BISMARCK (AP) — Federal wildlife officials have learned that conducting an aerial census of American white pelicans nesting in south-central North Dakota is better done when the big-billed birds are visible on the ground — and not in flight.

The nesting colony at Chase Lake National Wildlife Refuge is the largest in North America and biologists believe the birds’ population is down only slightly from last year, despite air-to-ground photos that showed only a meager turnout of breeding adults.

Acting Refuge Manager Drew Williams said the only time biologists could book a pilot and an airplane equipped to take pictures of the nesting area was during a June afternoon, and at the same time most adult birds were probably feasting on small fish and salamanders from prairie lakes up to 100 miles away.

The pelican colony at the refuge north of Medina has been the largest in North America, peaking at 35,466 birds in 2000. This year’s aerial photo showed only 12,000 birds, down from the 29,000 that were captured on camera in 2018.

Biologists have been doing aerial surveys since 1972. Aerial counts historically have been done in the morning, when adults are caring for their young, Williams said.

“There are definitely more birds than what we got in the photo” based on ground observations, Williams said.

“We’re a little bit lower than last year but well within the long-term average,” he said. “There is no reason to be concerned. It is not a trend.”

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.

The white pelicans are among the largest birds in North America, measuring 6 feet from bill to tail. The birds have a wingspan of nearly 10 feet and live for about 25 years.

They spend winter mainly in the Gulf Coast states, but some fly to the North Dakota nesting grounds from as far away as Florida and California.

Pelicans typically begin returning to Chase Lake in early April and stay through September, caring for their hatchlings. The lake is alkalized — free of fish and other food sources.

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.