Is there an achievement for finding all the birds, if not why not? Just curious. Also, how can a user know when they have caught all the bugs? Is it retroactive?

There isn't an achievement for finding all of the birds, but there is the "A World of Birds" achievement for finding birds specific to 8 different regions.

Also yes, the bug catching achievement should be able obtained over multiple seasons, and I think it does get updated when new bugs get added. There isn't really any way to tell which bugs you've already turned in, unless you're keeping track yourself. I'm not sure why that one is different from the bird one.

Thanks, I think the bird might have been from some event.