The birds that winter-over, such as the northern cardinals, white-breasted nuthatches and dark-eyed juncos, are exceedingly vocal now. The March migrants — American robins, red-winged blackbirds and eastern bluebirds — are adding to the chorus.

The singing of birds increases manyfold in March, not only due to migrants returning north but also because the year-round residents are responding to increased daylight. We have gained nearly four hours (three hours and 54 minutes) since last Dec. 21, the winter solstice with its shortest daylight and longest night.

For centuries, people have believed that birds sing to announce the arrival of spring. But there is much more to the ritual. It’s important for birds to mark off territories and attract mates.

Competition for nesting sites and food can be fierce, and birds often sing and display themselves prominently, warning other birds of their own species that the territory has been taken. If too many American robins were in the same territory, earthworms and fruit would soon disappear. Birds don’t need to be concerned about other species nesting in their territories if those other birds eat different foods.

It’s not unusual for a mallard, a black-capped chickadee and an American robin to nest in the same vicinity, because they don’t compete with one another for food or nesting sites.

Jim Gilbert taught and worked as a naturalist for 50 years.