Branching (birth–death) models are used to project the time to extinction for bird and mammal orders. While such projections are very coarse, they provide information on how long fundamental units of biodiversity, as represented by such higher taxa as orders, will persist at specified rates of species extinction. The results of a basic decay model indicate that, if all threatened species in each order go extinct within the next 100 years, and this species extinction rate continues indefinitely, then half the species in most orders will disappear in a few hundred years or less. This result is robust and is in agreement with other estimates. A more detailed branching model indicates that most bird and mammal orders will become 100% extinct in a few thousand years if current extinction trends are extrapolated. This projection is also relatively robust, as it is consistently produced using a number of different methods and estimates of future species extinction rates. Mammal orders generally have substantially shorter half-lives and time to 100% extinction than bird orders because of a combination of fewer species per order and higher species extinction rates.