
These are species with at least one recorded territory in United States, not a national checklist: a bird list from eBird or Avibase is more complete. The counts follow recording effort as much as wildlife, so a country with more visitors and more naturalists will show more of both.
The species with the most open occurrence records in United States, among those a photograph exists for. Most recorded is not most numerous: it rewards animals that are common, conspicuous and live where people go.

Cardinalis cardinalis

Turdus migratorius

Corvus brachyrhynchos

Zenaida macroura

Cyanocitta cristata

Haemorhous mexicanus

Melospiza melodia

Branta canadensis

Anas platyrhynchos

Melanerpes carolinus

Dryobates pubescens

Poecile atricapillus
The reference database has no regions filed under United States.
The field guide covers United States on the map, but none of its species have an entry written for them yet.