Silent Spring and Bald Eagles

My eagle is back! An eagle has hung around a point of land on Lake Metigoshe each spring and fall for several years. It may not be the same eagle, of course, but I have that impression, and each time I see that magnificent bird I thank Rachael Carson.

This past September was the 50th anniversary of the publication of her book Silent Spring. The book is arguably the most important book ever published on the environment, and widely regarded as providing the impetus that launched the environmental movement. If Rachel Carson would not have written Silent Spring there might not be any bald eagles around Lake Metigoshe today. I cannot be sure of course, but without Silent Spring, things would have played out differently.

The publication of Silent Spring catapulted the widespread use of pesticides into the national spotlight. It stirred up debate on a whole range of environmental issues, and among other things, led to the ban of DDT in the United States in 1972.

DDT has the rather unusual property of bioaccumulation, which means that the substance accumulates in animal tissues and increases in concentration at higher levels in the food chain. For example the DDT concentration in phytoplankton (microscopic algae) may be only 0.02 parts per million (ppm) but by the time it worked its way up through the food chain to zooplankton, minnows, larger fish, and finally a bald eagle, the DDT concentration in the eagle may be 124 ppm.

We now know that high concentrations of DDT upset the calcification of egg shells, resulting in overly thin shells. The egg shells of bald eagles and other birds at the top of the food chain became so thin that they would crack under the weight of the parent incubating the eggs. The widespread use of DDT led to plummeting reproduction in bald eagles and their placement on the endangered species list. From 1975 to 1988 not a single bald eagle nest was documented in North Dakota. However an article in North Dakota Outdoors from 2010 noted that 66 bald eagle nests were occupied in the state during the summer of 2009. The eagles are back!

Most everyone gets a thrill at the sight of our national bird, and let’s hope that thrill never fades. But each time we see one of those magnificent birds, we owe a bit of gratitude to Rachel Carson and her Silent Spring.

Chuck Lura

Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.