Zoos, that historically originate in the private entertainment of rulers and gradually moved on to the 'edutainment' of the general public are now trying to convince us that they are undertaking serious research and conservation work, perhaps as a reaction to increasing environmental and public awareness that the days of all animal abuse and incarceration facilities (including circuses, dolphinaria) are numbered. The green-(magic) imagemaking effort sometimes includes producing articles by prestigious researchers in prestigious journals, such as this one
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0080311 which claims that up to a quarter of species 'kept' in zoos are officially-considered endangered. (This by itself is one more way in which a designation of a species as endangered may be counter-productive under the currently prevailing socio-economic system (capitalism) as the designation increases its monetary value.).
If zoos really wanted to clean up their act, they would gradually transform themselves into research & conservation institutions, abolish public animal enclosures, return all existing non-endangered animals to wilderness areas, stop purchasing new ones (often from illegal purveyors), and turn former zoo facilities into nature and recreation / amusement parks but without animals.