Many serious birders keep a life list of the bird species they’ve seen. We can tell you exactly how many birds are on that list, and there’s great excitement when we can add a new “lifer.” We may also have a list of “target birds,” those not yet seen, and we often spend considerable effort tracking them down. But once a year, we have an opportunity to add a new bird or two without lifting a finger.
All year, ornithologists are busy debating bird taxonomy. They present evidence—behavioral, morphological, a new DNA analysis, etc.—to support their opinions as to which species need to be split into two, which need to be lumped together as one (perhaps as subspecies), which need to be moved to a different genus, and other taxonomic changes. Every July, the AOS (American Ornithological Society) publishes the agreed-upon changes, and we all scramble to update our life lists.