The back is blue-gray and the tail is long and narrow with black bands and a subterminal bar tipped white. Immatures are similar, but are generally brown dorsally, turning blue at the onset of sexual maturity. Coloration varies according to Gloger's rule. Therefore, peregrines tend to be pale in dry climates while dark in humid ones. Ventral barring is also more dense in humid and tropical ecosystems.
Females are usually larger than the males.
By the mid-1960s, researchers determined peregrines were no longer a breeding species in the eastern United States. Today several species of peregrine falcons are endangered. Nest robbing, trapping, and shooting first contributed to their downfall, followed in the 1950's by ingestion of chemical pesticides and industrial pollutants. Occupying a position high on the food chain, peregrines are still exposed to high levels of chemical residues if they migrate or eat migrant song birds contaminated in countries using pesticides now banned in the United States. As in all birds of prey, ingested chemical toxins accumulate in their bodies causing reproductive failure, leading to the decline and even eventual extinction of the species.