I'd allow it for the right tenant. I have about 20 chickens myself though so I'm partial to people being able to grow their own food. In reality, with the right density of birds, 3 - 5 birds in a yard are MUCH less risk than a dog in the house... depending on your city I suppose.
I would only allow it if its legal and I could charge a pet fee. Try to get whatever number you think they would pay. Then sign a pet addendum that states if neighbors complain, they will have 7 days to get rid of the birds. Make everything clear in writing and get some extra $ out of the deal.
Also, I would make sure they don't build any permanent structure. People can get ready made coops at the tractor store that you move around the yard. With the right management, its free fertilization and great entertainment... not to mention they are making the space more of their home. If you have good tenants, you want them to be able to feel like its home so they stay.
If they are not good tenants, don't allow it.