This post is just to show some details of the latest brooder version, which I'm using in a tractor. However I need to add a very strong caution about following my lead: be sure that no rain can seep underneath the nest area (a tray full of litter for the birds to sleep on may help), no draughts can blow through from any direction, and the ambient temperatures are not going to be even a little bit Siberian (though of course this depends on the age of the birds). In fact the best place to use a cold brooder like this one would be in a cosily sealed shed, with clean dry litter all over the floor.
As with all cold brooding remember too that the trick is to balance insulation and warmth against ventilation (alongside the ability to expand the brooding area as chicks grow). It's not rocket science but neither is it a total cinch get right first time. The safe thing is to remain vigilant until you know what you're doing. And also as with all cold brooding, chicks have to be taught to use it.
Here are some details of the unit. It was incredibly easy to make, and took my birds from the day they turned 3 weeks of age (when they outgrew their former brooder and run). I feel this is the right age to try using a brooder like this one, because the birds have some feathers and are that bit more hardy, and this brooder is really quite airy. Furthermore the metal legs can tend to dissipate heat away from the unit, whereas a wooden-legged brooder might not... (To stop that I've got the curtain running around on the inside of all metal parts.)
Anyhow, here is the basic skeleton (as a sketch):
It's basically a rectangular mesh box. Two of the extended sides form supports for an open area underneath. However these mesh sides aren't quite strong enough in themselves to keep the unit from flexing, so I've added 4 makeshift 'legs' made of cheap brackets wired to the box base. You can see how they sit below:
The height of these legs and hence the height the box of hay sits above the ground is dependent on the height of the chicks. For very young birds all cold brooders need to have their insulation (or ceiling) as close as possible to the chicks. My birds, being older, could cope with a space under the box being about head height while standing normally (younger birds would need a slightly lower ceiling, so they have to duck when entering). Naturally the area enclosed by the fringed curtain (or the perimeter of the legged box) also reflects the grouped size of that particular number of birds. There can obviously be some leeway with this; you don't want chicks to be packed like sardines so that some spill out through the curtain and others can't breathe.
I've put some fairly thick material as a curtain all the way around this brooder (attached by c clips to the base of the box), and along the front where chicks come and go I've cut the fabric into strips to make a fringe:
When first starting out with this kind of cold brooder it's vital to make sure that chicks can go nowhere else except into the nestbox — that is, they can't pace up and down the sides looking for a way in and eventually chilling. If the run is the same width as the nest area the birds will happily learn to go through the curtain into the warmer area under the hay.
The hay in the box on top isn't packed firmly, and is about 10cm deep:
As you can see from the above photo I've got the hay sitting on a layer of shadecloth so it doesn't all fall through the rather wide mesh.
The tarp on top of the unit wouldn't be necessary in a cosy shed, and in fact is a bit of a drawback. That is, as the chicks breathe the hay is meant to breathe too, allowing water vapour from their exhalations to rise up and dissipate without wetting the hay. However with a tarp on top the hay can get damp quite quickly. For this reason I've got the sides of the box above the layer of hay quite open, with the tarp pinned so that it really only covers the top, not any of the sides. The tarp is partly there to stop any moisture falling into the hay from above (my tractor isn't 100% waterproof on top) and partly to stop chicks scratching all their insulation hay out during the day (which they would otherwise love to do). However in a dry shed you could just lay a sheet of mesh on top to discourage scratching.
Remember this cold brooder is being used for birds around 3 weeks of age... However it's getting down to about 5C (last night was even colder) so I'm pleased to say it's working well for birds at that age.
Just one more thing: I've seen similar brooders to this one being recommended for younger chicks. In particular I remember one made of timber, with wing-nuts that allowed the 4 legs to be lengthened or shortened depending on the age of the chicks. I'd be a little careful using this kind of brooder in really low temperatures — I feel they can be a little too airy for really young chicks. But on the other hand chicks do need ventilation, and it's very easy for my other cold brooder (the square timber box version) to become filled with dampness and ammonia from a couple of nights' brooding.
Ultimately this is an art for people with a bit of free time and a great willingness to fiddle and fidget... But I must say, when the chicks emerge at a happy run exactly as they do from under a hen, it can be a very rewarding way to brood. :-)