There is a product on the market that traps and kills rodents by
electrocution. The mice or rats are concealed from view because they are
contained within a box once they are killed. By baiting them with
something more tempting than your grain (peanut butter maybe?), you may
succeed in keeping them away from the exhibit.
I took the liberty of looking it up for you. The company is called
Agrizap, and the product is called the "Rat Zapper." Thier web page is:
http://www.agrizap.com/
I have no idea what these devices cost, but they can't be too expensive. I
hope this helps.
Regards,
------
Christian C. Burke
Project Manager
Inter-Con Security Systems, Inc.
Los Angeles County Museum of Art <http://www.lacma.org>