Interesting. How do they manage in icing conditions?
I do believe they have found the bodies of crashed birds which have experienced severe icing in thunderstorms and have literally dropped out of the sky!
For normal icing conditions they use their feathers like boots on aircraft expanding and contracting the feathers to remove high levels of ice build ups.
Too severe and the ice build up on their backs and exposed surfaces becomes too much and they crash.
It really depends like aircraft on the specific bird type on how prone to icing they are.
Swans are notoriously bad as are ducks. Water based birds which have large swimming legs and webbed feet designed for water propulsion ice up badly.
Water landings in such a situation have caused many problems with the birds unable to extend their legs for landing with fatal consequences.
Pace