Throughout the conifer belts of North America's western interior mountains, Cassin's Finch can be one of the most common and conspicuous breeding birds. John Cassin, of the Philadelphia Academy of Natural Sciences, considered this species to be the “greatest bird in the lot” among a collection made by the Pacific Railroad Survey in the southwest mountains in the early 1850s. At Cassin's request, Spencer Baird named the striking new finch for his friend and colleague. Cassin's namesake differs subtly from its closest relative, the Purple Finch (Carpodacus purpureus), in morphology, plumage, and voice, in its restricted western distribution, and in its preference for higher elevations when breeding. Its rollicking song and liquid tidilip calls are familiar to visitors to the Rocky Mountains, eastern Cascade Mountains, eastern Sierra Nevada, and high points in between.
Males in their first breeding season, still wearing drab brownish female-like garb from their first Prebasic molt, sing much like older males, giving the false impression that both sexes sing with equal complexity. Cassin's Finches often produce an astonishing string of imitations of the sounds of other birds as a cadenza to their typical warbly song; many a field worker has searched in vain for another species, fooled by these excellent mimics.
In late summer and early fall, Cassin's Finches assemble into groups, often foraging in the company of crossbills (Loxia) and other mountain birds, and frequently visiting mineral deposits on the ground to satisfy a salt craving that this species shares with other family members. In winter the birds drift southward and to lower elevations. At this time they may visit bird feeders regularly until they depart for the high country early in spring, though their occurrence at any given locality is highly variable from year to year. Despite the prevalence of this finch, remarkably little detailed work has been done on it, leaving many possibilities for new discoveries.