ON SECONDARY SEXUAL CHARACTERS IN BIRDS. 663 



48. The Distribution o£ Secondary Sexual Characters 



amongst Birds, with relation to their Liability to 



the Attack o£ Enemies. By J. C. Mottram, 

 M.B.(Lond.)*. 



[Received June 25, 1915 : Read November 9, 1915.] 



Index. 

 Morphology : Page 



Secondary sexual characters. Distribution among birds ... 663 

 Ethology : 



Secondary sexual characters. Relation to enemies 663 



Geogeaphical : 



Secondary sexual characters. Insular and continental 671 



This paper endeavours to show that there is a correlation 

 between extra-sexual dimorphism among birds and their vulner- 

 ability to enemies. No attempt is made to account for this 

 correlation. However, in the light of this correlation, the more 

 important theories which have been put forward to account for 

 the presence of secondary sexual characters are briefly con- 

 sidered. 



Collection of Material. 



In collecting material for classification, works were selected in 

 which many facts were collected on broad lines and with no 

 particular object in view. References are given for all the 

 material used. The work from which by far the most material 

 was taken is ' The World's Birds/ by F. Finn. This is a small 

 volume, in which the families are briefly described in a syste- 

 matic manner under the headings — ^diagnosis, size, form, plumage, 

 young, nest, eggs, incubation, courtshijj, food, gait, flight, notes, 

 disposition and habits, economic qualities, captivity and dis- 

 tribution. A wealth of reliable information is here laid down 

 in a condensed manner, and is, therefore, at once available for 

 classification. 



Apart from this work, descriptive monographs have been 

 chiefly consulted. Only families coiataining more than ten 

 species are considered, as it was thought that a lesser number 

 could not give a reliable picture of the characters of a family. 



In oi^der to escape any misunderstanding, the factors to be 

 correlated must first be clearly defined. A species is said to 

 show extra-sexual dimorphism when there is a difference between 

 the somatic characters of male and female. Attention is usually 

 directed to differences in form and colour, but it must be remem- 

 bered that almost any chai-acter may show a difference. It is by 

 no means uncommon to find birds in which the form and colour- 

 ing of the sexes are identical, but it is quite rare to observe birds 



* Communicated by the Seceetakt. 



