More than 100 birds have been seized and 13 people arrested during a police raid on an east London pub.
Wild finches sell for up to £100 but many die while being caught
RSPCA and police officers raided the Fountain pub on Sceptre Road, Bethnal Green, at Saturday lunchtime.
It is thought most of the birds were wild UK finches.
The suspects, aged between 30 and 50, were taken to police stations across the capital for questioning over alleged offences of animal cruelty.
As well as seizing birds - thought to include bull and green finches - cages and nets were also recovered during Operation Kongo.
Chief Superintendent Barry Fryer, of the RSPCA special operations unit, said: "There is a very legitimate fraternity who breed finches and a legal trade of those bred in captivity.
"But there are people who are introducing wild bred birds into their breeding for reasons such as they have brighter feathers or a nicer song."
As well as the RSPCA, members of the Met's Wildlife Crime Unit, Territorial Support Group and Tower Hamlets officers were involved in the raid.
The arrested men, who will also be interviewed over alleged offences under the Wildlife and Countryside Act, were from various parts of the country.