> Birds such as chickens have gravel-containing gizzards that 
> help them break down seed hulls, but even chickens can 
> develop nutritional deficiencies from eating too much cereal 
> grains. 

Wolfgang Lutz conducted experiments with chickens. He showed that they
develop fatty plaques on blood vessel walls when eating a diet dominated by
grains while free range chickens do not develop these plaques. I think the
study was published in German, but he mentioned his findings in: Lutz, W.
(1995). The Colonization of Europe and Our Western Diseases. Medical
Hypotheses. 45, 115-120  

> Also, does anyone know the 
> purpose of phytase in ruminants, given that they can only 
> handle small proportions of grains in their diet--that is, 
> did they adapt evolutionarily to digest SOME grain, or do 
> non-cereal grasses also contain phytates, just in smaller 
> concentrations?
Not only do grains contain phytic acid, most seeds and nuts also contain
them. Further, legumes contain quite a lot of phytates
http://www.springerlink.com/content/t00214j2135x0r37/
 
Best Wishes, 
Ron