A nature centers main purpose is not to exhibit species either. When the main purpose of the center is to preserve wild lands and their entire captive collection is more or less a single small room with a handful of tanks and/or a few makeshift aviaries, it's not a zoo. It's the same as if you went to a local public park and saw that they keep a couple of random rescued animals captive and had a Koi pond, then tried to count it as a zoo visit. That's where the duck pond comparison came into play. From my limited experience with European nature centers vs US ones, European centers with captive collections will often have an actual zoo on-site operating as a zoo separate from the preserve (ex. WWT), whereas US centers just have a couple of locally rescued animals on display in the main visitor center (ex. Audubon). It's the same logic for why museums with a couple of live animals here and there have never traditionally counted. They do not operate as a zoo nor is it their main purpose.
As I mentioned this is not my rule, they've never counted as zoos as far as I've seen. Rule #6 is a word for word copy from every other past challenge, but has still never included these small centers. Last year for the North American challenge I visited an Audubon Center that had a much larger captive collection than most US nature centers tend to have and double-checked with
@jayjds2 to see if it would count, who also insisted on it not.
~Thylo