Being as honest as I can and with experience, I'd probably ignore them and go about my zoo day out.
However, if I happen to 'have a bee in my bonnet', then I would approach them and ask
1. What they are doing it for?
2. What they intend to achieve with this protest?
3. What they do themselves for any animal wellbeing or conservation themselves that prompts them to be here today?
I had a good old 'ding dong' with some of these people in the late 1990's outside Southport zoo. They where just people who had read a magazine (pre internet) and thought they were in the right. When asked what should we do with these animals in the zoo, they said let them go free in the wild.
The wild of what? Southport?