The Christmas Bird Count (CBC) is a long-standing program of the National Audubon Society, with over 100 years of citizen science involvement. It is an early-winter bird census, where thousands of volunteers across the US, Canada and 19 countries in the Western Hemisphere, go out over a 24 hour period to count birds. Current information can be found at http://audubon.org/bird/cbc/index.html
Based on 40 years of observations of 305 species, the winter distribution of 177 of these species has shifted north. Audubon has published their analysis of this in Birds and Climate Change in 2009. CBC data was also used in identifying Common Birds in Decline and WatchList 2007.
NEW Audubon members have participated for years, in groups organized by a local coordinator. We have summary reports from some recent surveys. If your group isn't listed, let us know how your count went this year.