Researchers found the New Zealand storm petrel egg on Little Barrier Island, off the country's North Island, China's Xinhua news agency reported Friday.
The storm petrel is a sparrow-sized bird that spends most of its life at sea, and before its rediscovery in 2003 had been thought to be extinct for more than a century.
Scientists took the opportunity when the female was off the nest to check the egg, determining it was fertile and expected to hatch in April, the New Zealand Department of Conservation said in a statement.
"It was exciting to see the egg of a bird once thought to be extinct," Graeme Taylor, the department's principle science adviser, said.
The white egg with a dusting of pink spots measured just 1.2 inches long, the scientists said.
"The fact it has taken until 2014 for scientists to observe one of these tiny eggs reflects how much we still don't know about New Zealand's natural environment and particularly for marine species," Taylor said.