Red-shouldered hawks are on the move

HARPERS FERRY — Red-shouldered hawks nesting along the Upper Mississippi River are adding to the mounting evidence that chronic high water is damaging and destroying wildlife habitat in bottomland forests.

Red-shouldered hawks have abandoned their historic nesting sites along the river’s main stem, said Jon “Hawk” Stravers of McGregor, who has studied the secretive raptors for the past four decades.

“This is significant since red-shoulder activity had been documented in several of these sites for each of the previous 37 years,” said Stravers, the primary investigator in research co-sponsored by Driftless Area Bird Conservation and the Raptor Resource Project and supported by Upper Iowa Audubon Chapter and Iowa Audubon.

Stravers said he and research colleagues Sophia Landis of Prairie du Chien, Wis., and Kathleen Carlyle of Ettrick, Wis., believe the exodus “may be related to long periods of extreme flooding that has occurred on the Upper Mississippi River during the previous decade.”

Many wetland habitats that red-shouldered hawks depend on have been inundated for extended periods during previous nesting seasons, Stravers said. The prolonged flooding also has killed thousands of trees in the Upper Mississippi River National Fish & Wildlife Refuge, he said.

Stravers said nesting sites along tributaries such as Paint Creek and the Yellow River remain active, which suggests red-shouldered hawks are shifting to more reliable upland forest tracts.

Stravers cites pool 10’s Sny Magill complex as an example of the red-shouldered hawk’s changing nesting territories, which shrunk from three in 2017 to zero in 2020.

“We found the same results in all other sites along the main stem of the Mississippi River in pools 10 and 11,” said Stravers, who locates nesting territories primarily by ear and claims at least a partial understanding of hawk vocabulary.

“Keeah. Keeah (My territory. My territory.) That’s the wildest sound in nature,” said Stravers, who knows a wild sound when he hears one and has heard them all.

John Howe, executive director of the Raptor Resource Project, a co-sponsor of the red-shouldered hawk research, said their diet consists primarily of reptiles, amphibians and small mammals.

“When their dinner table is under water they have to move,” he said.

Perhaps the worst aspect of the chronic high water, as it pertains to raptors, has been the proliferation of black flies, which typically have explosive hatches in springs characterized by flooding.

Females, which require a blood meal to fuel egg maturation, swarm raptor nests, killing chicks or driving them from their nests before they are ready to fledge, Howe said.

Since lock and dam construction in the 1930s, higher water in the flood plain has shifted the distribution of forests to higher and drier adjacent areas and encouraged the proliferation of more flood-tolerant species such as silver maple, which now make up more than 80 percent of river bottom trees. But even silver maples can’t withstand the chronic inundation of recent years.

A forest inventory conducted by the U.S. Fish and Wildlife Service documents the decline of river bottom trees on pool 10 and correlates it with chronic high water.

Wildlife biologist Billy Reiter-Marolf, who’s coordinating the long-term study, said the widespread tree mortality throughout pool 10 “is very alarming and concerning.”

Hydrograph data show that in two fairly normal years, 2016 and 2017, water levels in each year exceeded the 16-foot flood stage on seven days and exceeded the 13-foot action stage (at which point most trees are surrounded by water) on 53 and 67 days, respectively. In 2018, the river topped the pool 10 flood stage on 21 days and the action stage on 72 days.

Then came 2019, the longest-lasting Mississippi River flood on record, with a devastating 91 days above pool 10 flood stage and 181 days above action stage, followed by a more normal 2020 with 14 days above flood stage and 53 days above action stage.

The effects of the high water are reflected in the results of tree inventories. In 2015, for example, the inventory found 65.4 percent of the trees surveyed were described as “vigorous,” while just 7.7 percent were classified as dead.

In ensuing years, the percentage of vigorous trees declined steadily while the percentage of dead trees climbed, so that in 2020 just 12.3 percent of trees surveyed met vigorous criteria, while 33.7 percent were classified as dead.