The way I was tought:

"Juvenal" is an adjective with a precise meaning to an academic
ornithologist (as previously quoted).

"Juvenile" is used by those same ornithologists as the corresponding
noun, sometimes as an adjective when referring to the bird's age group
as opposed to its plumage.

I.e., a juvenile bird (typically) has juvenal plumage.

Nonacademics tend to use "juvenile" as both an adjective and a noun,
without as precise a meaning, more-or-less as a way of conveying the
notion of avian youth.

If you have the urge to be a pedant (that's not a bad word, anybody),
and use e.g. "first pre-basic plumage" freely, then I daresay you will
want to get it `right'. Many of us, however, probably don't give a
hoot (i.e., we're mere juvenile owls, but we'd be juvenal owls if we
were wearing baggy trousers).

;-)
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P W (Bill) Smith
Grays Harbor, Washington USA
pwsmith at dc.seflin.org