226 ADAPTATIONS TO NOCTURNAL ACTIVITY 



Only one bird, the black skimmer {Rynchops nigra) is known to have 

 slit pupils, despite vague mentions, in popular and even highly technical 

 literature, of such pupils in owls. The pupil is most peculiar in Rynchops, 

 in that the two halves of the iris seem to swing inward independently, 

 like a pair of gates, to form the slit. One can understand the presence of 

 a slit pupil in a sea-bird — the surprising thing is that there are not more 

 such cases. Water of any depth is a dim-light environment, calling for 

 extra retinal sensitivity; and it is observed that the pupils of diving birds 

 are more responsive to light than those of others. The penguins have a 

 great range of pupil size. Contracted penguin pupils are never quite 

 round, and they can all become very small. That of the king penguin, 

 Aptenodytes patagonica, contracts to a perfect square, dilating through 

 a succession of polygonal shapes, like an iris diaphragm, to a huge circle. 

 It opens widely at night or when the eye is shadowed in daytime (though 

 the pupil of the other eye may then be a mere speck) , and it presumably 

 dilates widely under water. Penguins dive deeply out of sight; and 

 Brandt's cormorant has been trapped at forty meters, where the light is 

 much reduced, and is believed to go even deeper. 



The skimmer does not have its sensitive eye and slit pupil for under- 

 water vision, however. Simple nocturnality seems to be the whole expla- 

 nation — the bird rests in coves by day and goes to sea in the evening to 

 feed all night. If this one nocturnal bird species can have a slit pupil, it 

 is perhaps strange that the owls, oil-birds, snipes and so on have failed to 

 produce one. The skimmer is scarcely the 'logical' species to be an ex- 

 ception in this regard, whether it be compared with nocturnal land birds 

 or with other oceanic birds. One would rather expect the genus of the 

 boobies, Sula, to have taken the lead here : 



The red-footed booby, S. sula, is called by Robert C. Murphy the most 

 nocturnal of all sea birds. It has a notably larger eye than any other bird 

 in its family, but it does not have a slit pupil. A close relative, Morus 

 {=Sula, in part) bassana, the northern gannet, has been netted in 

 twenty-seven meters of water. One peculiarity of booby pupils mentioned 

 by one or two authors is the apparent sexual difference in size, the female 

 seeming to have a much larger pupil than the male. If true, this would 

 suggest a sexual difference in retinal sensitivity or eye-size; but Dr. 

 Murphy explains it as an illusion caused by a ring of black blotches at 

 the pupil margin of the otherwise yellow iris of the female. The male iris 

 being entirely yellow, the pupil seems smaller and more regular. Sula 

 nebouxii shows the feature strikingly; probably other boobies have it. 



