The bird-song orchestra is tuning up for spring. House finches chortle from treetops, goldfinches sing from perches, and sparrows twitter in the shrubs.

This time of year, two of the most persistent songsters are among our most familiar birds: the northern cardinal and black-capped chickadee. Both of these birds, which hang around in winter, are fairly quiet in the cold. But once the weather warms, they're ready to break out in song that does more than just celebrate spring.

Fight song

During the cold months, cardinals seemed relatively indifferent to other cardinals. They gather in groups of a dozen or more at sources of food or water without raising their crests or exhibiting other threatening behavior. Likewise, chickadees spend the winter together in flocks, hunting for hidden insect eggs or bushes with berries.

But peaceful coexistence ends when winter does. When spring approaches and hormones ramp up, resident songbirds start sending out songs to cajole or threaten. Male chickadees sing "fee-bee" to entice a female to leave the flock and set up housekeeping. That same call, whistled more loudly by a male, can also mean, "Stay away from this side of the yard, buddy!"

The cardinals filling the spring air with their exuberant "wha-cheer" song are engaging in early courtship behavior, too, but with a twist: Both males and females are singing. That's unusual in the world of songbirds. Cardinal pairs sing duets, burbling "purdy, purdy, purdy, wheet, wheet, wheet," back and forth, before moving on to other combinations in their repertoire. In early spring, their songs are part of the courtship ritual. Later, they later become aural fences, defining the territory a pair intends to defend.

Sing a claim

And spring is all about territory. In fact, choosing and defending a territory is a major factor in determining how many young birds survive to leave the nest. The two essential characteristics of a good territory are plenty of insects to feed nestlings and plenty of shrubs and trees in which to hide from predators.

By the time migratory birds begin returning in May, cardinals, chickadees and other "winter" birds will have carved up the local real estate. But that doesn't mean migrants have to settle for whatever space is left.

Songbirds sing to entice or warn but only birds of their own kind. Cardinals don't want other cardinals setting up housekeeping nearby. But cardinals and chickadees don't much care what other species are doing. They'll tolerate orioles and warblers and a host of other birds because those other species have their own particular food and nest needs and aren't viewed as competitors by our resident birds.

Sounds of summer

During the busy nesting season, when adult birds are busy feeding their young, they sing less often and more softly. Researchers think that these pianissimo songs may be one way cardinals and chickadees tell their mates that they are bringing food to the nest.

Young birds must learn the songs sung by their species. In late summer and fall, you may hear the same notes over and over. That's how young birds practice a song, repeating several notes over and over until the song matches the one in their mental template. Like a kid learning to play the violin, these sounds can be discordant, until they finally become music to our ears.

Val Cunningham, a St. Paul nature writer, bird surveyor and field trip leader, can be reached at