Warbler - British Trust for Ornithology

6 INTRODUCTION
Scope ofthe Work
This is not a formal taxonomic review, and my main purpose
has been to ferret out points which I think: will assist the identification
in the hand of those birds, species or subspecies, which
have even an outside chance ofdrifting to the British Isles. It has
nevertheless been necessary to adopt a taxonomic approach, since
at one or two points I have found myself in disagreement with
the most recent review, Charles Vaurie's The Birds ofthe Palearctic
Fauna-a Systematic Reference. This excellent work deals most
competently with relationships and distribution, buthas the drawback
for the field-taxonomist that it lacks the plumage descriptions,
wing-formulae and measurements which are fundamental to
correct identification. It should, however, be in the library of
every bird observatory, and access to it cannot but enhance the
competence ofall who trap and ring migrants on any scale. The
reader is referred to this work for a full statement ofdistribution,
since only the bare outline is given in the Guide.
I have kept plumage descriptions to a minimum: in the case of
the more familiar birds, adequate data are given in The Handbook
and The Popular Handbook, whilst illustrations can also be found in
the field-guides ,by Peterson, Mountfort and Hollom, and Fitter
-and Richardson, to mention a few recent works. For this reason,
more space is allotted to a discussion of the unfamiliar extralimital
forms, and those in which confusion with a commoner
species could arise. In these genera the sexes are alike in plumage;
in general, rst-winter birds are separable from adults, and the
main points of distinction are given. The information on the
colours of bill, legs, inside of mouth and iris has been culled
partly from the literature, and partly from collectors' notes on the
labels ofmuseum specimens, written when the birds were freshly
killed. Since the work is intended for the use ofthose who handle
birds, I have not added to its bulk by setting down field-characters
(except in a few special cases) but have included references to notes
on this subject in the journals.
Many of these birds have narrow, dark 'fault-barring' across
the tail; such bars arise during the growth ofthe feathers, resulting
from their irregular protrusion from the protecting sheaths, and
their presence is not therefore mentioned under the separate
species. In some cases, where it may aid identification, the wing!
tail ratio (or tail-length expressed as a percentage of winglength)
is given under 'Measurements'.
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