The birds are singing now, and for good reason: breeding season is in full swing.
It's the males, of course, providing the music (females make alarm calls and other vocalizations but not songs). And while a male will sing for several reasons, the main function of a song is to inform other birds of the singer's existence. Specifically, a song informs rival males and potential mates of the bird's species, and subtle variations to the song identify the bird as an individual. These variations are usually imperceptible to humans, but studies have shown that birds respond differently to the songs of their neighbors than to recordings of birds from other areas.
Studies have also shown that female birds respond more favorably to complex songs -- provided the renditions still convey basic identity information. In other words, it pays to show off, and this has led to the evolution of some very elaborate songs. Not surprisingly, birds possess some very sophisticated vocal instruments.
Bird vocalization comes from an organ called the syrinx, which is located in the breast (thus even headless ducks and chickens can quack or cluck). The syrinx is a branched structure through which air passes, and each branch can be controlled independently. As a result, a bird can produce two distinct sounds at the same time, essentially harmonizing with itself or, in some cases, even adding percussion. The haunting melodies of the thrushes and the overlapping phrases of the Brown Thrasher are great examples.
Another physical attribute that contributes to a bird's ability to produce complex songs is its specialized breathing apparatus (click here for details). By manipulating air sacs and lungs independently, a bird can inhale and exhale simultaneously. That's what allows the tiny Winter Wren to produce its long, complicated songs.
Click here to listen to songs of the Winter Wren and hundreds of other birds.