Spring weather has finally arrived, ushering in a sensory feast of nature's sights, sounds and scents after the barrenness of winter. We humans feel joyful at the prospect of long days filled with warmth and promise, and we assume birds do, too.

And why not? They sound joyful: Think of a male cardinal perched on top of a leafless tree, beak pointed upward as he endlessly burbles his "wha — cheer" song. Or a robin sitting on a branch, whistling his bright, lilting notes each sunlit morning. It's easy to infer that they're happy that spring is here.

But those lovely spring bird songs have very little to do with expressing pleasure. Instead, they're all about asserting a bird's place in a small universe, the territory he's chosen to raise his brood.

Like arias

Birds lead very operatic lives, putting into song their most intimate thoughts and violent threats. A male cardinal sings softly to his mate, likely the same female who shared a nest with him last year. He's telling her that he's still able to warn off competitors and be a good provider for their offspring. Then he turns around and robustly roars to other cardinals that they'd better stay out of his way.

As for the robin, he's just back from the South and looking to attract this year's mate. He and the cardinal and all the other singing males of springtime are of one mind: Get and keep a territory and gain and keep a mate.

Although a number of other living things engage in singing (humpback and beluga whales, humans, some bats and mice), birds are the most accomplished in this line. About half of the world's 9,000-plus bird species are songbirds, including orioles, tanagers, wrens and thrushes. What makes songbirds unique is their divided voice box, or syrinx, sitting deep in their chest. Birds manipulate muscles to tighten or loosen the sides of the syrinx and pass air through them to make their sounds. They can vibrate the two sides of the syrinx separately or in tandem.

One-bird duets

Think about what this means: Many songbirds are able to sing duets with themselves. Listen to a veery (a kind of thrush) singing and you'll swear that two birds are singing at once (hear one here: www.allaboutbirds.org/guide/veery/sounds). But it's just one veery, creating his hauntingly beautiful song by moving air through both voice boxes at once. Listen closely to a robin and you may detect a slight "echo" within its song. Cardinals can switch from one voice box to the other in the middle of a song.

Since birds' songs are formed in their chest, they can sing even while carrying food or nesting material in their beak.

We used to think that bird song was instinctual, hard-wired into bird brains as they emerged from their eggshells. A great deal of research has gone into studying how birds make their songs, and it's becoming clear that most singing species must learn their songs from adult birds, most often their father.

Babbling like babies

At about three weeks of age, soon after leaving the nest, young male birds listen to and store their father's songs in their brain. They spend their first summer and early fall practicing over and over until they're finally satisfied that they're matching the song template in their brain.

What about a young bird that hatches late in the summer, after birds have stopped singing for the year?

Donald Kroodsma, one of the premier researchers in the field, says the "song-learning system in songbirds is flexible enough to adapt to different lifestyles and needs" (from his fascinating book, "The Singing Life of Birds"). If it's too late in the season for a chipping sparrow to memorize his father's song, he'll have to wait until the next spring, after migration, to learn from another male.

Birds in one region, even one neighborhood, can develop dialects after listening to each other. And even birds that all sound the same to us, such as robins, create their own, unique song by adjusting pitch and combining notes in changing patterns.

Very soon we'll hear all those males sending their songs out into spring mornings, making much more noise than at any other time of day. Is it because fewer predators are awake at this time, so it's safer to call attention to oneself? Or is this the best time for singing because the air is usually still and songs carry farther?

Maybe both are true, but we can be sure that birds are singing for serious reasons. If this happens to bring us joy and lift our hearts, there's no harm in that.

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 val​writes@comcast.net.