The Bald Eagles of “the lower 48” (the continental US) have made a truly amazing comeback after approaching near extinction from the use of DDT. As the chemicals from DDT ran off into our waters, small plants and fish began accumulating DDT in their bodies, and it began working its way up the food chain as smaller fish were eaten by bigger fish, each time concentrating larger amounts and effects. Being predators at the top of the food chain, this caused this high level accumulation of chemicals to pass into the bodies of raptors, especially adult eagles, which lead to thinning egg shells which crushed easily under the weight of incubating parents. Through ignorance and carelessness, we almost lost the symbol of our country. But with years of very dedicated conservation efforts, breeding programs, and protective laws, the Bald Eagle is once again soaring our skies, becoming a fairly common sight along our waterways. So watch the skies and trees when you travel near water. In their adult plumage, the Bald Eagle’s white head and tail, plus their size, makes them easy to identify.
You may notice that the male bald eagle in this family seems smaller than the female. There is a reason for that, and it is called “sexual dimorphism.” The definition of sexual dimorphism is a visible size difference between the sexes of a particular species. In most animals that exhibit sexual dimorphism, that means that the male is noticeably larger than the female (think bears, elk, cattle, etc). However, in eagles, as in all raptors (including owls), this is reversed! A female bald eagle can be up to 1/3 larger than her mate! There are various theories of why this is true for raptors. One is that the female spends most of her time on the nest during incubation and in the early days of tending chicks, and her greater size provides more protection for eggs and nestlings. Another theory is that it is nature’s way of providing for the ever-increasing appetites of fast growing chicks. By spreading out the size of prey that can be taken by each parent, each can focus on different prey species within their territory.
This impetus for this project was a commission from Na Aina Kai Sculpture and Botanical Gardens in Kauai, Hawaii, intended for their new Alaska-scape exhibit. Their plan, however, called for a reduced scale for sizing the creatures of their exhibit. To accomplish the desired scale, these birds are about the natural size of our nesting Colorado Bald Eagles.
Speaking of Size…
Unlike Golden Eagles, which are found worldwide, the Bald Eagle is strictly a North American raptor, ranging from above the Arctic Circle to northwest Mexico. An interesting size fact for Bald Eagles is that the farther south you go, the smaller the birds become. Alaska and northern Canada have the largest bald eagles, while the desert southwest has the smallest subspecies. This may have to do with the availability of food, or perhaps the necessary body mass for the extremes of heat and cold. In any case, when it comes to winter migrations, a portion of local populations will move farther south for the winter, so one might even see Alaska birds wintering as far south as Colorado.