MILWAUKEE — Experts say swarms of black flies appear to be the best explanation for why endangered whooping cranes are abandoning their nests at a Wisconsin wildlife refuge.

But the Milwaukee Journal Sentinel reported Sunday that the pesky insects at the Necedah National Wildlife Refuge near Tomah aren't the only reason the birds are struggling.

Whooping cranes were reintroduced at the refuge in 2001 in the hope they would once again flourish in the eastern United States.

The effort has included a multimillion-dollar investment of public and private funds, a collaboration of government and citizen groups, and bands of dedicated crane watchers.

There have been successes. Five chicks have hatched and joined the wild population. Ultralight aircraft have successfully guided young birds south for the fall migration while other young whooping cranes have followed older cranes in successful migrations south and north.

But the work has yet to produce a self-sustaining flock.

The whooping crane population in the eastern U.S. stands at 106 migrating birds, numbers that have been built up by the release each spring of chicks born in captivity.

Most of the 600 or so whooping cranes in existence are part of the only other migrating population, which winters along the Texas Gulf Coast and summers in Alberta, Canada. That population is faring relatively well.

In the east, however, "we would have hoped that we would have a trajectory by now where they were well on their way to a sustainable population," said Jeb Barzen, director of field ecology for the Baraboo-based International Crane Foundation, part of the Whooping Crane Eastern Partnership, formed in 1999 to lead recovery efforts.

The 5-foot-tall cranes, the tallest birds in North America, were once frequent targets of unregulated hunting. The world's migrating population was down to 15 in 1941.

A few years ago, crane watchers noticed that the reintroduced birds would pair off at Necedah, build nests and lay eggs. But then they would abruptly abandon their nests. The leading hypothesis pointed at the hordes of biting black flies at the marshy area. As an experiment in 2011 and 2012, biologists treated two rivers near the refuge with a soil bacterium known as Bti, which is used an alternative to chemical pesticides. Black fly numbers fell significantly.

Four chicks hatched in 2011, but all eventually died. Several more chicks hatched in 2012, and two survived.

Bti treatments were not used this spring, and the black flies returned. So far, 18 nests have been abandoned amid dense clouds of black flies. No chicks have hatched this year.

While the flies appear to be the problem, Barzen said, researchers still need to learn more about the role of predators, habitat quality and food supply, whether adults have sufficient energy stores to mate and raise chicks after migrating back to Wisconsin, whether they've gained enough experience at nesting and parenting, and whether cranes raised in the wild make better parents than those raised in captivity.

Given the struggles at Necedah, starting in 2011 the partnership released whooping cranes in the Horicon Marsh near Waupun and the White River Marsh State Wildlife Area near Berlin, where Barzen said conditions might be better. There are no black flies. There are no wolves and there are fewer other predators such as otters and bobcats. The soil is also richer which might mean more food.

"Our hope is that in another five to 10 years, we will know what it takes to make it work," Barzen said.