The first chicks hatch around mid-May, a month after their parents fly in from Gulf Coast states and as far away as Florida and California. The pelicans normally stay through September, feasting on fish and salamanders from small ponds within a 100-mile radius of the refuge.
Shook said the island has been shrinking by a couple of acres annually since the early 1990s but appears to have mysteriously decreased from 24 acres last year to about 15 acres this year, a loss in land mass equal to about nine football fields. The decrease of island real estate comes despite relatively average rain and snowfall and it has even shrunk in dry years, he said. It's not clear why it's happening.
"As the water level at Chase Lake continues to rise, we're losing a little of the nesting island each year," Shook said.
Chase Lake historically has had high alkaline levels that made it unable to support aquatic life, but Shook said that's changing as the lake level increases.
"The lake is changing and becoming more of a freshwater lake even to the point that it now has a few minnows and salamanders but not enough for the pelicans to feed on," he said. "I don't think change in water chemistry is enough to make the birds leave but loosing nesting habit that could be an issue."
Biologists aren't overly worried yet. The main nesting island was first used by pelicans in early 1990s after an island previously preferred by the pelicans was swamped by rising lake water. But other islands over the decades have cropped up when parts of peninsulas are flooded, he said.
Pelicans have been monitored at Chase Lake since 1905, when the birds numbered about 50. President Theodore Roosevelt designated the site as a bird refuge in 1908, after many were being killed for their feathers and for target practice.
Biologists have been doing aerial surveys since 1972. But the pelican rookery has presented puzzling situations for biologists before.
In 2004, nearly 30,000 pelicans departed 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.
Since that scare, the nesting population has been on the rise.