Possible Clues:

• Birds roost here through sitting under a railway
• Where to observe flight patterns
• Enclosed area where birds are kept
• High-ceilinged enclosure
• Enclosure originally approached by way of track
• Place for birds
• A change one introduced — it has some flapping
• Where flights are limited start to advance by railway
• A seesaw covering middle of big bird cage
• Area against airy building designed for high-flyers