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

Pycnonotus sinensis

Otus spilocephalus

Passer montanus

Egretta garzetta

Zosterops japonicus

Spilopelia chinensis

Acridotheres javanicus

Streptopelia tranquebarica

Heterophasia auricularis

Psilopogon nuchalis

Dicrurus macrocercus

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