PACHYCEPHALA. 
case the white throat remains, but a red shade appears on the breast and then 
extends over the abdomen also, ehminating the yellow. The bird may become 
larger or the bill may become bigger and stronger. In another case the yellow 
may extend over the whole under -surface. It is obvious that these yellow 
species must be more closely allied to each other than they are to the grey- 
backed pink -breasted species, though structurally there is much more superficial 
difference between some of these than between the yellow and pink -breasted 
forms. The females also vary in a similar manner, tending to become yellow like 
the male, and as the evolution proceeds the plumage changes become lessened 
and the birds may take on the more highly developed plumage from the nest. 
I am compelled to conclude from a study of these birds that colour values 
are of much more importance than so-called structural characters, and that 
such have been the chief factors in all the groupings utilised by the workers 
who have pretended that they used structural features alone. 
As above stated, though the males differ but slightly, the females show 
marked specific characters. I am unable to recognise geographical representa- 
tives as subspecies only, when they present no intergradation, and show marked 
and constant differences. In these true Pachycephala many species must be 
recognised, though Messrs. Rothschild and Hartert suggested these were sub- 
species only, on account of the similarity of the males. Yet these writers would 
value as specific such a subtle difference as is named between the Willow and 
Marsh Tits of England. 
The three species of PacJiycephala, (formerly classed all as gutturalis) are 
differentiated very easily by means of the females, and so far I have seen no 
gradation. The males are separable by their size and size of bill and tail. 
An artificial Key might be prepared thus : 
Males : 
Bill small ; tail long, black with grey base or even 
all grey ; green edgings to primaries of all save grey- 
tailed birds ... ... ... ... ... ... ... pectoralis 
Bill small ; tail short, black ; grey edgings to primaries melanura 
Bill large ; tail short, black ; grey edgings to primaries, 
wing longer robusta 
Females : 
Under -surface with no yellow, varying from greyish to 
reddish; tail greenish to grey, not black ... ... pectoralis 
Under-surface white with yellow under tail-coverts ; 
tail green ... ... ... ... ... melanura 
Under-surface, breast to under tail-coverts bright 
yellow ; tail black robusta 
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