this question basically infuriates me. allow me to inform you based on all our time working at the zoo:
most (not all but most) animals you see in captivity at the zoo are there for one of two reasons: vital breeding program, or sustained injuries that make it impossible for it to compete in the wild against other members of its species (or any other species). broken collarbone that never healed right, cracked flight bone, just because people can't see it doesn't mean the animal is perfectly healthy and ready to go hunt it up in the wild or whatever. thanks to the migratory bird act (which actually covers all birds (except house sparrows, and pigeons lol), whether they migrate or not, migratory in this case only refers to the fact that birds move around even within their area) the marine mammal act, and the exotic species act, they pretty much ensure that every single animal at every single zoo has an extensive paper trail detailing where it was removed from the wild and how (and reasons) or how it was otherwise acquired. people need to quit picking on zoos and go pick on shit like uncle jimbo's illegal exotic animal jamboree. most animals you see from zoos these days are placed there from other zoo's breeding stock. ie the reason the sf zoo has no elephants anymore is because no zoos have bred new elephants with the intent to place them in the sf zoo and no elephants have been removed from private captivity in california recently. if it were still the days of walking into a jungle and willy-nilly grabbing whatever animals your zoo lacked, we would have had new elephants 7yr ago when our last elephant died. (don't even get me started on replacing tatiana RIP FOREVER!). one of the most annoying things people say at the zoo is "you should let that ___ go" because pretty much fully 90% of the time if you let that animal go in the wild it would immediately fall prey, often to one of it's own species if it is a hunting animal. you wanna know the wildest, most ability to survive outside the zoo animals there are at any zoo? the fucking squirrels. and not the ones in cages. we need animals to breed (to repair the damage humanity has already done to them) and animals to learn from (for biologists and zoologists to figure out which mistakes not to repeat for one). if you REALLY want to help animals and repair animal habitats, burn down your house and let nature reclaim where ever you live.
eta: this absolutely isn't directed at other kinds of animal chitchat or activism. only to people who want to free all the fluffy animals from the zoo. people need to think about the shit they are spouting off because not thinking is what got us into this whole "endangered species every which way" scenario in the first place. or to use another analogy, don't be a PETA and end up killing all of the thousands of animals you wanted to save.