It's not until spring when we expect male birds to break out the melodies to woo females and establish their territories. That's because a songbird's winter activities are focused on finding food and keeping warm. They may alert each other to new food sources through calls, and stay in contact with the group through chirps and chips. Other than that, we just don't expect to hear actual bird song in the winter.
Except sometimes you do catch a bird song, even when there’s no sign of spring in sight. A Black-capped Chickadee can start whistling “Hey sweetie!” as early as mid-January. Other birds, like the American Robin, the Northern Mockingbird, the Fox Sparrow and the Northern Cardinal will sing many weeks ahead of the nesting season. Why would they go off script and start singing now?
Under one theory, according to National Geographic, birds may be responding to the increasing day length. Since these winter songbirds evolved to survive in harsh winter conditions, it makes sense that their brains are attuned to the changes in seasonal light. This increasing amount of light may trigger something in the brain that the time is coming to start thinking about spring territories.
One study explained in The Atlantic Monthly has an interesting theory, but about migratory birds: Some of these birds "spend all winter practicing their love songs,” as the headline states. Researchers studied The Great Warbler, which spends its winter in Africa and returns to Europe in the spring. During the winter months, the males repeatedly practice their tunes — at slower tempos, for long stretches of time.
The scientists then analyzed other research about migratory birds that sing in the winter, and discovered those that do so have more complex songs to master, or they are drab in color and therefore rely on their pipes to impress the ladies.
It’s not fully understood why birds break out into song in the winter. As the research continues, it will no doubt uncover much more to appreciate about our feathered friends. Enjoy the tunes when they happen, and note how they progressively increase in frequency as spring draws near.