The journal 'Science' says that <br />the population has shrunk by <br />three billion over the past 50 years. The report adds that most of the <br />drop, around 90 percent, relates <br />to just a handful of types. These include <br />finches and sparrows. Other recordings show a one-third fall <br />in shorebird numbers. The population of grassland types has <br />also dropped over 50 percent. To get their results, the study's authors observed migration patterns <br />over a period of 10 years. They also used analyzed surveys from <br />bird watchers in North America. Researchers say there are many reasons <br />for the drop, including habitat loss <br />and toxic pesticides.
