Having spent a huge amount of time messing around with Plumed and Tawny
Frogmouths in the field, I largely second what Greg says.
I think the first clue is that you saw the bird at all! Usually, Plumed
Frogmouths (Marbled is better applied to the northern form which has quite
different calls) are ridiculously hard to see. To put this in perspective,
in 10 years of working with Plumed Frogmouths in the Conondales Ranges (one
of the best areas for them) I only twice saw birds without any effort - one
a bird sitting on a roadside post (just as a Tawny would) and once in
flight. Usually they stay deep in the rainforest and typically high up, as
well. Of course, that's not a diagnostic feature, and who knows, perhaps a
bird wandering out into the Eucalypt forest would be more likely to be
visible ( they do wander out of the rainforest, but not often, but when I
have seen them do so, they were still very difficult to see). In practice,
you nearly always hear a Plumed long before you see it.
Plumage and eye-colour wise, I don't think anything you have said is going
to help either way. Although Plumed typically have redder/oranger eyes than
Tawny, there is a lot of variation in both species. The fact that you liken
it to a Boobook further suggests Tawny to me. It would be harder to think
that of a Plumed with its long, tapered tail. The "neat" barring in the
primaries sounds more like Tawny - in Plumed, the primaries are nearly
always barred with three colours not just dark and light as in a Tawny, so
the barring actually looks less conspicuous and more complex.
Both Tawny and Plumed vary a great deal in plumage, and at least part of
that is based on sex. One of the most reliable features, in my view, is that
Tawny's always show bold black streaks along the middle of each feather on
the crown and most of the back, whereas in Plumed, these streaks are much
finer and less conspicuous. There is a matter of degree in this, but it is
one of the features which makes Tawny's look streaked while Plumed looks
mottled (usually). But you would most likely have to be looking for that
feature to notice it.
The eyebrow is different. In Tawny's, the white stripe is typically more or
less linear as it separates the darker crown from the darker area through
the eye. In a Plumed, the supercilium is curved above and seems to arch over
the eye. With experience, this is usually a conspicuous field mark, but you
have to appreciate that just the conspicuousness of the supercilium is not
enough. Furthermore, in both species, the shape of the supercilium can
depend on how the feathers are being held.
The underparts vary enormously, and especially between the sexes in Plumed.
The main point I would make is that a bird which is boldly streaked with
black will almost always be a Tawny.
Overall, these species are not usually at all difficult to distinguish, once
you are familiar with both. Almost always, the whole experience is more of a
guide than anything else - you will hear a Plumed calling and put a lot of
effort into chasing it down or attracting it with a recording, finally
getting to see it (if you are lucky) high up in a rainforest tree. A bird
you see without any effort will almost always be a Tawny. But there will be
exceptions, of course!
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