Mass Audubon’s Broadmoor Wildlife Sanctuary in Natick offers 9 miles of trails through forests, marshes, and meadows. In the spring the sanctuary is alive with bird song, as the migrants announce their return from winter holiday. The unseasonably warm weather this year has drawn them home weeks earlier than usual. On a recent afternoon, I followed Broadmoor director Elissa Landre and a dozen other bird enthusiasts into the woods. We heard the “peter-peter” of the tufted titmice, the “PHOE-be” of the phoebes, and the two-note courtship call of the chickadees. A great blue heron sat motionless across a pond.