Every year, the American Ornithological Society (AOU) reviews a number of proposed changes to the classification of North American birds. This affects the definitive list on which we birders base our life lists. Species may be added or deleted. Sometimes, a species is divided into two or more new species, such as the split of the Western Scrub-Jay a few years ago. (I gained a new life species as a result, as I have seen and photographed both the California Scrub-Jay and Woodhouse’s Scrub-Jay.)
Other times, species may be lumped together—what was once considered two species is now thought to be only one, with various subspecies. Sometimes, entire genera are moved into new families. Or, a species remains unchanged, but the name is updated, as when the Gray Jay became officially known as a Canada Jay.