13G 



SEXUAL DIMORPHISM 



But if concealment was the object to be attained, probably 

 the absence of the erected feathers would be more effective. 



I have not found a detailed description of the function of 

 these feathers in courtship, but Audubon describes how the 

 males of certain species of the family, presumably American, 

 lly about with great rapidity and very sudden turns, making 

 in the movements a singular noise by the vibration of their 

 feathers. They perforin these evolutions in order to attract 

 the attention of the female, and when she has paired with 

 one the others are driven away. It does not follow that 

 the single elongated feather in the wing of Macrodipteryx 

 is used to produce a sound during flight, but it is developed 

 only in the males and only in the breeding season, for 

 which reasons I conclude that it owes its development to 

 some special stimulation which a knowledge of the habits of 

 the bird would soon disclose. In the fork-tailed goatsucker 

 of the Brazils, Macropsalis, the outer tail feather on each side 

 is elongated, and these feathers diverge and converge during 

 flight. The elongation of these feathers is peculiar to the 

 male, but whether they are present only in the breeding 

 season I do not know. The curious habitual movement of 

 the outer tail feathers during flight perhaps explains their 

 hypertrophy, and the movement seems to be associated with 

 sexual excitement, since it is confined to the males. 



Trogonidce. The birds of this family, generally of brilliant 

 and beautiful plumage, inhabit tropical forests in Asia, 

 Africa, and America, but they are most numerous and most 

 magnificent in Central and Tropical America. In the Asiatic 

 species the sexes are, if not exactly alike, usually similar, and 

 not very brilliant in colouring. In some of the American 

 species the males are resplendent, while the females are rather 

 dull-coloured like the Asiatic forms. In other American 

 species, again, the males are very brilliant, and possess en- 

 larged and specialised feathers, while the females are adorned 



