The Vertical Slit Pupil of Black Skimmers

Slit Pupil of Black Skimmer

Unlike any other bird, when Black Skimmers are exposed to bright light the pupil is reduced in size into the form of a  vertical slit. According to Zusi and Bridge (J. Field Ornithol. 2: 338-340, 1981) this was first reported by Taczanowski in Proc. Zool. Soc. Lond. 1874. I do not have access to Taczanowski's report but I was able to read a  note by Alexander Wetmore published in the Proceedings of the Biological Society of Washington in 1919. A clip  from the note is shown below.

Excerpt 1919

Proceedings of the Biological Society of Washington archive (1880-2004) is available on-line at the Biodiversity Heritage Library.

The Zusi and Bridge paper has drawings of the pupils of the Black Skimmer and Common Tern under five different  light intensities, but contains no photographs. The vertical slit pupil is mentioned in many descriptions of Black  Skimmers on the internet, but I found no photographs of the cat-like pupil. On my recent Denver > Pensacola trip, I saw numerous large flocks of Black Skimmers and decided to try to  photograph the vertical slit pupil. I found it very difficult to do. The dark brown iris is surrounded by black feathers, and the eye is almost invisible. Using my car as a mobile blind, I approached the flocks on the hard sand beaches on Galveston Island and Bolivar Peninsula slowly and in small increments. I was able to get very close to the birds. Using a Nikon D500 camera and 500mm f/4 lens with 1.4x teleconverter attached I focused on the eye region of a skimmer and intentionally overexposed the white areas of the birds to get a decent exposure of the eye. 

Slit Pupil of Black Skimmer

Several problems were encountered in trying to get photographic images of the slit pupil. The eye occupies a small region of the photo acquired with the equipment mentioned above. The photos of the pupil in this post were cropped from the 5568 x 3712 pixels of the original photo to approximately 300 x 300 pixels and then enlarged in Photoshop to 600 x 600 pixels to be more visible on the web. There is not much contrast between the brown iris and the black opening of the pupil. I have enhanced the contrast using a couple of methods in Photoshop. Other problems are illustrated below.

Reflections often obscure the slit pupil. The reflections of the clouds, my Subaru Forester and a utility pole are easy to detect in this image. The upper part of the pupil is not obvious because of the reflections of the front of the car, the clouds and sky.

In this image, the upper part of the pupil is obscured by the back of the car, clouds and sky.

The reflection of the car and utility pole are to the left of the pupil but the upper part of the pupil is not distinct because of the reflection of the clouds and sky.

Nictitating membrane

The slit pupil is not visible when the eye is covered by the nictitating membrane!

I finally found photos of the slit pupil on the web. Mark B Bartosik has 3 very nice images on pbase. 

David Sparks

I retired in 2005 after 40 years of research and teaching at the University of Alabama in Birmingham (24 years), the University of Pennsylvania (8 years) and the Baylor College of Medicine (8 years). Photography is my retirement hobby.

Nature photography, especially bird photography, combines a number of things that I really enjoy: bird-watching, being outdoors, photography, travel, messing about with computers, and learning new skills and concepts.  I now spend much of my time engaged in these activities.

David Sibley in the preface to The Sibley Guide to Birds wrote "Birds are beautiful, in spectacular as well as subtle ways; their colors, shapes, actions, and sounds are among the most aesthetically pleasing in nature."  My goal is to acquire images that capture the beauty and uniqueness of selected species as well as images that highlight the engaging behaviors the birds exhibit.