ON SEXUAL DIFFERENCES. 
31 
Thrashes, as, for instance, the Blackbird group, and many of the 
Chats, differ very greatly in colouring ; the Robin chiefly in the 
colouring of the throat and the feathers all round the base of the 
bill. I have not yet had an opportunity of carefully comparing 
the Warblers, but many of them show tolerably well-defined 
differences of plumage. 
So far as T have examined the Babbling Thrushes, I am 
disposed to think that the characters of bill, width of skull, and 
length are indicative of groups, and are not constant to the whole 
family (TimeMdoe ). I think, as a rule, the males are rather larger 
than the females, but have stouter bills, though the latter difference 
is sometimes very ill-defined. In many cases there are marked 
distinctions in the plumage. 
Head of lit.rE- Bird. Female Blackbirds. 
The Starlings and many other groups I have hitherto not been 
able to examine critically;* but in the true Crows I believe the 
males all have stouter and shorter bills than the females, whereas 
the reverse is the case with the W histling Crows, which are 
considered to be related to the Shrikes. In Pinches the structure 
of the head varies remarkably, according to the habits of the species, 
though in somo the differences are so slight as to be of little 
practical use. When neither colouring nor form of beak assist 
one, the length of the wing is well worth examining, that of the 
male being usually (if not invariably) greater than in the female. 
Whim opening the wings for comparison v\ ell-defined colour 
differences may sometimes be noticed, as in the Tunnel, and I ope- 
Cardinal. I think if the wings of the sexes in birds were carefully 
compared it would be found that, in proportion to their relative 
size, the wings of male birds are generally longer than those 
of females ; that cock-birds are in fact mere powerf ul in flight than 
hens. It is probably on this account that during their migrations 
the flocks are often found to be all of one sex, and thus a large 
importation of some common species may consist entirely of either 
cocks or hens. A few years ago there was a large importation of 
* Since writing the above I have compared the sexes in various groups of 
Starlings and find that the differences, though loss strongly defined, are similar 
to those in Thrushes. 
