Here are some of the significant changes found in this research
regarding birds in Colorado:
 
>>AMERICAN ROBIN (Turdus migratorius)
This common bird of woodlands, gardens and other habitats throughout
Colorado has experienced a northward shift of 206.0 miles all across its
continental range. At the same time its numbers in Colorado during the
winter have increased by 1704.4%, as more robins winter farther north
than in the past. As woodland birds, like the robin, move north, they
may be at increased risk from sudden cold snaps. Robins and Wood Ducks
are examples of more southern species moving north and wintering in
greater numbers in Colorado.
 
WOOD DUCK (Aix sponsa)
The colorful Wood Duck, which inhabits wetlands, lakes and rivers in
mostly eastern Colorado, has exhibited a movement northward of 37.1
miles throughout the U.S. The population in Colorado has grown at a
rate of 3158.3%, indicative of the northward movement of birds that
used to spend their winters south of Colorado. Many wetland/aquatic
birds are also wintering more in inland states like Colorado as they
find more ice-free conditions there. But these birds may be
particularly susceptible to the impacts of unusually cold winters or
harsh storms.
 
BLACK-BILLED MAGPIE (Pica hudsonia)
The population of this familiar bird has moved 85.6 miles north over
the past 40 years. Indicative of this northward shift, its population 
size inColorado has declined by 22.0%, even though its numbers have 
actually increased by 17.3% acrossNorth America. Like other generalist birds, its
northward movement has been less dramatic than that of woodland birds.
This species and the next are examples of more northern species moving
further north and losing numbers in Colorado.
 
ROUGH-LEGGED HAWK (Buteo lagopus)
This large hawk of North American grasslands and tundra breeds far
north of Colorado but winters here. The continental population of this bird 
has moved 178.7 miles north over the past 40 years and the number wintering in
Colorado has declined 49.2%, which far exceeds the continent-wide decline of 2.4% for the species.
Like other grassland species, these birds are threatened by habitat loss and 
degradation from human encroachment and climate change.
 
"Nearly 60% of species that winter in North America have moved
northward or inland, with climate playing a likely role in shifts that
can exceed hundreds of miles. Changes can be seen among the birds of
Colorado. Though implications for individual species can appear
benign, the long-term environmental disruptions they portend are
serious for birds, other wildlife and people alike."
 
More info at http://www.audubon.org/bird/bacc/