About the American Bald Eagles
The Bald Eagle is a magnificent bird of prey that is also known as the national bird of the United States of America. It was once a ubiquitous bird on the North American continent. Yet, during the 1950’s and 60’s their numbers dropped so dramatically that they were on the verge of extinction.
The effects of DDT on the American Bald Eagles
It was soon discovered that environmental pollutants (most notably DDT) were one of the primary factors responsible for the massive decline of the Bald Eagle population during the 1950’s and 1960’s. From populations recorded in the 1700’s of 12,500 nesting pairs only 412 nesting pairs were found in the whole of continental United States (excluding Alaska) in the 1950’s.
The bioaccumulation of DDT
Bald Eagles were on the top of the food chain and so they were especially susceptible to the effects of DDT due to bioaccumulation. Bioaccumulation occurs when a fat soluble chemical builds up in the food chain. The higher up an animal is in a particular food chain the more chemical they inadvertently consume. The American Bald Eagle was on the top of their particular food chain so they accumulated a disproportionate amount of DDT in their system.
The effect of DDT metabolism on Birds
DDT affected bird’s calcium metabolism interfering with their capacity to lay healthy eggs. As result their eggs were often too brittle and cracked easily under the weight of a brooding adult bird. Wildlife experts found this to be the case with Bald Eagles. They found that the eagles ability to reproduce was severely compromised because of the affects of DDT.
A number of measures were quickly put in place to stop the rapid decline and the population of the birds has rebounded since. In 2006 the bald eagle was recorded having more than 9800 breeding pair populations in the U.S.
How the case of the Bald Eagles developed
In the Great Lakes region the Bald Eagle population climbed dramatically from 26 mating pairs to 134 between 1977 and 1993 and it seemed on the face of it that all was well with the local population of American Bald eagle.
But studies undertaken by the U.S fish and wildlife service biologists seemed to suggest the rapid growth of the Great Lakes population was due more to immigration of eagles from cleaner areas than a recovery of local Bald Eagles.
Chemical contaminants are still present in the Great Lakes region
New recruits from around the region often breed successfully at first, but they too succumbed to the level of chemical contaminants that had built up in the food chains of the region.
Studies have found that contaminants like DDT and PCB accumulate quickly in the bodies of the top predators like Bald Eagles and the level of chemicals eventually lowers their breeding success.
For some reason it seemed that birds were more sensitive to the effects of toxic synthetic chemicals than other forms of wildlife. Bald eagles as well as a number of other fish eating birds displayed a variety of abnormalities ranging from severe birth defects which included missing eyes, clubbed feet and crossed bills to a strange wasting disease that would suddenly strike a healthy chick rendering them helpless and withered within a few days.
Environmental pollutants were found to be responsible
Toxicology tests revealed that the birds all contained elevated levels of PCB’s, dioxins and furans in their tissues. The mechanism by which these chemicals interfered with the development of these birds is still unclear, But the correlation between the presence of these chemicals and the obvious physical abnormalities of the eagles as well as other local birds was undeniable.