Ice Build Up on Birds

It has certainly been darn right freezing of late. So cold that I have seen several birds with ice build up on them. I can only find very limited information about this and most of it relates to an anecdotal account or two, like a layer of ice on the wings of geese which render it unable to fly, or Great Blue Herons and Rock Sandpipers (the most northerly wintering sandpiper) with ice encrusted legs.

Since I am not seeing it in all birds, it leads me to believe that it possibly could be because of a breakdown in the necessary functions of some individuals. Certainly a breakdown in natural oiling seems plausible when there is icing on feathers. But what about the bare parts, such as the bill or legs? Birds have a heat-exchange system that prevents loss of body heat. Did this breakdown?

Does air temperature have anything to do with this? What about relative salinity? Both the photographic examples that I have shown, the Tundra Swan (Lake Ontario, ONT) with a sheathe of ice around its bill, and the Black Scoter (Barnegat Light, NJ) with the two ice pellets on its back were taken on sub freezing days with temperatures near zero degrees F. The swan was one of about 7 in a group of about 20 in freshwater that had ice in some form on their bills. The scoter was by itself in saltwater and the only duck I could see that was in this condition.