The white pelican colony at the 4,385-acre Chase Lake National Wildlife Refuge north of Medina peaked at 35,466 birds in 2000. The pelicans winter mainly in the Gulf Coast states but some fly to North Dakota from as far away as Florida and California to nest the island that also serves as a rookery.
The big-billed birds normally stay in North Dakota through September, raising their young and feasting on crawfish, small fish and salamanders from small prairie ponds within a 100-mile radius of the refuge.
Last year, an aerial count found 31,534 breeding adults, up from 20,854 in 2011, said Neil Shook, a U.S Fish and Wildlife Service biologist and the refuge manager.
An aerial count this year shows a little more than 15,000 nests, Shook said. Biologists estimate two adults per nest.
"We're down a little but still above the long-term average," Shook said.
The white pelican breeds once a year, and males and females take turns caring for their young. The white pelican is one of the largest birds in North America, measuring 6 feet from bill to tail and weighing up to 20 pounds, with a wingspan of nearly 10 feet. They have a lifespan of about 25 years.
Shook said the birds dodged storms this year at the refuge that could have caused high chick mortality.
"There was a huge rain event and a hail event that just missed the island by yards," Shook said.
Pelicans have been monitored at Chase Lake since 1905, when the birds numbered about 50. U.S. President Theodore Roosevelt designated the site as a bird refuge in 1908, when many of the birds were being killed for their feathers and for target practice.
Biologists have been doing aerial surveys since 1972.
Shook said the nesting pelican population has been increasing since a scare in 2004, when nearly 30,000 pelicans left the Chase Lake refuge, leaving their chicks and eggs behind. A year later, the refuge saw a massive die-off of pelican chicks, followed by an exodus of their parents.
Predators, weather, diseases and other factors were considered but biologists have never pinpointed the cause of the pelican deaths and departures. Shook said that it may have just been a natural correction.
"What transpired, we don't know," Shook said. "But the population has pretty much increased every year since then."
Follow James MacPherson on Twitter at http://www.twitter.com/macphersonja
Copyright 2013 The Associated Press.