Officials from the Department of National Parks, Wildlife and Plant Conservation (DNP) captured some of the birds from Silom to check their health including tests for bird flu.

The birds were tagged before being let free again.

Officials want to monitor and gather information on migratory birds, some of which are believed to fly from China and Russia in search of friendly weather conditions.

Available records suggest the number of barn swallows in the Silom area has decreased. In 1994, a survey estimated the population at 270,000. Late last year, officials surveyed the population and found just 1,500 barn swallows in Silom.