Bait hives

They're used to catch swarms. They're baited with "swarm lure" i.e., lemon grass oil or other pheromone/lure such as queen juice (queens soaked in alcohol). They can be made of about anything but should be roughly the volume of a single deep hive body- 35-40 cubic liters or so. I'm actually using some old deeps for the purpose. I'm also putting a couple of old combs in as an additional attraction. You can buy bait hives (aka swarm traps) made out of pressed paper (like egg cartons) that are light, cheap, and convenient.

They're most attractive to bees looking for a home in the shade, 8-10 feet or more off the ground. Place them where you think there will be swarms, perhaps 1/2 mile or so from where you know there are hives that may swarm and check them periodically during the swarming season. If you're lucky, you'll find some bees have moved in.

Free bees

I'd gladly repeat what George has said. The pressed paper is highy favored by the bees. Unfortunately, the "box style" traps (which I still use) that held frames were discontinued due to problems with warping. The only pressed paper traps are now the "flower pot" style. For some unknown reason, these don't warp.

These flower pot traps work, but once the swarm moves in, they want to get right down to business and draw out comb, pack nectar and get that queen laying eggs. Unless you catch this swarm within one or two days of moving into the flower pot trap, you have a mess trying to move them into a regular hive. The fresh comb is extremely fragile and any brood is usually lost.

All my swarm traps use conventional frames. When I take the trap down and move it to my bee yard, I simply transfer the frames into a Langstroth hive.

This way, there is no disruption or damaged comb.

Just my 2 cents. WARNING: trapping feral swarms is a highly enjoyable, tremendously interesting aspect of beekeeping and should be considered potentially addictive.

Grant
Jackson, MO (hopelessly addicted)