@SivatheriumGuy definitely recommend it. The two Berlin zoos are amazing. I went in winter and Tierpark they are a no show but thankfully there was the lone male at Berlin to see.
@SivatheriumGuy depends on yourself but I did both in a day each, Tierpark is a stretch in a day but do able with a quick walk. We only had 4/5 days in Berlin so wanted to see some of the other sights of the city while there.
Loved the city and loved the zoos. I want to go again but might wait until after the big construction works have finished.
@SivatheriumGuy I think it depends how many groups of animals you are interested to observe.If the Mammal collections is your main focus, with bit of luck, one day in each collection is fine. If you go all-in with all the Birds and Reptiles + Berlin Aquarium, than I think one day is definately not enogh.
@twilighter My main goals are the ungulates and birds, so the four-day trip makes sense to me. I've already visited the city of Berlin, so I wouldn't miss much. And yeah, one of the zoo days would also include the Berlin Aquarium.