Bird-watchers often like to track birds, keeping life lists of which birds they have seen and paying close attention to details, such as the locations, traits and behaviors of birds. Amateur birders turn out to be the perfect army for a global ornithological network called eBird. A project of the National Audubon Society and Cornell Lab of Ornithology, this crowdsourced online checklist is providing scientists with a big picture of bird populations around the world, year-round. Since 2002, eBird has compiled 141 million observations, and the project has allowed them to create heat maps of the migration patterns of distinct populations. “It’s a really neat tool,” said Bob Martinka, a retired state wildlife biologist and keen bird-watcher. “If you see one bird or a thousand, it’s significant.”