Vol. 32, pp. 195-202 December 31, 1919 



PROCEEDINGS 



OF THE 



BIOLOGICAL SOCIETY OF WASHINGTON 



GENERAL NOTES. 



A NOTE ON THE EYE OF THE BLACK SKIMMER (RYNCHOPS 



NIGRA). 



The pupil of the eye in the majority of species of birds is rounded or cir- 

 cular in form, whether expanded or contracted. Few exceptions to this have 

 been known. In some of the gallinaceous birds the pupil when contracted 

 is said to become slightly elliptical 1 and in owls this condition is at times 

 slightly more accentuated. 



Recently while handling a living black skimmer (Rynchops nigra) I was 

 astonished to note that when the eye was turned to the sun the pupil con- 

 tracted to an elongated vertical slit as in the eye of a cat. The opening was 

 reduced very little in its vertical length but narrowed greatly laterally so 

 that when contracted the opening was nearly as high as when expanded. 

 When fully opened the upper and lower points of the pupillar aperture were 

 marked by distinct angles so that even at this time the opening was not 

 circular. The swinging motion as the sides of the pupil moved toward one 

 another in contracting was very peculiar. 



It has been stated by Gadow (in the citation given above) and others that 

 owls also have the opening of the pupil a distinct slit as in the eye of a cat. 

 Recently I have examined living individuals of the Snowy, Great Horned, 

 Barn, and Screech Owls finding, however, as I had expected from previous 

 observations, that the pupil did not differ markedly from that in other birds. 

 For example immature individuals of the Great Horned and Screech Owls, 

 when suddenly exposed to the light from a high candle power electric light 

 bulb, contracted the pupil until it was very small, while in form it varied 

 only from round to slightly elliptical. In the latter case the axis of the 

 slightly elongated opening was longitudinal and not vertical. Adult in- 

 dividuals examined in larger enclosures by the aid of high power binoculars 

 exhibited the same peculiarities and though I have handled many owls of 

 the common North American species I have yet to see one with form of the 

 eye differing from that described above. 



The peculiar development found in the Black Skimmer therefore is, so far 

 as I am aware, unique, though there is little doubt but that the same peculi- 

 arity will be found in the other species composing the genus Rynchops. It 

 may be that this slit-like eye opening like the peculiar compressed bill with 

 its elongated lower mandible is another adaptation brought about by the 

 strange feeding habits of these birds. — Alexander Wetmore. 



iGadow, H., Klassen und Ordnungen des Thier-Reichs : Vogel, Vol. I, 1891, p. 436. 



40 — Pboc. Biol. Soc. Wash., Vol. 32, 1919. (195) 



