Over a 48-year period, wintering bird species have collectively shifted their winter range northward in California. 

Many bird species regularly migrate — mostly north and south between breeding and wintering grounds — to take advantage of available resources and favorable conditions at each location. During the fall, birds migrate south to wintering grounds in response to shorter day length, lower temperatures, and changes in food supplies. Climate is an important factor that defines the geographical area or range in which a species can be found. For more information, download the Bird Wintering Ranges chapter. 

Flock of Snow Geese take flight above a field on Twitchell Island in the Sacramento-San Joaquin River Delta. Of the wintering bird species studied, snow geese showed the greatest northward shift (326 miles) over the past five decades. 

Credit: Kelly M. Grow, California Department of Water Resources DWR 

• Some California bird species have shifted their wintering grounds north by an average of about 7 miles over the past 50 years. These findings are based on observations in the Christmas Bird Count, the longest running census of birds conducted by volunteers from December 14 to January 5 each year. 
• Not all birds moved in the same direction: Of the 234 bird species studied, roughly a third moved northward, a third moved southward, and a third showed no significant change.  

• Climate change is one of many factors threatening bird populations. Understanding how birds are changing their geographic distribution will help efforts to protect them.  
• When birds change where they spend their winters, human and ecological communities experience the loss of not just the birds themselves, but also the important functions and benefits they provide. For example, western bluebirds eat insects that damage crops, hummingbirds pollinate flowers, and woodpeckers build nests in tree holes that can then be used by other animals as shelter. 
• When bird species move to, or increase their numbers in, a new location, they may disrupt how species interact and negatively impact the native wildlife in that location. 

• Temperature, food and water availability, habitat alteration, topography, and interactions with other species can affect bird wintering ranges. 
• In California, the 7-mile northward shift in bird wintering ranges was closely associated with warmer December temperatures. 
• Warmer and drier conditions do not always result in range shifts to the north. Some bird species have shifted their ranges to higher elevations or toward the coast. 

• Audubon California, Survival by Degrees: 389 Species on the Brink 
• Audubon, Christmas Bird Count