If you get far enough west of the city, the mighty Charles River isn’t so mighty, snaking tentatively through the low plains and wetlands, unsure of its plans. In its serene basin sit some of most bucolic acres in Greater Boston. Mass Audubon’s Broadmoor Wildlife Sanctuary in Natick offers 9 miles of trails through forest, marsh, and meadow. In the spring, the sanctuary is alive with song, as migrant birds 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 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. Two mute swans shot us intimidating looks. Garter snakes rustled in the brush. Beavers and painted turtles made ripples in the water. Dry leaves crunched underfoot. Tall white pines formed a canopy of stillness. In each moment, a gift of peaceful escape.