Black-capped Chickadees are already singing -- that two-note whistle you are now hearing is their song, rather than the "chickadee-dee-dee" call notes they use all winter.
Woodpeckers have started drumming -- tapping on trees to select a suitable one to excavate for a nest hole.
Even Blue Jay vocalizations have changed in the last week. Their tooting, trumpet-like calls are a sign that spring territoriality is kicking in.
Another early songster is the Brown Creeper who's beautiful high-pitched warbling song is already echoing through the chilly, still-snowy woods.
What triggers this spring chorus? . . . day length. Increasing solar exposure triggers hormones in the birds. Soon it will be time to find a mate, defend a nesting site and reproduce.