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.