i. 270;

monogamous, becoming polygamous under domestication, i. 270; eagerness of male in pursuit of the female, i. 272; wild, numerical proportion of the sexes in, i. 306; secondary sexual characters of, ii. 38; difference of size in the sexes of, ii. 43; fights of male, witnessed by females, ii. 49; display of male, to captivate the females, ii. 50; close attention of, to the songs of others, ii. 52; acquiring the song of their foster-parents, ii. 55; brilliant, rarely good songsters, ii. 56; love-antics and dances of, ii. 68; coloration of, ii. 74 _et seqq._; moulting of, ii. 80 _et seqq._; unpaired, ii. 103; male, singing out of season, ii. 106; mutual affection of, ii. 108; in confinement, distinguish persons, ii. 110; hybrid, production of, ii. 113; European, number of species of, ii. 124; variability of, ii. 124; gradation of secondary sexual characters in, ii. 135; obscurely coloured, building concealed nests, ii. 169; young female, acquiring male characters, ii. 180; breeding in immature plumage, ii. 214; moulting of, ii. 214; aquatic, frequency of white plumage in, ii. 229; vocal courtship of, ii. 331; naked skin of the head and neck in, ii. 377.

_Birgus latro_, habits of, i. 334.

BIRKBECK, Mr. on the finding of new mates by Golden Eagles, ii. 105.

BIRTHPLACE of man, i. 199.

BIRTHS, numerical proportions of the sexes in, in animals and man,