The North American letterbox once ranged across the continent in herds so large as to darken the plains from horizon to horizon. These gentle beasts had no fear of humans and would frequently wander into population centers. It was once common to find clusters of letterboxes on street corners in urban areas.
By the beginning of the 21st century, overgrazing and invasion of their habitats by other communication media had resulted in a massive decline of the wild letterbox population. Today the largest populations of North American letterboxes are found in zoos such as the one below.

The letterboxes shown here are part of a herd being kept at a postal center in Rockville where scientists are attempting to repopulate the species through a captive breeding program.