For attracting swarms
A bait hive is an empty hive that is set up to attract a swarm during the swarming season.
For about a week before a colony swarms it sends out scout bees to find a new home. Although we shouldn't try to humanise bees there are several criteria that seem attractive to them, these being:-
Most beekeepers have seen bees inspecting an empty hive or a pile of supers during the summer. At first there is just the odd bee or two, then there are more, often becoming quite agitated. This normally happens for about a week then either ceases or a swarm appears. If activity ceases, the swarm could have found another home, the beekeeper has dealt with the colony or collected the swarm when it settled.
I have always put bait hives out and been successful at attracting swarms. I set them up as follows:-
If bees become interested in a bait hive the first thing to do is to check your own colonies. Even though you may think they are alright, there is a possibility there may be supersedure cells or queen cells you have missed. If they are not your bees they could come from another beekeeper or a feral colony, so take the usual precautions to avoid the possibility of them being infected with foul brood.
If the bait hive is at home where I can see what is going on, I only use one old comb. When the swarm arrives I can shake the bees off, clip the queen and fill up the brood box with foundation. If the bees have put honey/nectar in the comb it can be burnt. If I use one comb away from home where I don't see what is happening I am likely to get a box full of wild comb, so I fill the box with comb and take a risk on the possibility of getting foul brood.
There is a suggestion that bait hives should be placed at head height or above, but In my opinion this doesn't matter.
I have always had quite good success at attracting swarms into bait hives, but this has improved since I started putting them in places where ley lines cross.
Some beekeepers use swarm lures, but I think that is cheating and prefer to rely on my own skills.