Birders often use a checklist to document which species they see during their outings. Most birders today document all of their sightings and outings with eBird. The reason is quite simple-it allows you to maintain a chronological history of every location you've ever visited and every species you've ever seen. The Cornell Lab of Ornithology’s eBird website makes creating and sharing your records very easy.
Besides being fun, this is citizen science at its best. Your data, once documented, is then shared with Universities, government agencies, conservation groups, and the public. This data is critical to understanding bird movements, population trends, and other aspects of avian conservation worldwide.