30 BIEDS. 



to one, which is quite short and covered by a spur-like claw ; the digital formula 

 therefore becomes 1, 3, 4, 5. 



(ii.) In the Swifts of the genera Cypselus and Panyptila (though not in the 

 rest of the family) the number of phalanges in each digit external to the hallux 

 is three ; the formula being 2, 3, 3, 3. 



(iii.) In the Sand-Grouse (Pteroclidce) and true Caprimulgidce (Nightjars) the 

 fourth digit wants one of the normal phalanges ; the formula being 2, 3, 4, 4. 



Position of the Toes. 



In the vast majority of birds three toes are directed forwards (second, third, 

 fourth) and one (the hallux, or first toe) backwards. 

 Exceptions : — 



1. All four toes may be turned forwards, as in some of the Swifts {CypseUdce). 



2. The first toe may be versatile, turning backwards or forwards, as in the 

 Coliidce. 



3. The outer (fourth) toe may be versatile, or capable of being turned back- 

 wards or forwards, as in most Owls and the Osprey {Pandion). 



4. The toes may be permanently in pairs, two before and two behind, as in 

 Woodpeckers, Toucans, Cuckoos, and others, a condition described as Zygodactyle, 

 or yoke-toed. In these cases it is always the first and fourth that turn backward, 

 except in the Trogons, where it is the first and second. The third toe is in- 

 variably directed forwards (Nat. Hist. Museum). 



Leading Modifications of the Foot of Birds. 



There are three functional plans or types upon which birds' feet are constructed, 

 each with various modifications and with transitional forms connecting them. 



I. The Perching or Insessorial type, in which the foot is best fitted for the 

 grasping function required in perching on trees. 



(a) The typical Passerine modification. All the toes free and movable, and 

 the hinder one well developed ; the foot is thus like a hand with three fingers 

 and an opposable thumb. Examples: Song-Thrush {Turdus musicus), Sky -Lark 

 {Alauda arvensis), "Wood Pigeon (Columba palumbus), Nightjar {Caprimulgus 

 europceus) and Lyre Bird {Menura superba). 



(b) The Raptorial modification. The toes armed with powerful curved sharp 

 claws and widely spread, so that they may have greater power in seizing and 

 holding their prey. Their under surface bulbous, scabrous, or even spiny, for 

 greater security in grasping. Examples: Osprey {Pandion haliaMus) and Sea- 

 Eagle (Haliaetus albicilla). 



(c) The Syndactyle modification, in which the outer and middle toes are 

 coherent for most of their extent, with a broad sole in common. This occurs in 

 perching birds, as Kingfishers, which scarcely use their feet for progression, but 

 simply for sitting motionless. Examples: Kingfisher (Ceryle torquaius), Hornbill 

 (Buceros rhinoceros), Cock of the Eock {Rupicola crocea). 



(d) The Zygodactyle, or yoke-toed modification. Two toes are turned forwards 



