424. BRITISH BIRDS. 
domed nest with the entrance at the side, a feature distinguishing them 
from all the genera alluded to except Abrornis. ‘Typical examples of the 
Flycatcher Warblers have a very wide flat bill abundantly furnished 
with long rictal bristles, and have rounded wings, with the first primary 
half the length of the second, or nearly so. Many species, however, are 
so exactly intermediate that, when I wrote the volume of the British- 
Museum Catalogue of Birds contaiing the Sylviine, I should most 
certainly have considered whether some of them ought not to be included 
in the genus Phylloscopus, if my colleague for the time being had not cut 
the Gordian knot for me by having already absorbed them into the volume 
containing the Muscicapide. 
From the other allied genera, the various species of which build cup- 
shaped nests, the yellow axillaries of the Willow-Warblers are a sufficient 
distinction, with the exception of two aberrant species of the genus 
Hypolais, which also have yellow axillaries. We must therefore find 
another character, and are obliged to fall back upon the comparative size 
of the bill and wing. The Tree-Warblers have large bills, the length of 
the culmen being about a fifth of the length of the wing ; whilst the sub- 
generic group of the Willow-Warblers, which, like the Tree-Warblers, 
have no pale bar across the wings formed by the greater wing-coverts 
having pale tips, have small bills, the length of the culmen being about 
one sixth of the length of the wing. 
The Willow-Warblers are essentially fly-catchers in their habits, and 
may constantly be seen catching flies upon the wing; but probably the 
greater part of their food is picked off the leaves. Accordingly their bills, 
if not very wide, are considerably depressed at the base, and the rictal 
bristles are well developed. Most of the species are migratory, the range 
of their seasonal distribution frequently extending over thousands of 
miles; consequently their wings are long, flat, and pomted, whilst the 
bastard primary is very small. The migration of others is confined to the 
plains within sight of the mountains where they breed. In these the wing 
is shorter, more concaved to fit the body, and rounded, whilst the bastard 
primary is larger, the vital energy required for its production not having 
apparently been absorbed in lengthening the adjoming primaries. ‘The 
tail is generally even and frequently forked. All the species of the 
genus are soberly coloured, the upper parts varying from yellowish 
green to buffish brown, and the underparts from yellow to buff or 
white. . 
Probably all the species build a semi-domed nest on or near the ground, 
and lay white eggs spotted with red. 
In their breeding-range the Willow-Warblers are Palearctic, ranging 
from the Atlantic to the Pacific. Several species extend their range 
-further north than the limit of forest-growth. Those species which do not 
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