I guess i should say it is just the "default". In the case of Dupage county, it is written into the code:
37-403.5: - USES NOT SPECIFICALLY PERMITTED IN DISTRICT/PROHIBITED.
Except as hereinafter provided, when a use is not specifically listed as a permitted or conditional use in any specific...
i think the consensus is that you can have chickens in unincorporated Dupage if:
1) you have 5+ acres and can be classified as "ag"
2) you have 40k sq ft (~1+ acre) and are in the 4H program
OR
3) you have less than a acre, your neighbors are cool and you don't get caught
chickens are not...
From what I can tell, the Downers Grove ordinance states you can have up to 4 mature birds, but the coop must be at least 50ft from all neighboring property lines. Since most average lots around here are 60 x 150 (give or take), it essentially prohibits them.