Ok - first of all, you might wonder why a psychotherapist is answering this! Basically, it's because I had a late in life career change, and previous to that, I was an environmental health specialist, and at one stage, looked after Vecor Control for a major company in the Middle East.

First of all, with the rat population you are talking about, snap traps are simply not going to do it.

You are going to have to use a two-pronged approach. You could always call in a pro Pest Control company, but I suspect that you have already ruled that out.

Now, you have to try to arrange a fairly quick kill of the majority of your rat population. You need to use a chronic, NOT acute poison, so that the family (and pets are not at risk)

Something based on Warfarin or Coumarin, whare you can place small quantities inside, for example, drain tiles along the rat runs in your property. Remember, rats do not like to cross open spaces, so place them along the fence lines of your chicken coop.

Secondly, make sure that you stop any drips or leaks from water pipes around.

Thirdly make sure that the mesh around the chicken area is in perfect condotion, and small enough to exclude them. A juvenile rat can get through an opening not much larger than a ballpoint pen. Make sure that any spilled chicken food is removed promptly, and that any cat or dog food bowls are removed imediately after the animals have eaten.

Basically, deny them harborage, access to food and water. Make it easy for them to access your poison baits - but make sure that these are more or less inaccessible to kids.

Don't EVER use an acute poison unless you are qualified. This should work for you, but if all else failes, call in the professionals!

There are no poisons that are really save, but you'll find a safe way to use them here

Other than poison, intensive trapping and good proof proofing is about all you can do.