No, category-nesting is not possible ( for the hard coded categories) .
One article can be located in the "_default:" category ( without any "…:" namespace before the page name)
AND can be attached to MORE but different (!) categories - giving always different URL's.!
What you can use are
A) the soft_categories ( like wikipedia does ) :
Every article - a page in the "default" cat. or "specific:" like your "parrots: cat :) - can have a link to a "category- page" like your
- "parrots:amazons" or
- "parrots:Macaws"
- "parrots:whatever" - where the aricles belongs too.
These are pages holding the "headers" of your categories and shows all "linked" pages inside these groups by using the "module backlinks"
See * http://www.wikidot.com/doc:backlinks-module
B) the tags for indexing your pages:
You can give every article some tags like "parrots amazons Macaws whatever…" . This helps not only in the tag cloud to find some pages on an easy way;
but you can use the "ListPages" module on some "header" pages ( like in the section A) before) with a (fixed ) tag selection like "+parrots,+amazon" which gives you all articles tagged wizth these 2 tags only!
Hope this helps to imagine what is possible?