I'll be honest. Sometimes they're photographer's worst nightmare. However, by following these tips, they can become a dream come true [that is, a good dream, like swimming in a pool of pudding. Not a bad dream like getting chased by a giant vacuum cleaner].
1. TIME IS KEY
It's the golden rule of photography-- when possible, shoot when the lighting is good. The main difference between awesome and, well...not so awesome photos is usually lighting. Choose your time of day wisely. Usually in the morning before 10am and in the evening after 7pm is the best time to take photos. During other times of the day, the lighting is too bright and harsh. Unfortunately, there's not a lot even the most experienced Photoshop user can do to fix yucky mid-day lighting. Try to go to your local zoo as soon as it opens. The animals will be more active and the highlights and shadows will be nice and soft [plus it smells better, too ;)].
2. CAMOUFLAGE THE CAGE
Wildlife photography is an artist's paradise. But, since not all of us can afford to travel to Africa, zoos are our next best bet. Nothing screams amateur like a cage showing through a picture, so this tip is key. Do whatever you can to get "through" the cage. Zoom in, step closer, put your lens right up against the chain-link, or try standing on a park bench to see over the fence [just don't do anything dangerous or harmful to yourself or the animals]. When glass is available, turn off your flash and use a polarizing filter to get rid of reflections. You may notice your photos look really washed out because of the light's reflection on the glass. To fix this, you can in crease the blacks and contrast in your photo via Photoshop.
3. OWN THAT TELEPHOTO LENS
Don't be afraid to zoom in on your subjects. Capture the detail on those butterfly wings or try a head-shot of that zebra. When you have to zoom in to avoid cage lines [see above] sometime you can't fit the entire animal in the shot. It's ok. Own that telephoto lens, girl. [I love my 55-200 mm. It's enough zoom without being too heavy.]
4. TAKE LOTS OF PICTURES
As you already know, animals move. a. lot. To compensate for this, try using your camera's sports setting [or on manual, a fast shutter speed] and take lots of pictures. I like to do about 15 shots of each type of animal. Of the fifteen, usually only 1-2 turn out in focus and with the animal's face looking in a desirable direction. [Yes, I do have my fair share of shots of animal backs and tails. haha]
Following these easy steps, your zoo photos can go from drab to fab.
Happy photographing, and enjoy the zoo!