
These are species with at least one recorded territory in Croatia, 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 Croatia, 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

Larus michahellis

Egretta garzetta

Microcarbo pygmaeus

Corvus frugilegus

Passer montanus

Tringa nebularia

Himantopus himantopus

Alcedo atthis

Cettia cetti

Podiceps cristatus

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