SAN DIEGO — An egg found in an abandoned eagle nest could herald the return of the California condor to Mexico, which hasn’t had a breeding population of the iconic giant of the skies for about 75 years.

“This is a momentous occasion,” Mike Wallace of the Zoological Society of San Diego said Monday. “We’re all excited.”

The California condor, once on the brink of extinction, is the largest bird in North America with a wingspan of almost 10 feet.

Mr. Wallace and colleagues found the egg March 25 on a cliff in the Sierra San Pedro de Martir National Park, located in the arid interior of the Baja California peninsula more than 100 miles south of the American-Mexican border.

Mr. Wallace climbed to the nest and took photographs and measurements of the egg, shining a bright light through the shell to determine that the egg was 45 to 50 days old. Condor eggs incubate for 57 days, meaning the chick could hatch any day. There was also a chance the egg was dead, but Mr. Wallace said he did not smell any sulfur, and the parent condors were still tending to it.

“We are all sitting on pins and needles waiting to see where the situation is going,” said Mr. Wallace, who works for the zoological society’s center for Conservation and Research for Endangered Species. The society also runs the San Diego Zoo and its wild animal park. The California condor was once widespread, swooping above the western United States, parts of Canada, and Baja California.

A type of vulture, the condor scavenges dead fish and animals. As coastal population of seals and otters declined, so too did the bird. The use of poison to kill California’s grizzly bears in the 1800s also devastated their numbers and lead shot remains a potential source of poison. Hunting, egg collecting, and power cables were also blamed for hurting the creature’s numbers.