The Bait Hive
Allow me to introduce...the first beehive in the new garden.
This is my bait hive - an empty hive set up with the intention of attracting a swarm. I've set it up now, before the swarming season kicks off, because at that point I'll be fully occupied with the "real" hives. And because I was itching to set up a hive in the garden!
It is intended to attract a swarm from my own colonies (when they are here) but may also attract a swarm that someone else has lost. Better for any swarm to take up residence in my bait hive than in the chimney, the attic, or the garden of somebody who panics and reaches for a can of insecticide.
It has another purpose too, serving as a way to detect when bees are thinking about swarming. Scout bees from a colony will visit possible new nest sites several days (possibly weeks) before the colony swarms. So if I see scout bees checking out this hive, I will know that some local colony is thinking swarming thoughts, and will watch mine all the more closely in case it is one of them. If I could be sure that a swarm from my apiary would select the bait hive as its new home then I could rest easy and let them swarm naturally, which would be a great pleasure. But as it is, the bees may well choose somewhere else to go, so I'll still have to try to prevent them swarming.
I'll hope to see a few scout bees checking out the bait hive, even if no swarm moves in. If I don't see any then it probably means the hive isn't very attractive. Whereas If I see dozens of scouts there at a time, I'll know there's a beard of bees hanging in some nearby tree and giving the bait hive serious consideration, so I'll stick around in the hope of seeing the swarm descend and pour itself into the hive.
Height
As you see, I set the bait hive on top of an empty water butt. The extra height should give it more "swarm appeal" - bees naturally seek new homes higher up than is convenient for the beekeeper to reach.
Tom Seeley's book Honeybee Democracy has a chapter called "Dream Home for Honeybees", in which he describes how he figured out what characteristics bees look for in a nesting site. Height is one of them, which makes sense when you're a flying insect whose nest is preyed on by ground-dwelling mammals like bears.
From that point of view, the bait hive is rather closer to the ground than the bees would like. Tom Seeley found the average height of nest entrances in wild bee trees to be 6.5 metres (21 feet) from the ground. But I need to be able to see the hive and safely lift it from wherever I put it, so we had to make compromises.
Smell
Inside the hive are some empty frames. Old brood comb is apparently attractive to swarms, but I didn't have any suitable combs to put out, so just used empty wooden frames. Even without comb, this hive should have a good "bee smell" - it's the old Smith Hive and has a nice propolis/honey scent left by its previous occupants.
You can get pheromone "swarm lures" to really draw the swarms in, like the brilliantly named commercial product "Swarm Commander". Or by popular belief you can use lemongrass. But I don't want to overdo it and lose the monitoring function. If the hive smells too interesting, it risks attracting general worker bees not just scouts.
Capacity
An advantage of the empty frames is that they don't divide the internal space of the hive like combs would do, making it easier for the bees to estimate the size of the brood box cavity (by flying from wall to wall and measuring the time of flight) and hopefully they will find it to their liking. Tom Seeley found the average cavity size of wild bee nests to be 45 litres. I've not measured the internal volume of the bait hive (I'm too big and clumsy to pull off that wall-to-wall flying trick) but it'll be in the right range.
Bait hive, before addition of mouseguard
Entrance - aspect and size
The hive faces South/South West - apparently honey bees like a sunny porch. I've put a mouseguard across the entrance to reduce the risk of squatters. The holes of the mouseguard are quite small, but hopefully not so small that the scout bees reject the hive.
I will anyway try a few different things, and see what works best (or at all), but this is what we're starting out with.
Bait hive on water butt. The black water butt stand (next to it) provides a handy step to reach the hive, and can be a tea-drinking stool at other times.
Update 20th April 2018
It seems to work...we have scouts!