Or maybe the angle of the sun. It surely isn't the temperature here (below freezing at 5 am) which made every local bird wake up this morning and begin singing or twittering his territorial song for the first time this year.
I heard Robin, Tufted Titmouse, Chickadee, Carolina Wren, Cardinal, and Song Sparrow singing. Most migratory breeders won't be here until the buds pop open, and the tree buds aren't even swollen yet.
Today was really just a warm-up for the welcome Springtime morning chorus to come. It will last the next 6 weeks or so. Once their eggs are laid, they tend to go quiet so as not to attract attention.
Photo is a Tufted Titmouse, a fairly common songbird in New England.