All of the local eBird reviewers set their own filters based on county levels. These filters allow reviewers to catch eBird observations that have potential for data errors, such as rare or easily misidentified species. The eBird needs alerts and rare bird alerts are based on observations flagged by these filters. When you see common species showing up on these lists, it is likely that a birder chose to report a subspecies that triggered one of the filters. Most eBird reviewers have subspecies flagged to ensure the proper written description or evidence is provided when identifications are made to that taxonomic level.
Below I've linked an article about the review process that might help further answer your question. It's one that I'm sure other eBirders have had as well. Cheers!
On Mon, Jun 29, 2020 at 9:27 PM R Stewart <2cnewbirds...> wrote:
> Can someone please inform why certain very common birds keep showing up on > the Rare Bird Alert - such as Am Robin, (migratorius Group) , House Wren, > Turkey Vulture, Common Yellowthroat, Barn Swallow, Song Sparrow, etc? > AND why a Yellow-billed Cuckoo seen up N. is on the list and not the YBCU > reported in the southern 'sun belt?" > > -- > Ruth Stewart > E. Dorset VT >