SEXUAL CHARACTERS IX BIRDS. 673 



the case of passerine birds, powerful species are only to be found 

 amongst those showing no sex dimorphism, e. g., crows, drongoes, 

 shrikes. Swallows, which are aerial birds, present little or no 

 sex dimorphism, 



The classification of sexual dimorphism chosen for analysis 

 refers almost entirely to sexual dimorphism of colour ; neverthe- 

 less, it will be advantageous to consider briefly other secondary 

 sexual differences — song and precopulator}^ displays. 



As regards song, this character is frequently to be found in 

 birds presenting no sex difl:erences in colour and form, but which, 

 nevertheless, do not appear to be especially free from the attack 

 of enemies — for instance, among warblers, American warblers, 

 wrens, cuckoos, larks, pigeons ; whilst among birds not lia.ble to 

 attack, song-birds are very uncommon, if not entirely absent. 



It thus appears that this dimorphic character is coi-related to 

 vulnerability to enemies, and accounts for some of those cases 

 of birds presenting little or no sexual dimorphism of form or 

 colour, but which are yet liaV)le to the attack of enemies. 



Prexoj)ulatory displays^ or the displays of courtship, is another 

 secondary sexual character open to investigation. Here, again, 

 birds showing little or no display are birds not liable to attack, 

 whereas birds showing great display are especially liable to attack. 

 In the 'World's Birds,' under the heading "Courtship," sexual 

 displays are briefly described in 25 of the 50 families under con- 

 sideration. In the case of 19 of the 25 the usual actions aie 

 described : against diurnal birds of prey there is the remark 

 " never seen the disjDlay," divers " said to be on the wing," 

 horn bills " little known "; whilst against bustards "very elaborate," 

 game-birds " elaborate," cranes " verj'' elaborate." Ci'anes, then, 

 form an exception, for they possess weapons of defence. Never- 

 theless, certain factors might lead one to suspect that they 

 are not entirely free from attack ; they are terrestrial birds, they 

 are excellent-eating (their food being mostly vegetables, seeds, 

 etc.), and in the East they are a favourite quarry of the hawker ; 

 they are also usually not sociable during the breeding-season, 

 and, lastly, in most species both sexes take equal part in the 

 displays, so that in truth they do not present secondary sexual 

 dimorphism in this respect. 



It is very fortunate that cranes needed this detailed consideia- 

 tion at the end of the examination, which has covered a great 

 deal of material, as it is easy to draw attention to the fact that 

 this is almost the only occasion (ducks also) when such treatment 

 became necessary. 



Summing up, it may be said that for certain reasons — namely, 

 because of aerial, aquatic, and oceanic habits, possession of defen- 

 sive and offensive weapons, of diving-habits, of ill-flavour to man, 

 of large size, of insular habitat, of social disposition, of carni- 

 vorous habits — certain birds must be relatively free from the 

 attacks of enemies ; and it has been shown that these bii-ds 

 present less secondary sexual dimorphism than birds which for 



