I have not been to a zoo party, but as the mother of 4 kiddos (meaning I've had one year olds 4 times) - quite honestly, having to take a one year old to the zoo and go on an organized tour would have really seemed like torture to me, lol. Ok - not torture, but not fun. And the birthday child will not even remember that birthday. I think these days it is so easy to get carried away when it comes to birthdays - but no matter what we have done over the years, the birthdays my kids remember the most, were the parties we had at home where they could just play and hang out with their friends.
If you are inviting other moms with one-year olds, I HIGHLY recommend an at home or closed room party, moms of one year olds go to parties b/c they want to socialize with other moms, lol. Carting kids around the zoo doesn't offer them that opportunity. One-year old birthdays are more for the parents than the child, so do something the parents will enjoy but that kids can be safe and entertained :)
Good Luck!