I see a lot of complaints about the UI and don't get me wrong, I'm complaining too (devs - there was a huge post during the beta, it covers it all nicely), it's annoying trying to figure out which animals would be the best to sell or release when there's 5+ of them in a habitat, don't know the family tree etc. (plus the native names are impossible to remember), let alone peafowls where you have 50 of them in a second. I use "compare mates" where you get all the relevant info to decide what animals I'll sell/release and mark them by giving them birth control.
Then go to zoo management, filter by species and order by maturity and select all the marked animals, do what you want from there.
Then go to zoo management, filter by species and order by maturity and select all the marked animals, do what you want from there.