Cornstarch does not do anything against a really good bleed from something CAG-sized (I found out the hard way, just like how painfull the bird finds it if you clip the nail to the bleeding part /just nailfiling for me now! ).
I had a hell of a time finding quickstop.
That I had no idea what is was called did not help either

(Only could get it online - from Belgium!- as an added substance to cottonwool -really not something I want around birds. Stupid EU and its stupid rules again? The vets have it, but are not allowed to sell it or even let it out of the practice...and as a non-vet you can't get any.)
That is why I stumbled on 'old timers'- advice to sort of melt the nail shut (using a hot spoon / some people actually had a soldering iron lying around for that purpose...).
Never did it myself - sounded too scary, but apperently is still being done in case of emergency.
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to my defense:
I only had my macaw under aneasthesia because she needed a super-thourough check-up (with an extra assesment for the busted up wings as wel as drawing blood for the every scary-disease-in-the-book tests, DNA was a bonus) - so would not do it for fun .
... but she needs to got back for more testing in a few months and since she is touchy as hell with her painfull wings (which hyperreact to being constrained with a towel as well -> extra partytricks anyone?) I don't seem to have any other options (save a miracle of course)
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You are absolutely correct:
DNA is DNA indeed, but every testresult is only as good as the material sent in!
And not every sample is as uncontaminated as it should be...
(that is the *only* reason why the lab preferred bloodsamples over feathers)
Anyway- I think we all agree that for a firsttimer or someone who has no acces to a vet or experienced bird-person featherpulling is a safer way to get DNA for testing than deep-clipping a nail.
(OP's question in the first place)