Deputy Director-General Theerapat Prayoonsit said the Customs Department found 11 otters in a passenger's baggage at Suvarnabhumi Airport's departure terminal.
He said authorities are now identifying the owner of the seized baggage. It was believed the baggage belonged to a foreigner who was filmed on the airport's closed circuit television.
Initial investigation found that the otters were being smuggled to Japan for a circus show.
Mr Theerapat said the department confiscated protected rare birds at Chatuchak weekend market including white-rumped shama and red-whiskered bulbuls.
The owner of the birds did not show up at the scene.
Department agents inspected bird vendors at the statue of Brahma at Ratchaprasong intersection, in the city centre, and seized a number of rare birds. Legal action will be taken against vendors who violate the Wildlife Preservation and Protection Act, Mr Theerapat said.
He urged the public not to buy birds from the vendors for freeing them as a merit-making, saying that the more the people buy caged birds for merit, the more they encourage illegal trade.