GENERAL ORNITHOLOGY 



to be yoke-toed (Fig. 45). In all yoke-toed birds, excepting the 

 trogons, it is the outer anterior toe which is reversed ; in trogons, 

 the inner one ; the latter are called hetcrodadylous. In nearly every 

 three-toed bird, all three toes are anterior ; an exception is in the 

 genus Picdides, where the true hind toe is Avanting, the outer anterior 

 one being reversed as usual in zygodactyls. No bird has more toes 

 behind than in front. Birds' toes, and their respective joints, are 



Numbered, in a certain definite order, as follows (see Figs. 34, 36): 

 hind toe -first toe, It; inner anterior toe = seco7id toe, It; middle 

 anterior toe = third toe, 3t ; outer anterior toe = fourth toe, 4/. 

 Such identification of It, 2t, 3t, it applies to the ordinary case of 

 three toes in front and one behind. But, obviously, it holds good 

 for any other arrangement of the toes, if we only know which one 

 is changed in position, — a thing always easy to learn, as we shall 

 see at once. In birds with the hind toe reversed, bringing all four in 

 front, the same order is evident, thongh then It is the inner 

 anterior, 2t the next, etc. ; for it always happens, when a hind toe 

 turns forward, that it turns on the inner side of the foot. Similarly, 

 in yoke-toed birds (excepting Tror/onidce), it is the outer anterior 

 which is turned backward, as above said ; then, evidently, inner 

 hind toe = It; inner front toe = 2^/ outer front toe = 3^; outer 

 hind toe = it. In Trogonidce, with inner front toe reversed, the 

 correction of the formula is easily made. Moreover, when the 

 number of toes decreases from four to three or two, the digits are 

 almost always reduced in the same order : thus, in three-toed birds, 

 It is the missing one; in the two-toed ostrich, \t and 2t are gone. 

 Exceptions to this generalisation are afforded by two exotic genera 

 of kingfishers, Ceyx and Alcyone, in which 2t is defective ; and by 

 the anomalous passerine Cholornis of China, in which it is in like 

 case. The rule is proved by the 



Number of Phalanges, or joints, of the digits. The constancy 

 of the joints in birds' toes is remarkable, — it is one of the strongest 

 expressions of the highly monomorphic character of Aves. In (dl 

 birds, excepting Procellariida', It when present has two joints (not 

 counting, of course, the accessory metatarsal). In all birds, 2t 

 when present has three joints. In nearly all birds, 3/ has four 

 joints. In nearly (dl birds, it Ims five joints. Thus, any digit has 

 one more joint than the number of itself. (See Fig. 34, where the 

 digits and the phalanges are numbered.) The exceptions to this 

 regularity consist in the lessening of the number of joints oi It or 

 St by one, and of it by one or two. So when the joints do not 

 run 2, 3, 4, 5, for toes 1 to 4, they run either 1, 3, 4, 5, or 

 2, 3, 4, 4, or 2, 3, 3, 3. (These statements do not regard the 

 anomalous cases of Ceyx, Alcyone, and Cholornis — see above.) This 

 variability is nearly confined to certain Picarian birds : examples 



