Late spring would be a good time, Tina. So many of the animals are cooped up inside now; many out of view.
Unfortunately, many are separated from view by glass ... as was the case in the "ape house" ... and the zoo doesn't follow "photographer standards" in keeping the class clean. This Mandrill photo suffers from being shot thru grimy and smeared glass plus a pretty high ISO.
All in all, I think the Milwaukee County Zoo is one of the very best locations for a photographer. Most of the outside exhibits are quite large and I usually use a 70-300mm lens. It's a very slow lens but not a problem in daylight. When I move inside I switch to a 28-75mm ... not as long as I would like but it's a constant f/2.8