Every year approximately 30,000 American white pelicans descend on a tiny island in North Dakota to breed and raise their young. The annual migration brings pelicans from as far away as the Gulf Coast and California to Chase Lake National Refuge, the tiny island boasts guano-rich soil and a bevy of minnows and salamanders to feed on.

However, this year the island is submerging into the lake. The island usually measures about 24 square acres, but this year, rising water levels have cut that space down to 15 square acres. The sharp reduction in land size has alarmed biologists monitoring the pelicans; American white pelicans are the some of the largest birds in America (weighing 20 pounds, sporting a 10 foot wingspan and measure 6 feet long from beak to tail), and are now forced to breed in close quarters.

So far, biologists monitoring the situation say the lost real estate has not affected nesting activities. However, there is no doubt the surrounding ecology of the Chase Lake National Refuge is undergoing drastic transformations, with multiple factors compounding to exacerbate problems for the pelicans. For example, the rising water levels of the lake are beginning to affect its alkalinity, which in turn alter the surrounding ecology and food chain:

“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,” said Neil Shook, a U.S Fish and Wildlife Service biologist and refuge manager to the AP. “I don’t think change in water chemistry is enough to make the birds leave but losing nesting habit — that could be an issue.”

This is not the first time the pelican rookery has been the subject of mystery: ten years ago 30,000 pelicans abruptly fled the Chase Lake refuge, leaving their chicks and eggs behind. The next year, there was a massive die-off of pelican chicks, followed by an exodus of their parents from the wildlife refuge.

Predators, weather, diseases and other factors, such as a natural correction, were considered but biologists have never been able to decisively figure out the cause of the pelican deaths and departures.

Biologists are not overly worried yet about the fate of the breeding pelicans. The main nesting island was first used by pelicans in the early 1990s after an island previously preferred by the pelicans was similarly swamped by rising lake water. Also, other islands over the decades have cropped up when parts of peninsulas are flooded, so there should always be plenty of options for pelicans to nest in the area.