112 FALCONID.E. 



and the irides become bright yellow. Young females 

 have the whole of the under parts from the throat to 

 the under tail-coverts of a uniform reddish bay, without 

 any of the darker-coloured streaks observable in adult 

 females. 



Having thus described the last of the British Falconida, 

 it may be desirable, before quitting this group, to exhibit a 

 representation of the breast -bone, or sternum, of one of the 

 types of the Falcons, in order to show in the form and 

 magnitude of the principal bone, and the others attached to 

 it, the power of flight possessed by these birds, of which 

 the breast-bone affords good comparative indication. 



The power of flight is one of the decided characteristics 

 of the organization of the class of Birds ; and the division 

 of the Falconidw includes those birds which appear to pos- 

 sess this power in the highest degree of perfection. The 

 conditions necessary to produce this power in its fullest ex- 

 tent are, large and strong pectoral muscles ; great extent 

 of surface, as well as peculiarity of form, in the wing ; and 

 feathers of firm texture, strong in the shaft, with the fila- 

 ments of the plume arranged and connected to resist pres- 

 sure from below. The extent of surface, the form and 

 other peculiarities of the wings, have been already noticed, 

 and the anatomical part only requires to be briefly de- 

 scribed. A certain degree of specific gravity is necessary 

 to rapid flight, and this is imparted by large pectoral mus- 

 cles ; the power of these muscles may be estimated by the 

 depth of the keel, and the breadth of the sides of the 

 breast-bone or sternum, as affording extent of surface for 

 the attachment of those large muscles by the action of 

 which the wings are brought down. 



As an illustration of this form, the figure here inserted 

 as a vignette is a representation about one-fourth less than 



