However, note that the NACC and the SACC are not consistent among one another! In fact, there is no consistent AOU taxonomy! The North American and South American taxonomies differ in subtle ways.
This puts eBird in a bind, since eBird needs to be consistent across the database. The result is that eBird has chosen to follow the newest and best information when possible, even if the NACC (or SACC) has yet to consider it. This sometimes applies to scientific names, common names, or sequence. Our taxonomy article summarizes all differences between the AOU and eBird on this page, which discusses the taxonomic rules we follow for eBird.
Another case where eBird and NACC may be slightly out of sync is for Old World birds. For example, NACC has not yet considered the splits in Dusky Thrush (Dusky and Naumann's Thrushes) and it considered--and narrowly rejected--the split of the Stonechat (into European, African, and Siberian Stonechat, which eBird follows), so eBird has chosen to follow the British Ornithologists' Union decisions in those cases.
In 2016 the NACC made radical changes to the sequences of Orders in birds. eBird was not able to follow these changes (e.g., placing Procellariiformes in the middle of the checklist sequence, rather than near the beginning). These large-scale higher order changes will be tackled in August 2017.
Other key differences from NACC include:
- Sequence of hawks, eagles, and kites in Accipitridae
- Sequence of swallows and martins, including placement of Northern Rough-winged Swallow
- Species-level splits for Gray-faced Petrel, Band-rumped Storm-Petrel, Dusky Thrush (Naumann's Thrush), Stonechat (Siberian Stonechat), and a few others.
Note also that the eBird taxonomy and the Clements Checklist are integrated into a single project, the eBird-Clements taxonomy. The two projects are 100% in sync, although each list contains some birds (e.g., spuhs, slashes, hybrids, intergrades and domestics for eBird and subspecies for Clements; eBird and Clements both have identifiable subspecies groups).
See more on the Clements checklist here: http://www.birds.cornell.edu/clementschecklist