I'll keep my comments to composition, as otherwise things look quite promising. My comments may seem a bit blunt, but they are honest and I hope you will find them useful...
1. For me this scene is a cluttered mess. Your subject is small in the frame. The background is a busy and clearly contains man made objects and constructions, which I try to avoid including if at all possible. You've got all that dead space above the subject and yet have succeeded in just chopping off the feet. The other birds in the frame contribute nothing to the subject and are just untidy distractions. Some cropping might help, but why shoot this at 85mm when you have at least 200mm in your arsenal? I would suggest finding a better shooting position and tightening your composition.
2. Again we have the obvious man made structures in the shot. To capture that poor creature staring blankly at those wooden slats/planks or whatever they are just seems so sad. Why not wait until the animal is in a better position, or try to coax it to move.
3. Again we have that distracting thing going on in the background. This needs cropping to focus attention fully onto the tortoise. Timing could be better too. You've got the top of its head. Where is the eye? Where is the face? Where is the character of the animal?
4. Now this one I like. Great pose, nicely framed, cheeky look.There are no distractions in the frame and the subject is of good size within it. Lovely. It does look a bit soft though and you might be able to sharpen it up a little and perhaps just increase contrast a tad too. Easily the pick of the bunch for me.
5. I really don't like the shooting angle. I like to try to get close to eye level with my subjects if at all possible. If not possible then I probably won't bother with the shot. I don't especially like seeing the tops of their heads. I like it to appear as though I am in amomngst the creatures, not peering over a wall at them. One of the penguins has a brilliant pose, all arms (I suppose flippers really) in the air, but the magic is lost because you are not engaged with the animal and the raised flipper is kind of lost in with the body and almsost obscuring the eye. Eyes are really important.
6. Not bad, but I don't think the spare meerkat adds to the image. It detracts. I'd simply wait until the coast was clear, or shoot a different animal that was better positioned. Ignoring the spare meerkat I think the main subject is a bit too centred with an excess of space above and behind the animal. Still, as far as the main subject goes I think this is your second best shot. You have the eye, the face and it fills the frame reasonably well. If you'd got rid of the spare part on the right it would have been much better.
7. Wrong eye. The kind of shot which if I ever take will invariably be deleted. It is not interesting to me. It's not artistic or funny. Now, three in a row all doing the same thing, perhaps with a nice reflection, might be artistic, but not this. It's a snapshot. Sorry.
Key points (of course there are always exceptions, but these are good guidelines).....
1. Avoid a distracting background or other distractions in the frame. Change your shooting angle if you need to, or use shallow DOF creatively to blur out the background.
2. Make the sbject large in the frame. Apart from being more interesting, usually, this will help remove the background from the frame.
3. (nearly) Always try to capture the eye, the face, the character.
4. Get on level terms with the animal. Make it look like you are there with it.
5. Timing is important. Wait for the subject to adopt an interesting pose/expression. If it looks towards the camera then even better. If there is something in the way then wait for it to move out of the shot.
6. If you are there for photography, rather than to see every animal on display then take your time. I'd rather come home with 10 or 20 cracking shots than 50 or 100 mediocre ones.
If you want to see the sort of approach I take to zoo photography then have a look at my shots in this thread -
http://www.talkphotography.co.uk/forums/showthread.php?t=288707. There's always room for improvement, so if you have any feedback I'd welcome it.