Handbook of the Birds of the World online resource. Requires a subscription. Requires personal subscription starting at $7.99/month. ISU has the print volumes.
Members can upload and share bird sounds from all over the world. Free registration. From the Xeno-canto foundation (or officially Stichting Xeno-canto voor natuurgeluiden), a charity (Dutch "ANBI") from the Netherlands.
Migration data from the Cornell Lab of Ornithology, Smithsonian Migratory Bird Center, the U.S. Geological Survey, Georgetown University, Colorado State University, and the National Audubon Society.
App tries to identify the correct bird species using AI. Enter your bird photo or bird recording and AI compares them to its extensive library to suggest possible identifications.