
These are species with at least one recorded territory in Bulgaria, 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 Bulgaria, among those a photograph exists for. Most recorded is not most numerous: it rewards animals that are common, conspicuous and live where people go.

Corvus cornix

Microcarbo pygmaeus

Garrulus glandarius

Pica pica

Parus major

Larus michahellis

Dendrocopos syriacus

Picus viridis

Tachymarptis melba

Circus aeruginosus

Pelecanus crispus

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