Even before canaries were brought into coal mines to alert miners to the presence of poisonous gas, birds were giving us early warning calls signaling the earth’s deteriorating environmental health. Worldwide, some 1,212 of 9,775 bird species—one out of every eight—are threatened with extinction. Destruction and degradation of habitat is the number one danger, threatening 87 percent of these vulnerable birds.

As an ever-expanding human population has altered natural places around the globe—wetlands, grasslands, and forests—bird numbers have fallen. Global bird populations have shrunk by up to 25 percent since pre-agricultural times, largely because of conversion of habitat to farms. Over the past 300 years, farmland has expanded from 6 percent of the earth’s surface to nearly a third.

 
Read Janet Larsen:
http://www.earth-policy.org/Updates/2005/Update50.htm