Why do bees beard?

[flyboy]

I notice that two of my hives have bees bearding on the face regularly.

[biggraham610]

Humidity, heat, drying honey, space. Its natural. Kind of cool huh. G

[Dallasbeek]

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.

[Michael Bush]

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...