I use the birdwing and the oak silk moth - oak silk moth cheaper but I don't see so many.
Only stock up on those larger than ave size (the ave size is in Liquefy's walk thru faq on gamefaqs) - can't remember what it is for the oak silk moth for the birdwing its approx. 247mm - ie I only keep birdwings over that size and sell the others.
Nat - can just give you an ordinary item for a birdwing - you don't know the colour & lustre, just the size - so make sure you only keep the above ave size ones - up to now this has always got me bug furniture.
where to get them - in quantity I use my own island set it up with flowers & one centre island tree (at night the tree can get oak silk moths).
During the day, birdwings will be plentiful, go in and out of building to respawn them.
If you have access to another town its far easier to get the furniture, the visitor presents the birdwings
- keep the winning record low (you scupper yourself if you don't) and every birdwing so long as it breaks the record gets you furniture but does not change that record (so long as you just do with the visitor).
Easy when you have 2 towns, otherwise try and pair up with a friend.
If you do thebug tourney in your own town: each birdwing you give nat can alter the record, making it harder to get the bug furniture.
So birdwings don't work for long. Start low with darners etc and work up.
You have to keep a written record of each birdwing's size, (& know which one is which in your store) & present to nat in increasing size. so you slowly break your winning record. Possibly having to end up presenting an above ave Hercules or golden stag.
If its a visitor giving the bugs - you don't need to know the exact size, just that its above the average size for that bug.
The visitor does not change the winning record, just have to give above ave size & beat the current winning record. (so keep that low).
Using a visitor makes getting the furniture a bit too easy, feels like cheating. It isn't, its quite valid for a visitor to participate in your bug tourney.
I checked Liquefy: ave size of birdwing is 250mm (so I've been chancing it! You can win with a smaller size if the colour/lustre is good but that is game generated, no way of knowing that up front)
ave size of oak silk moth 320mm, so you want to keep those that are over. I don't get that many, but they are a lot cheaper.