68 Mr. H. Seebohm on the Phylloscopi 
in the western hemisphere, and that probably an accidental 
straggler on its first autumnal migration. 
The principal points to be observed in determining the 
various species of this genus are :—( a ) the size of the bill and 
the colour of the under mandible; ( b ) the size of the bastard 
primary (in the following description the exposed portion 
only is measured) ; (c) the wing-formula, especially the rela¬ 
tion which the second primary bears in length to the other 
primaries; (d) the comparison between the lengths of the 
wings and tail; ( e ) the presence or absence of one or two 
bars across the wings, formed by the wing-coverts being paler 
in colour at their tips; (/) the presence or absence of a pale 
mesial line on the crown, which is generally accompanied by 
the intervening space between it and the superciliary streaks 
being darker than the back; {g) the colour of the axillaries 
and wing-lining; and [h) the colour of the tarsus and feet*. 
In some cases colour alone can be relied upon to deter¬ 
mine the species; and the difficulty is increased by the great 
seasonal changes to which both the upper and underparts are 
subject. The autumn plumage of most of the species, more 
especially that of birds of the year, is very yellow, sometimes 
approaching buff, which frequently disappears entirely in the 
breeding-plumage of old birds, especially in the colder lati¬ 
tudes. The bars on the wing, and the mesial line on the 
crown, are occasionally indistinguishable when the plumage 
has become much abraded. There is also considerable varia¬ 
tion in size between individuals of the same species, and es¬ 
pecially between the sexes. An average variation in the 
length of the wing of the males will probably be about a 
quarter of an inch. The largest females are usually equal in 
size to the smallest males; and as the females vary equally in 
length of wing, the total margin of variation between the 
smallest female and the largest male is half an inch—a very 
great variation in the length of the wing of such small birds. 
* The comparative lengths of the tail-feathers does not seem to be a 
character of much value. Most of the species of this group have the tail 
both rounded and forked; *. e. the two outer and the two centre feathers 
are the shortest. 
