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Birds ... are sensitive indicators of the environment, a sort of "ecological litmus paper," ... The observation and recording of bird populations over time lead inevitably to environmental awareness and can signal impending changes.

Roger Tory Peterson (2012). “Peterson Field Guide to Birds of Pennsylvania”, p.12, Houghton Mifflin Harcourt
Birds ... are sensitive indicators of the environment, a sort of ecological litmus paper, ... The observation and recording of bird populations over time lead inevitably to environmental awareness and can signal