See page 282 at
Bird Migration - Google Book Search
Birds v. Planes - TIME
http://elibrary.unm.edu/sora/Auk/v11...0771-p0774.pdf
One suggestion is that while they may be able to fly in cloud they dislike doing so because they have difficulty ridding themselves of moisture produced by exertion in the prevailing humidity. Some book I read many, many years ago of WWII bomber crews who carried pigeons, stated that a pigeon released in cloud would adopt a gliding posture until becoming visual.