I notice that two of my hives have bees bearding on the face regularly.
Humidity, heat, drying honey, space. Its natural. Kind of cool huh. G
Don't forget that sometimes it's a sign of an impending swarm. I don't know how to tell when that's the case, though.
http://www.bushfarms.com/beesfaqs.htm#bearding
Any hive that isn't bearding in hot weather is probably a weak hive. I would not take it as a sign of an impending swarm. But the same circumstances, other than the heat, that lead to bearding might be a sing of some of the circumstances that lead to a swarm. If one hive is bearding and the others are not, you might want to see if they need a super...