The gallery included exhibits of other extinct species such as the passenger pigeon. The internal anatomy of the passenger pigeon has rarely been described. It is the only monument in the United States dedicated to the passenger pigeon. The passenger pigeon had no known subspecies. He cited Edward's description of the mourning dove and Catesby's description of the passenger pigeon. Brisson's description was later shown to have been based on a female passenger pigeon. In North America, the extermination of the highly gregarious passenger pigeon has been well documented. Rarities include sets of passenger pigeon, Carolina parakeet, and Bachman's warbler eggs. The passenger pigeon was exceptional for a number of reasons. The passenger pigeon was sexually dimorphic in size and coloration. The passenger pigeon changed its diet depending on the season. Similarly, the extinction of the passenger pigeon, which numbered in the billions, also caused alarm. Some spectacular migrants have already gone extinct, the most notable being the passenger pigeon. This amounted to about one passenger pigeon per day for each person in the fort. For instance, while the passenger pigeon was extant, forests were dominated by white oaks.