The mystery of whether Alabama's rare yellow northern cardinal will pass on his unusual coloration to his offspring may never be solved.

The two Alabama residents who spotted "Mr. Yellow," and followed his adventures finding a mate and rearing two hatchlings say the cardinal chicks have left the nest.

The cardinal family has been observed daily by Charlie Stephenson, who spotted the bird at a backyard feeder at her Alabaster home, and Jeremy Black, Stephenson's friend who has helped photograph the bird and spread the word about it.

Stephenson and Black have created a Facebook page for the bird, posting frequent updates about its condition.

Geoffrey Hill, an Auburn University biology professor who has written books on unusual bird coloration, says the bird is a male northern cardinal with a "one in a million," genetic mutation that prevents him from processing certain pigments that give most male cardinals their brilliant red feathers. As a result of this mutation, the bird's plumage is bright yellow instead.

The hatchlings are estimated to be about 8-10 days old, which is about the time young cardinals leave the nest. It's not clear whether the young birds flew off happily on their own or fell victim to predation or other misfortune.

"We hope that they are safe, unharmed, still growing," Black and Stephenson wrote on Facebook. "And although we missed their monumental moment of them leaving the nest, it meant the world to be able to document their growth this past week."

When last spotted, the young birds were not old enough to determine whether they were male or female or whether they had inherited the mutation that caused their father's unusual coloration.

According to Penn State University, northern cardinals typically make their first flight at about 10 days old, and the males will develop their bright red plumage by their first winter.

Cardinal parenthood is an intense process, with the male and female parents working together to feed the babies up to eight times an hour, mostly with insects.

Cardinals have an average lifespan of about three years, according to the Penn State biology department, but have been observed living for up to 15 years in the wild. They do face a number of potential predators, including house cats, snakes, small hawks, and owls.

Cardinals do typically mate for life and Black and Stephenson say they've seen Mrs. Yellow begin building a new nest and that they will keep watching for signs of the babies.