WEST-B Practice Test 1

Read the passage below.

"Each winter, tens of thousands of volunteers across North America spend a day counting every bird they see or hear within an assigned circle. Most are not scientists; many are retirees, families, or students. Because the counts have been repeated in many of the same areas for more than a century, researchers can compare results across decades and see which species are expanding their ranges and which are declining. No single research team could afford to gather information on that scale."

Which statement best expresses the main idea of the passage?