Fewer Osprey Nesting Along Upper Clark Fork River

Ospreys aren’t nesting along the Upper Clark Fork River between Deer Lodge and Missoula as much as they used to, and no one knows why

By Associated Press

BUTTE – On July 8, three osprey chicks were abducted from their Warm Springs Pond nest by the Montana Osprey Project.

Erick Greene, Dalit Guscio and Geffen Guscio put bands on the chicks’ legs while Maria Hyvonen took pictures. To weigh the birds, they slipped little sweater-like sleeves onto the chicks, who looked indignant at the whole process. Dalit took a feather from three tails and blood samples from three wings.

The ospreys were returned and the samples went into a cooler. They were tested later for heavy metals.

The Warm Springs nest was the first of six nests the project banded and sampled that day. At that nest, osprey blood is healthy, and parents return to raise chicks every year.

Yet all is not well in the house of Pandion haliaetus.

Ospreys aren’t nesting along the Upper Clark Fork River between Deer Lodge and Missoula as much as they used to, and no one knows why.

“We’re beginning to see disturbing trends in some areas,” said Greene, a University of Montana professor and Montana Osprey Project founder. “I’m pretty worried about it.”


Because ospreys sit at the top of the food chain, when there’s trouble in the ecosystem, they feel it.

“They’re sentinels telling us that there are problems in the system,” Greene told The Montana Standard.

Compared to 10 years ago, the number of active nests from around Deer Lodge to Missoula has dropped by about 65%. There used to be about 37 nests in the area, but there were only 13 this spring.

Osprey numbers vary from year to year, but in 15 years of surveying ospreys along the Clark Fork River, Greene has never seen this magnitude of flux. He’s confident he’s seeing a drop in the population. It started with a slow decline, then turned sharp in the last three to four years.

Greene has been studying the Upper Clark Fork ospreys since 2006, so he knows the nests well. Many have occupants that return from year to year, and upstream of Deer Lodge, they’re still reproducing every year. But downriver, nests that used to hatch chicks are going empty.

The cause is unknown, but Greene suspects several factors are at play.


Fish populations have been low, which Greene said could be impacting the ospreys. He suspects the fish decline is due to lower flow and hotter water temperatures.

“We associate ospreys with nice, clean, cold streams,” Greene said.

The Upper Clark Fork went without fish for quite some time due to mining waste. According to Trevor Selch, water pollution biologist with Fish, Wildlife and Parks, the Upper Clark Fork has gone from having no fish to having fish in all of its reaches since 2007. The fish population reached historic lows in 2019 and 2020 and is still recovering, according to Caleb Uerling, FWP fisheries biologist for the Upper Clark Fork. Brown trout numbers are declining sharply all across southwest Montana, and biologists have no idea why.

“Come back in a while, and we might have a Clark Fork full of catfish,” Greene said.

The Clark Fork is primarily inhabited by cold water fish, especially trout. An osprey’s diet is 99% fish, so if fish populations drop, they have nothing else to eat.

“We need to have a really healthy fish population in order to sustain ospreys,” said Will McDowell, stream restoration director with the Clark Fork Coalition.

Uerling said the flow of a river is mostly affected by snowpack and timely rains. A big snowpack lasts longer, so the flows get low later in the year. Flow varies from year to year.

On July 3, the Clark Fork near Garrison had a water temperature of over 75 degrees. This is above the “upper lethal temperature” for every trout species in the river, meaning these temperatures would kill 50% of the population after 60 days of exposure. The heat won’t kill these fish outright, but it puts them under a lot of stress. In the same area, river flow was at 35% of median levels as of July 3.

“Those fish aren’t adapted to those temperatures,” Uerling said.

Air temperatures have been rising over the past 50 years.

“I think it’s safe to assume that waters are warming,” Uerling said.

Warmer temperatures mean more rain and less snow, which leads to less snowpack. The snowpack melts off faster, causing floods that make the water turbid and hard to fish when osprey chicks are hatching, and then lower flows in late summer that stress the trout. Higher air temperatures mean humans need more water from the river, causing the flow to drop more.

Greene said some chicks were likely killed by the heat in the scorching week of June 28.

If, as almost all scientists predict, the climate continues to warm and air temperatures continue to rise, Greene foresees lower flows in late summer.

This would affect wildlife and people who depend on the river.

“Water in the West is a big deal,” Greene said. “We depend on it for our way of life.”

Water is in short supply all across Montana even in the best of years. However, Montana’s osprey populations are doing just fine outside of the area of concern.

“In the whole study area, there’s no shortage of ospreys,” Greene said.

He said ospreys may be leaving areas that are no longer suitable to seek out bluer waters. For example, Flathead Lake is still deep, cold and productive, so birds have less trouble nabbing fish.


Greene has another hypothesis: arsenic.

The Montana Osprey Project monitors levels of toxic metals in ospreys along the Clark Fork River. Since ospreys perch at the top of the food web, they ingest all the contaminants in every animal below them, and these chemicals are deposited in blood and feathers. Because of this, they serve as a barometer for toxins in an ecosystem.

Chicks are particularly useful, since they only eat food caught near their nest, so their toxin levels are area-specific. The project gets money from the National Resource Damage Program to use osprey chicks to measure the success of the Superfund cleanup.

Every summer, Greene and his team carry out what he calls “alien abductions:” They grab the chicks from their nests to band them and take blood and feather samples. They also measure heavy metal levels in river sediment.

The shores of the Clark Fork river contain “slickens,” patches of mine waste that turn slippery when wet and pose a danger to livestock. The long road of restoration is nowhere near finished, and the scars of the old mines still score the banks.

The slickens glitter with arsenic and copper salts, and the corpses of trees jut up like barbed wire on a battlefield. The metals will never degrade. The only way to keep them out of the river is through human restoration — and the Upper Clark Fork cleanup effort has only been underway since around 2013.

In summer 2018, a heavy thunderstorm season swept through southwest Montana. The intense rains washed many untreated tailings into the watershed. When Greene came to abduct osprey chicks afterward, he found abnormally high arsenic levels in their blood and feathers.

According to Cooke, a fall thunderstorm in 2019 caused a fish kill in the Deer Lodge area by washing metals into the river. When fish are contaminated, so are ospreys.

In 2018, the chicks at the Grant-Kohrs Ranch National Historic Site had almost 30 times more arsenic in their blood than they did in 2006.

Ever since then, the arsenic levels have remained high in riverbed sediment and the ospreys. Greene said other biologists have found high arsenic levels in fish and macroinvertebrates in the Upper Clark Fork.

While the increased arsenic is a new development, it’s not the only mine waste lurking in these chicks. Mercury levels have been high since the start of the study. Greene and his colleagues found high mercury levels can result in an egg mortality rate of up to 50%.

The average Upper Clark Fork osprey chick’s blood mercury level is about one hundred times higher than what is considered problematic in humans, and that level rises when they stop growing. No conclusive research has determined how much mercury is too much for an osprey.

The mercury comes from gold and silver mining, not copper mining, so it’s not included in the Superfund cleanup.

Around Frenchtown and Missoula, the river is so laden with dioxins, furans, and PCBs that humans are advised not to eat the fish. As ospreys do not read Fish, Wildlife and Parks advisories, they continue to eat these chemical-heavy snacks, which Greene said is another stressor.

Uerling said areas with a high concentration of metals also tend to have low levels of fish. He listed three big stressors for Clark Fork fish: metals contamination, river flow, and water temperature. These reasons line up well with Greene’s theories for osprey decline.

Ospreys really can’t win. When the flow is low, so is their food. But, according to FWP fisheries mitigation biologist Nathan Cooke, higher flows result in higher metals concentrations in fish as the slickens slide into the water.


FWP fish research biologist David Shmetterling pointed out the remediation to remove contamination from the river could also be affecting fish population in the short term, since the process simplifies habitat and removes cover.

Part of the restoration efforts involve purchasing water rights, reforming irrigation, and improving the floodplain.

“We do expect flows to improve over time,” Shmetterling said.

McDowell said remediation diminishes the population of migratory songbirds for a few years afterward, since it requires removing habitat to replace the soil. The cleanup effort in the Upper Clark Fork is too young for the river to see many of the desired improvements.

“The idea is to put it on a trajectory to a long term recovery,” Shmetterling said.

Cooke said the Natural Resource Damage Program has put erosion control measures in place to keep metals out of the floodplain, like putting hay bales in problem areas.

Executive director of the Raptor View Research Institute Rob Domenech stressed he doesn’t want to raise any alarms yet. He theorized the decline may be influenced by a robust bald eagle population, since bald eagles predate on ospreys and steal their fish.

FWP wildlife biologist Torrey Ritter also pointed to bald eagles as potential culprits.

None of this is an exact science, and Greene doubts eagles are to blame. He pointed out eagle populations haven’t skyrocketed enough to account for this osprey decline, and areas of high eagle numbers aren’t correlated with areas of low osprey numbers.

Domenech also speculated ospreys may be choosing not to return to the Clark Fork after migration, or they’re present but not nesting.

“It definitely warrants further investigation,” Domenech said.


Ospreys are the most human-tolerant raptors, which certainly makes further investigation easier. When their chicks are abducted by Greene and his team, the parents don’t attack or struggle. They just fly around, cheeping in concern.

Ospreys tolerate people very well — sometimes a little bit too well. They love to build their sprawling nests on top of power poles, which can cause fires or tragic osprey electrocution. NorthWestern Energy tries to combat this by building nesting platforms, installing deterrents on power poles, and removing any sticks found on power poles.

Osprey also find baling twine to be an enticing nest material, until they get their claws tangled in it and die. Baling twine can kill up to 10% of ospreys.

When the Montana Osprey Project goes banding, Kory Boggess with NorthWestern Energy cuts free any twine he can find in the nest. When the project went banding on Wednesday, they found four chicks tangled in twine at the Race Track and Chris Ford nests. They cut three free. One was dead.

The project runs a nest camera at Missoula College. Its regular occupant, Iris, may be the oldest osprey alive, at about 24 years old. Over 17,000 people follow the Montana Osprey Cams Facebook page, where Greene and the team post banding events, updates on the “Iris Soap Opera,” and updates and theories regarding the population decline.


If this decline is really due to warming waters caused by climate change, Greene pointed out neither he nor the Superfund can do much about it. If it’s due to metals, then the river cleanup will hopefully help — but it may not see results for some time.

Greene will continue to watch the ospreys and share his data with the Superfund. The Clark Fork Coalition will keep working to restore the river’s tributaries. The Facebook viewers will keep watching Iris, and Fish, Wildlife and Parks will keep tracking fish numbers.

The Upper Clark Fork is a watched pot. If it boils, the ospreys will sound the alarm.

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