Hi Trevor,
I'd consider the undertail to refer to the underside of the rectrices (the
actual tail feathers). They are dark in Soft-plumaged Petrel. In contrast,
the undertail coverts which are the smaller more flexible feathers that sit
below the rectrices and extend to the rear of the vent are white in
Soft-plumaged Petrel. When Onley and Scofield state 'white undertail' they
more accurately mean white undertail coverts. O&S may have justified the use
of 'undertail' as a more general term somewhere in a terminology section as
most tubenoses show fairly long undertail coverts that obscure the underside
of the rectrices (undertail) - it might have been a good way to save a few
words in most species accounts.
Here is a pic of a Soft-plumaged petrel that shows the dark undertail and
the contrasting white undertail coverts.
http://www.pbase.com/wildlifeimages/image/97002553
Cheers,
Rohan
Rohan Clarke
www.wildlifeimages.com.au
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