I did some digging around and discovered that the All Birds Wiki uses the antiquated NavFrame class. That being said, it is buried under a few template transclusions. The template that actually uses the class is here (Template:Hidden begin). I am unfamiliar with the class but based on what I have found so far, I believe the afore linked template is misusing the tags. From what I can gather, to hide the content, the NavContent div should have "display:none;" in the style parameter. What they may have been going for was something similar to "mw-collapsed" but I doubt this works on NavFrame and even if it did, they left out "mw-".
This is all a guess since I am not familiar with the NavFrame class. Perhaps someone who knows more about older versions of MediaWiki can provide a better answer.