"On a winter’s night in that first year he woke to hear wolves in the low hills to the west of the house"
Canis lupus baileyi
Creative Commons AttributionMexican Grey Wolf (Canis lupus baileyi) - Credit: Brian Gratwicke

When Billy hears the wolves sometime in the early to mid 1930s, he is hearing the last remnants of an all but extinguished species.

Perceived as a threat to domestic livestock, intensive efforts were undertaken at the turn of the 20th century to eradicate the Mexican Grey Wolf. These efforts were ultimately successful, and by the 1950s a wolf which had once ranged the Sonoran and Chihuahuan deserts from central Mexico to western Texas, southern New Mexico, and central Arizona, had been more or less eliminated from the wild.

In 1976 the Mexican Wolf was declared an endangered subspecies and remains so to this day.  Only an estimated 340 Mexican Wolves remain, living in captivity in 49 facilities at the United States and Mexico.