, a Cornell Lab staffer who's studied bird reactions to a robotic owl.

Look out, indeed: In his fascinating book, "What the Robin Knows," Jon Young writes about encountering Cooper's hawks (raptors that eat songbirds) building a nest. He observed the nest carefully for six weeks, from egg laying until the hawks' three offspring fledged. Young counted the number of small birds brought in by the parents for their nestlings, counting 66 songbirds for each young hawk, a total of almost 200 birds.

So it's not at all surprising that songbirds are on their guard, all of the time, and need a way to know when danger is close to hand.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at