@wyoDreamer has a good point, one completely empty open acre of land is not the same as one that accommodates a house and driveway and utilities and setbacks etc.
This is on par with a lot of ordinances I have seen. It has to do with a few things. One is protecting vegetation, the other is protecting our ground water (in terms of contamination from waste as well as how much is being used). You may be able to have a change where instead of just chickens...