I just encountered a news article titled "Populations of 20 Common Birds Declining."
Here's the link to the article: http://news.yahoo.com/s/ap/20070615/ap_on_sc/bird_declines;_ylt=Ak4_Jo3UQ3p_LV8enwJjoWwPLBIF
As an avid bird watcher, stories such as this are seriously depressing to me.
It might seem unusual for me to be talking about birds on this blog where I normally discuss topics related in some way to human culture, except that it is human cultural processes that are largely responsible for the decline of the North American bird populations mentioned in the article.
There is no single way in which humans affect animal populations such as these. Birds with specialist ecological strategies have been especially affected as their habitats have been fragmented by suburbanization and sprawl. Climate change, largely human induced, seems to be partly to blame for some species' declines. Globalization is involved as well, with the introduction of new invasive species that have outcompeted some native bird species, and in recent years with the introduction of the West Nile virus, which has devastated many North American bird populations. (The article doesn't discuss it, but corvids, such as crows and blue jays, have been especially hard hit by West Nile virus.)
There is some good news as well, though there are patterns to the good news. Some birds have done well because of changes in human cultural practices. Many species that had been particularly affected by past uses of the pesticide DDT have made dramatic recoveries since DDT was banned for use in North America (the article mentions the double-crested cormorant, but the U.S.'s national symbol, the Bald Eagle, is another example). Changes in farming practice over the past century have also benefitted some birds. Over the past century, farming has become more industrialized and concentrated. One result is that alongside larger energy inputs to farming and greater total crop yields, less total land is devoted to farming in North America than 50 years or a century ago, and there is more total forest land (even though much of it is fragmented by roads and sprawl). Some birds have been able to benefit from this, with the recovery of wild turkey populations over the past half century being perhaps the most dramatic example. Finally, birds with generalist ecological strategies have tended to do quite well and even increase in numbers. Suburbanization and sprawl has simply presented one new type of ecosystem for them.