482 MANUAL OF ZOOLOGY SECT. 



tail of many gregarious birds, such as Plovers, fully exposed 

 only during flight, and often widely different in closely 

 allied species, have been explained as " recognition marks," 

 serving to enable stragglers to distinguish between a flock 

 of their own and of some other species. 



The toothless jaws with the horny sheaths forming the 

 bill are universal in the class. But the dimensions and 

 form of the bill vary very widely in different groups of Birds. 

 It may be extremely short and wide for catching moths and 

 other flying Insects, as in Swifts and Goatsuckers ; short and 

 conical for eating fruit, as in Finches ; strongly hooked for 

 tearing the bodies of animals, as in Birds of Prey, or for 

 rending fruits of various kinds, as in Parrots long, conical, 

 and of great strength, as in Storks ; slender and elongated, 

 as in Swifts, Ibises, and Curlews ; broad and flattened for 

 feeding in mud, as in Ducks and Geese ; expanded at the 

 end as in Spoonbills ; immensely enlarged as in Hornbills 

 and Toucans. It is most commonly bent downwards at the 

 tip, but may be straight or curved upwards, as in the Avocet, 

 or bent to one side as in the New Zealand Crook-billed 

 Plover. It is sometimes, as in the Toucans, brilliantly 

 coloured, and there may also be bright coloration of the 

 cere, as in the Macaws, and of naked spaces on the head, 

 as in the Cassowaries. In the latter the head is produced 

 into a great horny prominence or " casque," supported by 

 an elevation of the roof of the skull. The cere is frequently 

 absent. The nostrils are placed at the base of the beak 

 except in Apteryx, in which they are at the tip. 



The essential structure of the wing apart from its feathers 

 is very uniform. As a rule all three digits are devoid of 

 claws, as in the Pigeon ; but the Ostrich has claws on all 

 three digits ; Rhea on the first and sometimes the second 

 and third ; the Cassowary, Emu, and Kiwi on the second ; 



