Thousands of swallows roost overnight in a single apartment block in Pasir Ris during the migration period. Every year during the migration period between October and February, certain areas in Singapore are visited by large flocks of swallows. It is not known why the swallows would choose only certain apartment blocks to roost for the night. Experts believed that this could be due to the urban heat effect, where the warmer urban areas actually provide these small birds their preferred temperature to roost through the night.