Data from Audubon's Christmas Bird Count (CBC) indicates that the American Robin is no longer a sure sign of spring in Colorado. Per the report, the portion of robins wintering in Colorado vs. migrating has increased over 1,500 percent since I was born in 1969. Sadly, it seems this delightful bird is becoming a much more worrisome first sign of global climate change in my neck of the woods.
Interested about your area? See the Audubon study here.