68 Mr. H. Seebohm on the Phylloscopi 



in the western hemispliere, 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 Avhicli the second primary bears in length to the other 

 primaries; [d) the comparison between the lengths of the 

 wings and tail ; (e) the jjresence 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 wdiich 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 ; i. e. the two outer and the two centre feathers 

 are the shortest. 



