The eye of a procellariiform seabird, the Manx shearwater, Puffinus puffinus: visual fields and optical structure

The Manx shearwater, Puffinus puffinus, is a pelagic sea bird which feeds from the surface of the sea and by shallow surface and plunge dives. Visits to breeding colonies are made at night. The mononuclear retinal field of the Manx shearwater eye is 148 degrees wide and is asymmetric with respect to the optic axis. The nasal and temporal hemi-fields equal 65 and 83 degrees, respectively. The binocular field is long and narrow, and the central placement of the bill suggests that vision is used to guide the bill position during foraging. A schematic model of the shearwater eye's optical system is presented, together with reanalysed data on the optical structure in the eye of the pigeon, Columba livia. The eyes of these two species are nearly identical in axial length and overall shape, but they are of quite different optical design. The shearwater eye has a shorter focal length and higher maximum image brightness, and the ratio of corneal:lens refractive power equals 0.4 and 1.6 in pigeon and shearwater eyes, respectively. In Manx shearwater eyes, the ratio of focal length:axial length and the ratio of lens refractive power:corneal refractive power may be correlated with a nocturnal life style. It is not clear whether the relatively low refractive power of the cornea is best regarded as a feature correlated with an amphibious life style or whether it is a consequence of adaptations for nocturnal activity.