Birds have been used to monitor the environment throughout history. Birds are one of the best, and in some cases the only, monitors of environmental change. Many species are easily identified, detected, and return to traditional breeding sites yearly, so large amounts of data can be reliably gathered from scientists and a concerned public alike. The longest running data gathered on wildlife populations over time comes from Audubon's Christmas Bird Count, providing nearly one hundred years of data on changes in North America's bird populations.
Birds are well known organisms.
Because the biology, ecology, behavior, and evolutionary histories of many birds have been studied extensively, scientists have a foundation on which to ask the most pertinent questions, base hypotheses, obtain answers, and cultivate solutions. This background knowledge reduces risk of misinterpretation, allowing scientists to use birds, sensitive to stresses in predictable ways, as a proxy measure of environmental change. Furthermore, this background knowledge yields cost-effective research, since studies using other groups of animals often requires several years of basic data gathering before monitoring can begin. Birds integrate and accumulate environmental stresses over time because they are usually high in the food chain and have relatively long life-spans. Bird populations and communities, and bird behavior and reproductive success often reflect closely the stability of an ecosystem.