I've made many trips around Europe with the main objective of visiting zoos (Spain, Portugal, Czech Republic, Poland, Italy, France, Germany, Belgium, Hungary, Switzerland, UK...) although it's true that I combine visits to zoos with gastronomic tourism and normal tourism (emblematic buildings, museums, parks, monuments...). My only trips outside of Europe were to Indonesia and California. In Indonesia I visited several zoos but they were absolutely secondary in the total computation of the trip. Instead, in California they were the main targets. In general, I try to combine zoos with all the other more normal goals, but itineraries are often organized around zoos.
I think that a very determining factor to visit zoos is the displacement. I think the most convenient is to rent a car, as many zoos can be difficult to access by public transport. A car allows you to save a lot of time and be more free.
Another very important factor is the people you travel with. It is not the same to travel with people who love zoos than with "normal" tourists. In Indonesia I escaped to be able to visit the zoos by myself, on the rest of the trips I was accompanied by Friends or family.
I think that a very determining factor to visit zoos is the displacement. I think the most convenient is to rent a car, as many zoos can be difficult to access by public transport. A car allows you to save a lot of time and be more free.
Another very important factor is the people you travel with. It is not the same to travel with people who love zoos than with "normal" tourists. In Indonesia I escaped to be able to visit the zoos by myself, on the rest of the trips I was accompanied by Friends or family.