Few sounds are more pleasant than the chorus of birds on a spring morning. Robins start in before the sun rises; their clear, bubbling songs echo across the darkened rooftops. Soon, song sparrows jump up on bushes to join them, and other voices from deeper hiding places. Raucous calls of jays and crows punctuate the melodies. Woodpeckers, lacking strong vocal talents, pound out rhythms on lampposts and house sidings. Whether in Cairo or Copenhagen, a similar chorus formed of local accents rises with the warming sun. Peaceful, isn’t it?

Not really. What we actually hear when stepping into the fragrant air is aggressive or lascivious shouting. As behavioral biologists Sanne Moorman and Johan J. Bolhuis have written: “The main function of song seems to be defense of territory and mate attraction.” In other words, each warble, trill and twitter in that mainly male chorale is either yelling “Stay back!” or “Come hither!” And these are no ordinary shouts. Ornithologists have shown that songbirds triple their daily energy expenditure to convey these messages. The birds are bellowing.

There is subtlety, though, because males must distinguish themselves and demonstrate their fitness for parental duties. Each bird’s song therefore differs slightly from another’s in tone, speed, inflection, accent and even vocabulary. Birds also vary their message according to context. When their young are hatching, for example, the “Stay back” holler wins out. These messages consist of multiple syllables joined into phrases, assembled as songs, and delivered in clusters. Typically, a bird repeats the same song cluster over and over with minor variations throughout the day and season. Some sing only a few syllables, while others offer up songs of astounding complexity.

The Pacific wren, a tiny brown sprite that lurks in the forests of the Pacific Northwest, sings perhaps North America’s most complicated birdsong. In a remarkable study, biologist Beatrice Van Horne found that an individual wren might use 10 or 11 different types of syllables (think of these as its alphabet) combined according to distinct rules into as many as 327 different songs. This means that their songs appear to observe rules of syntax. That is, their patterns of syllables carry specific meanings, much like properly constructed sentences do. More recent studies of California thrashers and Bengalese finches reinforce this conclusion: Birds mean what they sing.

Possibly the most remarkable realization these careful studies provoke is that birdsong and human speech follow similar phonetic sequences. Birdsong may sound sweet to the ear because birds prefer the kinds of patterns and rhythms we prefer. Birds and humans share, it seems, a basic “universal grammar.” Even so, it’s worth remembering that the robin singing from a treetop is not your friend; he’s bellowing “Stay back!”