SEXUAL CHARACTERS IN BIRDS. 



675 



important coi-relating factor — vulnerability to enemies or open- 

 nesting habits. Table XIII. deals with birds making open nests 

 and in no way helps to decide the point ; coui'sers, ibises, mouse- 

 birds, tinamous, touracous, pigeons, rails, shore-birds, according 

 to the " nesting-habit hypothesis," should present sex dimorphism, 

 but they also fail to fall into line with the other correlation. 



Table XIII. (Open-nesting birds.) 



If, however, one turns to closed-nesting birds (Table XIV.), 

 it is at once obvious that open-nesting habit plays no part, 

 because several of the groups present considerable secondary 

 sexual dimorphism, and quite a large number a small amount. 



Table XIV. (Closed-nesting birds.) 



1. Bee-Eaters, Hoopoes, Rollers, Motmots. 4 



„ Barbets, Jacamars, Kingfishers, Parrots, 

 Woodpeckers. 



3. Hornbills, Toucans. 



4. Trogons. 



Note. — Compiled from P. Finn's 'The World's Birds.' 



Table XV. continues the two previous tables, and also deals 

 with families of birds making both open and closed nests. It 

 shows that the relation between open-nesting habits and second- 

 ary sexual dimorphism is, if not entirely negligible, at any rate 

 not nearly so close as with vulnerability to enemies. It may be 



