It's a pretty common display technique to include either a small ungulate (pudu, muntjac, dik-dik, chevrotain etc) or a larger rodent (like agouti) to give some additional interest in an aviary, walk-through ot otherwise. Just mixes things up a bit (and, with my cynic's hat on, means visitors are less likely to write the exhibit off as 'just birds'..!).
Probably the most interesting animals I've seen displayed this way are the Royal Antelopes in one of the aviaries at Lowry Park.
Another small mammal that I have seen a lot in aviaries to add interest for people who aren't so interested in birds is having a fruitbat species, and I think that's quite an effective display too.
