Over the course of 13 years, the bird population in San Francisco bay has doubled from 100,000 to 200,000. Prior to 2002, the bird population was decreasing dramatically due to pollution and the sea salt levels used for food and medicine. Therefore, the increase in population is a clear sign of improved pollution (in a good way).
http://www.contracostatimes.com/breaking-news/ci_29008936/san-francisco-bay-bird-populations-doubled-since-2003
http://www.contracostatimes.com/breaking-news/ci_29008936/san-francisco-bay-bird-populations-doubled-since-2003