Possible Clues:

• Certain aerial feeders
• Do the birds move from the mouth to the oesophagus?
• Slows law up for the migrant birds
• Birds start to whistle in pussy willows
• 9s made up of the Walker family, according to Arthur Ransome
• Seasonal visitors behave like hippos aboard ship
• San Juan Capistrano returnees
• Takes in summer visitors
• Reluctantly accepts criterion that summer has arrived?
• Naively accepts birds of passage