OMG THEY ARE GETTING READY TO ATTACK CHILDREN!!!...hehe ok, horrible joke..... as Iddee said, it's just bearding and it is done for many reasons. Often when there isn't a nectar flow going on, field bees do not actually have enough to do and just sorta hang out even. If you see them on the landing or in the entrance and they are in patterns facing the same direction for instance, butts sorts in the air and not moving at all except the blur of their wings periodically then they are fanning, inside a hive, the temperatures can get very high actually and they cool it off that way. The entrance size can effect that also, often with a larger entrance, there will be less bearding and more airflow. I see you have a entrance reducer on. Also the more that go out there, no doubt the more 'in' thing to do is go out there and hang with everyone else and crawl all over them and stick your butt in their face or be part of the action. It's really a lot of what they do 'inside' the hive too. and a lot of it is sorta like your mom telling you to get out of the house on a nice day, instead of just raising the heat and taking up oxygen inside, and once again stopping the cooling breeze coming in. brood will die off in their cells if the temps are to high, not to mention new wax/etc gets awfully soft.
If your hive beards in early spring, it is often getting ready to swarm. if it is hot outside and it is in the afternoon they are usually just hanging out, and they sound differently also. a swarming beard is louder more aggressive 'sounding', and from that point on it'll only last a half hour or so and they will bee all over the place more and more and moving faster, then the queen joins them and they leave. a regular beard like you have just sorta hangs out and has normal comings and goings, with a bit of flight, usually in one area at the entrance.
increase your ventilation and you will see bearding go down more than likely. also if you have a well insulated hive, bees can keep it cooler in the summer and warmer in the winter. people often use 3/4 lumber and it works, don't misunderstand me, but it isn't really insulated like for instance the inside of a tree that has 5-6 inch thick walls. insulation works both ways....keeps the hot hot and the cool cool. Bees will find a space and then change their location to the micro climate within it to suit their needs....sorta like a cat going down in the basement or on the floor when it is hot outside and sleeping on the register when it is cold or basking in that one ray of sunlight that comes in through the window.
