bacteriology - Are there grass or fiber eating birds?

My understanding, that may be wrong, is that cellulose/fibre has little nutritional value to many animals because it's hard to break down thus making consumption inefficient. However, Ruminating mammals, other mammals such as black bears, and many insects such as grasshoppers digest grasses.

Are there any birds with the gut flora to break down fibre? Or even some that eat grass but digest it in another way?