A citizen science approach to ornithological research: Twenty years of watching backyard birds. Effective continent-wide monitoring of bird populations requires engagement of the public in order to gather data across large spatial and temporal scales. One long-term, continental-scale monitoring program, Project FeederWatch, has enlisted more than 40,000 people from all U.S. states and Canadian provinces since 1987. The project, operated by the Cornell Laboratory of Ornithology and Bird Studies Canada, has accumulated more than 1.5 million checklists allowing researchers to study changes in the distribution and abundance of species that regularly visit supplemental bird feeding stations in North America. Because of the efforts of FeederWatch participants, researchers have been able to identify species of concern, to track changes in the distribution and abundance of native and non-native species, and to understand how novel pathogens affect populations. Following two decades of successful public engagement, Project FeederWatch provides an ideal case study in the challenges and opportunities of public involvement in scientific research. This article was written by David N. Bonter and Wesley M. Hochachka of the Cornell Lab of Ornithology. It appears in the Proceedings of the Fourth International Partners in Flight Conference: Tundra to Tropics.