The Challenges to Yellowstone Osprey

While walking along the Yellowstone River, I stopped to observe these two attentive ospreys in their nest high on a cliff overlooking the river. The large size of the nest reveals that it has been reused and expanded over the years—perhaps by these two since ospreys mate for life. The female is the larger bird on the left with the chest streaking, sometimes called a necklace. This mated pair and others return to Yellowstone each April to breed. But Yellowstone’s osprey population has dwindled over the last thirty years because of human-caused challenges inside and outside the park. 

Inside the park, the challenge to ospreys began when someone illegally introduced lake trout into Yellowstone Lake twenty some years ago. Nonnative lake trout are a voracious predator of the lake’s native cutthroat trout, the shallow-dwelling fish that osprey once subsisted on. As the lake trout population swelled, the cutthroat trout population plummeted. Since ospreys eat only fish, and lake trout dwell too deep for ospreys to catch, and there were fewer cutthroat trout, ospreys had less food. With poor nutrition, their reproduction rate and population decreased. As the National Park Service manages an ongoing campaign to remove lake trout, the number of cutthroat slowly increases. Hopefully, more cutthroat will feed more ospreys and the bird’s population will increase. 

In a recent year, Yellowstone Lake had only one active osprey nest, while twenty nests were found elsewhere in the park. That small number of ospreys is a far cry from the number in the park’s early years. The park’s first naturalist, Melvin Skinner, reported that earlier scientific parties had observed abundant osprey near Yellowstone Lake and the Yellowstone River. His 1917 research found thirty osprey nests along just the west shore of Yellowstone Lake. He estimated that 120 pairs of osprey bred in the park.

The challenges outside the park are also caused by humans. The national population of ospreys (as well as that of bald eagles, pelicans, and others) crashed in the mid-1900s, when ingestion of the pesticide DDT caused birds to produce fragile, thin-shelled eggs that could be broken by the adult sitting on them during incubation, killing the unhatched young. After the 1972 US DDT ban, populations rebounded, and the osprey became a conservation success symbol. 

But human development still threatens. Many ospreys build nests in the tops of standing dead trees. But as power poles (another form of standing dead trees) have proliferated, ospreys have selected them as nest sites. After many birds were electrocuted, utility companies and concerned citizens installed safer and more welcoming nest platforms near power poles.  

Another threat is the twine used around bales of hay. Once bales are opened, the twine is often left on the ground. Ospreys like the soft twine and incorporate it into their nests not knowing that adults and chicks can become entangled in the twine and die.

The nest of this osprey pair I photographed rests upon a column of rock not a power pole. The pair has not used any bailing twine. They are not exposed to DDT. Having avoided these challenges, hopefully they’ll produce two to three young to add to Yellowstone’s struggling osprey population.

I write, speak, and photograph to protect wildlife and wild lands. My bestselling In the Temple of Wolves; its sequel, Deep into Yellowstone; and its prequel, The Wilds of Aging are available signed. My books are also available on Amazon unsigned or as eBook or audiobook.