Introduction; Variation. 
65 
The psychological differences of the sexes. — The rule found among mammals 
— that the male is more active and wars and works for the sake of the female, 
while the female is more passive and gentle and devotes herself more to the care 
of the young — holds good also for large numbers of birds, but in many others 
the sexes seem to he much alike in temperament and to share duties, while in 
some species the rule is more or less completely subverted, the male undertaking 
the “female duties”, and the female assuming the usual role of the male. The 
fact is important, as it shows that there are no mental peculiarities origin- 
ally bound up with the primary fact of sex. It appears, moreover, that 
these psychological conditions often (but not always) accompany the three con- 
ditions of development of plumage and structure mentioned above; namely, 
when the male is more highly developed than the female, he is noisy, com- 
bative and extravagant of display in his courtship, while the female builds 
the nest or most of it, incubates the eggs, and takes the chief or sole care of 
the young; when the sexes are alike, the males are less quarrelsome in the 
breeding season, less demonstrative in their courtship, and share the work of 
incubating the eggs and rearing the young; when the female is the more highly 
developed, she is noisy, pugnacious with other females and courts the male, 
leaving him to do most or all of the work of hatching the eggs and caring for the 
young. Thus the highly coloured males of the Trochilidae many Anatidae and 
Gallinae seem not to concern themselves for the brood to which the plain-look- 
ing female devotes herself most faithfully, whereas the large and handsome female 
Tiirnix roams about and calls and fights other females, leaving the smaller and 
plainer male to attend chiefly to the incubation of the eggs and the welfare of the 
chicks, though indeed she does most of the nest-building and assists a little in 
hatching the eggs (Krohn, Gefied. Welt, 1894, 190). The female of one of 
the Emus which is larger than the male and wears a slight top-knot has been 
observed in captivity not only to leave the entire work of incubation to the male, 
but apparently to use her utmost endeavours to destroy her young when hatched 
(Darwin, Descent of Man 1871, II, p. 205). 
Among species the sexes of which are much alike in appearance and which 
share the duties of incubation may be mentioned the Tits , Paridae ; many 
Warblers, Sylviidae\ some Larks, Alaudidae\ some Buntings, Emberizidae; certain 
Finches, Fringillidae; Woodpeckers, Picidae, and others; while in other cases the 
male feeds the brooding female and sometimes relieves her in sitting for a short 
time (cf. e. g. Naumann’s Vogel Deutschlands, 1824, IV, 93 and in many other 
places). But it is by no means always the case that the finer one sex in birds 
is in comparison with the other, so much the more he (or she) will abandon 
the nest, eggs and young to the humbler consort, and that the more similar 
they are in appearance, the more evenly will they share duties. As instances 
to the contrary may be mentioned the male Ostrich, which, though the finer 
bird, broods on the eggs of his wives at night ; the females of the Birds-of-prey, 
which are usually superior to the males in point of size (though not in coloration), 
Meyer & Wigleswortli, Birds of Celebes (May 5tu, 1898). 
9 
