FOOD GRADE (the other kind is a pool cleaner and fatal) Diatomaceous Earth. http://www.richsoil.com/diatomaceous-earth.jsp You can get it at any feed store and even order it online. That stuff is GOLD. We use it all the time and IMO everyone who has pets or a zoo like us, should have it on hand. Do a Google search on this amazing, natural product! The best part? Totally safe for dogs, cats, chickens, torts - you name it!
Yeah so there are many things to do. Repel or kill? The ants and yard will dictate what strategy you may implement for the best result.
Where are you, how are the pens set up, do you have paths around the enclosures are there many in series. An over all picture or diagram would be helpful.
Boric acid solutions are good, neem oil is good, the dry diatomaceous earth that is used for roaches can work etc., so can a clear swath of sand all the way around, but cleaned kiddy play sand, not river sand.
Regularly flooding the whole area can work too, but requires a great deal of water.
You can also type in "ants" in the GOOGLE Custom Search box in the upper right corner on every Forum page. There are quite a few good ways to cut down the ant population that we've talked about here.