What bird species live in your area? Which migrate and which stay year-round? How do bird populations change over time? Citizen science provides the essential tools to address these questions and more. With ever-growing databases such as Project FeederWatch and eBird, students can connect with people around the world as they make observations, pose questions, conduct investigations, and build deeper connections with the natural world. This article describes several different citizen science investigations in which students learn about adaptation, interactions among species and their environments, and change over time--fundamental principles in biology (NRC 1996, 2012). Students see the real-world relevance of science as they experience authentic processes of science, from posing testable questions to analyzing data and reaching defensible conclusions. (Contains 6 figures and 5 online resources.)