BIRDS 



417 



in Wilkinson's (1949) view, is the observation of the sun's arc with great accuracy 

 over a small excursion, and it is said that the pecten may play an important 

 part in the visual analysis thus involved by acting as a fixed point when taking 

 observations (Menner, 1938 ; Crozier and Wolf, 1943 ; Griffin, 1952). 



Areas subserving acute vision are the rule in birds and are more 

 elaborately constituted than in any other species.^ An area centralis 

 is almost invariably present, one fovea is the rule and two occur in 

 many species.^ The single fovea usually takes the character of a 

 remarkably deej) and well-formed pit, the depth varying with the 

 excellence of vision ; it is thus deepest in swift -flying diurnal birds 

 of prey. This central fovea subserves monocular vision. Only rarely 

 does a single fovea occur in the temporal part of the fundus (owls). 

 In bifoveate birds, usually diurnal birds of prey, the deep central 

 fovea is associated with a temporal fovea which is shallow and less 

 ^^'ell formed, except in hawks and eagles, where it is deep ; the temporal 

 fovea is used for seeing straight ahead and sometimes for binocular 

 vision. The kingfisher, Alcedo, is unique in that it uses its central 

 fovea for aerial vision, its temporal fovea for aquatic vision.^ In 

 addition to these macular areas with their fovese, a ribbon-like band 

 of specialized retina is sometimes associated (the infula),^ running in 

 the horizontal meridian through the fovea, particularly in birds that 

 seek their food in the ground {Strufhio, Saxicola) or in aquatic birds 

 ( Anseriformes : geese, swans, etc.). It would seem probable that tliis 

 band subserving accurate vision may be designed for food-searching. 



From the point of view of these areas for specialized vision, birds 

 may be classified as follows, a classification which depends less on the 

 type of bird than on its habits (Plate XII) : 



(1) Afoveal. {a) Domesticated birds and some ground-feeders. 

 There is a suggestion of an area centralis centrally but it is sometimes 

 absent and at best is poorly defined, and a fovea is absent. Typical 

 examples are the domestic fowl, Gallus domesticus, and the Californian 

 valley quail, Lophortyx californicus vaUicola. In the turkey, Mehagris 

 gallopavo, the guinea-hen, Numida jyucJierayii, and the pigeon, Columba, 

 there is an attempt at a shallow fovea. (6) Some sea-birds have a well- 

 formed area centralis in wluch cones only are fomid but a fovea is 

 absent — the shearwater, Puffinus, and the fulmar, Fulmarus glacialis 

 (Lockie, 1952). 



(2) Central monofoveal. Tliis applies to the majority of birds in 

 which a well-formed fovea situated centrally is surrounded by a large 

 macular area. 



1 Chievitz (1891), Slonaker (1897), Casey Wood (1917), Rochon-Duvigneaud 

 (1919-23), Franz (1934), Walls (1942), Bruckner (1949). 

 ^ Compare the lizard, Anolis, p. 365. 

 3 p. 641. 

 * Lat. infula, a band (Casey Wood, 1917). 



S.O.— VOL. I. 27 



Hawk, 

 Buteo 



Kingfisher^ 

 Alcedo 



Californian quail, 

 Lophortyx 



