I have dealt with a bad rat infestation before and I also own three domestic rats. Rats are extremely intelligent. They are one of the only animals recently found in the animal kingdom along with humans, dolphins, and great apes that are capable of meta-thought...or thinking about thinking. They understand the concept of "I Don't Know." This is why they are used for lab experiments for humans. We share nearly 99% of our DNA with them! Rats actually are more closely related to humans than they are to mice!
So traps or poisons of any sort are only effective on a very few out of the horde. Rats are extremely wary of trying new foods, because they are incapable of vomiting. They will "taste test" something to see if it makes them ill first. If it does, then they will not go back for more and they will warn their comrades not to go near it (with a special scent in their urine). This is what happens with the majority of rats and poisons. It's usually only the starving or young and inexperienced rats that eat enough to get themselves killed. They do pretty much the same thing with traps.
I haven't heard of the rat zapper, but if you are having as bad an infestation as it sounds, the ONLY way to get rid of rats effectively is to make where they are dwelling is inhospitable. Trust me on this, when we cleared out my mother's house (she was a hoarder) after her death, and it was so infested with rats they were flying out of boxes and climbing UP our bodies to get away. Repeatedly. We set traps and our exterminator went to work repairing the entrance holes.
We only killed 3 out of the hundreds. They avoided the traps and the poison.
However, we got that house completely cleaned up and cleaned out, and the rats disappeared entirely. The house sold and it's been six months and the new owners have said they have not seen hide nor hair nor poop of any rat since.
So my advice:
- Keep ALL food cleaned up and locked up tight in chew-proof containers.
- Make sure all debris they could hide or nest in is gone. Wood piles, trash piles, stacks of boxes, attic junk, and so on. Get rid of ALL clutter.
- Cut back brush and trees. Rats are quite arboreal, and they will climb trees and drop onto your roof with them and use any cover to get to your house/coop.
- Scrub thoroughly. Rats are nearly blind, so they use urine scent trails to get around and get messages to the colony. Erase those and you will disorient them and their activity will curb for a few nights.
- Make sure all entry/exit holes are patched up. This is a royal pain, but it's one of THE most effective things you can possibly do. Call around and try to find an exterminator that knows how to find and fix their entry/exit holes. Rats can squeeze through entries the size of a quarter and their paths are easy to miss.
It's a LOT of work, but it works. I wish you best of luck fighting the wee beasties.