Ospreys have bounced back in Minnesota from the deadly days of DDT

Fewer than two dozen animal species are found throughout the world, assuming appropriate habitat.

One of them is the osprey, nesting on all continents except Antarctica. That includes here, with osprey nesting on our ball-field light standards, utility poles, channel markers and the many artificial nesting platforms that dot the Twin Cities area.

All nests are located near water containing fish, which compose 99% of the birds' diet.

DDT, synthesized in 1874, was first used as an insecticide in 1939. When applied, mostly as an insecticide, the poison became part of the food chain, which can include fish.

DDT became a problem for osprey.

I asked Dr. Victoria Hall, executive director of the Raptor Center at the University of Minnesota, to explain how this threatened the birds.

"As a species that sits at the top of the food chain, osprey were extremely impacted by environmental contamination by DDT," Hall wrote in an e-mail.

"Osprey would eat fish contaminated with DDT, and with each fish more and more of the pesticide would bioaccumulate or 'build up' in the osprey's body.

"This buildup of DDT resulted in the osprey becoming less able to absorb calcium, and thus resulted in osprey laying eggs with extremely thin shells.

"These thin-shelled eggs were much more likely to break when the adult tried to incubate them, resulting in a dramatic decline in osprey populations," she wrote.

Ingested DDT accumulates in the tissues of the consuming animal because it metabolizes more slowly than nutrients. Four DDT parts per million is all that is needed to do damage.

Residual traces of DDT remain in the environment, still to be found in bald eagles, another consumer of fish.

"This leaves an important lesson about unintended consequences of chemicals in the environment," Hall wrote, "and the role we all play in protecting human, animal and environmental health."

Given a healthier environment and suitable places to nest, our osprey population recovered quickly. The metro area is a good example.

There were no known osprey nests south of Lake Mille Lacs when 144 young birds were captured in northern Minnesota and released in southern locations in 1984 and 1985.

The first wild nest attempt by those birds came in 1986. Last year, 2020, the eight-county metro area had 159 known active osprey nests. Parent birds raised 237 chicks, a gain of 43 from the year before.

Seventy-five of those nests were on nesting platforms, 30 on cell or radio towers, 26 on ball-field lights, 24 on power poles or transmission towers, two on other man-made structures, and two nests were built in the old-fashioned location — a tree.

Osprey returned here in mid-April for another nesting season.

Since nest monitoring in the metro area began 35 years ago, 2,704 chicks have been logged into the records. The numbers are compiled by Vanessa Greene of Minnetonka, with information gathered by a hardworking crew of volunteers. (osprey.mn@gmail.com)

Is there a limit to population growth?

"Ecosystems are fine tuned machines that constantly strive to be in balance," Hall wrote. "Osprey populations will be limited by things like nesting site and territory availability and food resources."

Lifelong birder Jim Williams can be reached at woodduck38@gmail.com.