A 2009 report from the Audubon Society analyzed decades of Christmas Bird Count (CBC) data and determined that 58 percent of bird species had shifted their winter ranges northward. More than 60 species were found to be nesting 100 miles north of their historical range.
The problem becomes more dramatic when you take into account high altitude nesters, such as Bicknell's Thrush. If the tree line moves up and conifers no longer grow, the thrush will have no place to nest. Luckily, the thrush does range to Canada, so will survive for a few more years.
The study conducted by ecologist Dr. Nicholas Rodenhouse of Wellesley College, looked at the Bicknell's Thrush and observed the Black-throated Blue Warbler, a small songbird that nests in northeastern US and southeastern Canada. While not as fragile as to changes in the nesting environment, the warbler thrives in a mix of hardwords with understory shrubs in which natural prey live.
A warmer climate spells disaster for all birds as there would be a serious reduction in habitat.
Dr. Peter Marra, of the Smithsonian Migratory Bird Center, stated:
. . .if you want to understand how climate change might influence birds in the future, you need to understand their exposure throughout the annual cycle. . .Both the Bicknell's Thrush and Black-throated Blue Warbler winter primarily in the Caribbean. What is happening there is drought, which may extend for 30-50 years. Birds suffering from dehydration and malnutrition due to the drought, may have difficulty in the breeding cycle.
Climate change threatens in unexpected ways. The Gray Jay, a bird of the boreal forests of North America, caches food in the winter and depends on that food in the spring. In warmer temperatures the food might rot, which means the jay may be less successful in the spring breeding cycle.
Common Loons have always left the freshwater lakes before they molt, which leaves them without flight. As temperatures rise, the loon may stay too late and have to overwinter in harsh unfamiliar habitats.
National Audubon Society
Sherry, T.W., and R.T. Holmes. 1995. Summer versus winter limitation of populations: conceptual issues and evidence. Pp. 85-120 in Ecology and management of Neotropical migratory birds: a synthesis and review of critical issues (T. Martin and D. Finch, Eds.). Oxford University Press, New York.
The Birds of Norther America Online http://bna.birds.cornell.edu/bna/species/087/articles/introduction