The American Southwest has a high diversity of bird species. Bird surveys are one of the most widespread types of long-term monitoring in the United States. In addition to being common to most habitats, birds also function as a useful indicator of habitat extent and quality. Some recent studies also suggest that bird reproduction rates vary with habitat quality, leading to higher densities in superior habitat. Birds are subject to pressures such as historic and current land use, human visitation, development, air and water pollution, and other changes in the surrounding landscape. Long-term population monitoring data can be used to examine the broad effects of human activities and measure the sustainability of those activities.