Just some food for thought. Birds can see UV light, and UV passes through clouds (you can still get sunburn on a cloudy day :o ), so maybe they see UV reflections and shadows, and use that to help fly/nav through cloud?
The same quesion could by asked about deep sea fish. Its pitch black, how do they know which way is up, and how do they nav. Pressure differences and magnetic fields perhaps (same as birds?)
