The birds of the north are moving to their winter headquarters in more hospitable parts of the hemisphere like the southern and southwest U.S., valleys and coastlines, all the way down to parts of South America. They move south in the winter to escape the cold that will soon envelop their breeding habitat and to find their particular food which will become scarce or non-existent in the winter. It’s a long and dangerous journey for some of them, but clearly worthwhile as evolution has created this behavior.
It’s estimated that 20 billion birds engage in some sort of movement in the fall, whether it’s just a local dispersion or short or long distance migration. Along the way, there are hazards. In eons past, mortality was caused by disease, predators, storms, or lack of food. Today those are minor hazards compared to what humans have created. The greatest source of mortality in North America is collisions with buildings, mostly windows, which kill perhaps 500 million birds a year. Power lines cause 175 million deaths. Poisoning, from pesticides, oil spills, etc, result in 75 million dead birds. And a study on cats in Wisconsin estimated that domestic rural cats killed 39 million birds in one year. That’s in one state and the research only looked at a subset of cats. Extrapolate that data and cats may kill one billion birds annually!
Birds have evolved to respond to changing photoperiod – the amount of daylight, which is predictable, rather than weather, which is not. So, although the exact timing fluctuates a bit, birds migrate about the same times each year which bring them to their breeding grounds when food is most available. Climate change is interfering with this historic pattern. Since the earth is warming, insects and plants, which respond to temperature, are peaking in abundance earlier than they used to. But the photoperiod has not changed, the result being that birds are arriving later than the time of ideal resources. We don’t know yet what kind of mortality this causes.
There is clear evidence that birds are changing their migration times and are changing their migration patterns in response to warmer temperatures. They are migrating north earlier and/or flying farther north. There’s an interesting article in Science Daily http://www.sciencedaily.com/releases/2012/02/120223142642.htm about this. Audubon also has something to say at http://web4.audubon.org/globalwarming/files/GW%20and%20birds%20041707.pdf. Some studies show birds are staying north later in the season http://www.upi.com/Science_News/2011/11/21/Global-warming-affecting-bird-migrations/UPI-85961321923613/
What’s nice about the migration habits of birds is that they make bird-watching dynamic. There are subtle changes in the avifauna of an area, like your backyard, almost on a daily basis. During migration times these changes become more pronounced and come more quickly. And you see different bird species and different plumages of the same species in the spring compared to the fall. The two most popular activities of nature-watchers are wildflower hunting and bird watching. Wildflowers are great, but they are here and then gone. Birds come and go and come and go in this amazing phenomenon called migration.