Most birds are either diurnal or nocturnal. The Manx shearwater is one of the very few that regularly perform such normal activities as feeding, hunting, and flying by day, and other activities, such as homing to the nest, by night. The retinal structure of the Manx shearwater is compared with that of a diurnal passerine (house sparrow) and a diurnal sea-bird (fulmar petrel), in order to determine what features are common to sea-birds and what, if any, may be specific adaptations to vision over a wide range of light intensity. Both sea-birds have a linear area centralis but lack a fovea. The rods of the dark-adapted fulmar and shearwater show great difference of shape from those of the light-adapted. The shearwater has more rods at the area centralis, a larger rodparaboloid in the dark adapted retina, and a greater ratio of total number of visual cells to ganglion nuclei than has the fulmar.

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