The bald eagle (Haliaeetus leucocephalus), our nation’s symbolism of freedom, is the second largest bird in North America. While the species can be found throughout North America, two subspecies can be found living in more distinct areas. H. l. alascanus is larger and lives in more northern climates while the smaller H. l. leucocephalus is found in the south. Generally the northern subspecies breeds north of latitude 40oN. The size of the birds varies considerably, but the largest birds are found in the northern most part of their ranges, but they average 81cm from tip of the bill to the tip of tail. Females are usually about 25% larger than males. Juveniles are larger yet lighter than adults of the same sex. They vary in weight from 3 to 7 kg. The bald eagle is protected under the U.S. Endangered Species Act (Environmental Protection Agency).
The lifecycle of the bald eagle begins in mid to late winter in northern climates. In Virginia, a much more temperate climate, the breeding, and nesting season starts in March. The courtship process includes an array of aerial displays in order to attract a female. Once a male does convince the female to bond with him, he has created an endless relationship. Bald eagles are monogamous and typically stay together year after year until one of the birds dies (Environmental Protection Agency).
After a pair has bonded they begin to build their nest. This process begins one to three months before the eggs are laid. Males bring sticks and other materials to the nest site and the female places them into the nest structure. The pair often reuses the same nest site for many years. One nest site was reported as being reused for up to 35 years. The couple may choose to use other sites within their territory, especially after a nesting failure or disturbance (Environmental Protection Agency).
In Virginia, eggs are laid in early April. Clutches can range from 1 to 3 eggs, but 2 is the most common size. They are laid one at time, a day or more apart, and being completed in three to six days. The female does 75% of the incubation. Incubation takes 35 days. The time period between egg laying allows time between hatching with the firstborn an advantage over the ones that follow. Clutches with three eggs generally do not produce three fledglings because the last born will die of starvation (Environmental Protection Agency).
By the eighth week, the eaglets begin to flap their wings in order to develop wing and chest strength, as well as coordination. They move from the nest and to adjacent limbs. Over the next six weeks, the young birds take their first flights out of the nest. Some first flights are failures, and the birds can spend days or even weeks on the ground until they can fly successfully. In the field, or returned to the nest, the parents will continue to feed them for several more weeks. After the young leave the nest, they spend about four years without a set territory and move with the availability of food and weather patterns (Environmental Protection Agency).
The digestive system of the bald eagle is similar to that of all birds. A strong, sharp beak allows the bird to rip and tear its diet of meat. Saliva helps the food pass down the esophagus into the crop. The crop holds the food and allows the continuous passing of food into the stomach. The stomach is the most active component of the system. The proventriculus or upper portion of the stomach secretes digestive juices to break down the food. The ventriculus or gizzard is the lower part of the stomach. Here food is ground up with the aid of gravel which the bird has previously eaten. Next, the food moves into the small intestine where it is mixed with bile and enzymes to break further down sugars, fats, and proteins. Nutrients are absorbed through the walls of the small intestine and enter the bloodstream. (Bald Eagle – Haliaeetus Leucocephalus)
All birds, the bald eagle include, have reptiles as ancestors. There are still debate about whether they arise from thecodonts, which were the precursors of dinosaurs or from small dinosaurs directly. The Jurassic period offers an excellent example of a reptile and bird intermediary. The Archaeopteryx had feathered wings and tail with the teeth and claws of reptiles. This animal died out about 65 millions years ago during the great extinction, but other bird species had already evolved and were able to thrive with the loss of the dinosaurs (Rochelle, 2006).
Today’s modern raptors appeared in fossils about 50 million years ago. The Eocene epoch also saw the beginnings of today’s kites. Kites include bald eagles and all eight sea and fish eagles. The sea eagles are thought to come from scavenging Asian and Australian kites and first appeared 25 million years ago. How long bald eagles have been their own species is unknown but fossilized remains found in the La Brea tar pits place them to at least one million years old (Rochelle, 2006).
Eight species of Haliaeetus offers a regional distribution which is almost entirely exclusive of each other. The birds are great dispersers so plate tectonics cannot be used to explain the exclusivity. The bald eagle is part of the kingdom Animalia, phylum Chordata, class Aves, order Falconiformes, family Acciptiridae, genus Haliaeetus and species Haliaeetus leucocephalus (Rochelle, 2006).
The anatomy of the bald eagle’s eye has adapted over time in order to allow for amazing eyesight, which is actually three to four times stronger than that of a human’s. The eyes are large, actually taking up most of the space in the bird’s head. There are a high number of cones, many more than a human’s eye, but a disproportion of rods and cones make the eagle’s vision poorer at night. Two fovea allow the eagle to see both forward and sideways. The bird is able to rotate its head 270 degrees. A transparent membrane can move across the eye to protect it from feeding their young or when catching prey (Hatcher).
- Bald Eagle – Haliaeetus Leucocephalus. (n.d.). Retrieved from Digestive Systems in Different Phylums: http://digestivesystemsxavier.weebly.com/bald-eagle—haliaeetus-leucocephalus.html
- Environmental Protection Agency. (n.d.). Species Profile: Bald Eagle. Retrieved from Environmental Protection Agency: http://www.epa.gov/region1/ge/thesite/restofriver/reports/final_era/B%20-%20Focus%20Species%20Profiles/EcoRiskProfile_bald_eagle.pdf.
- Hatcher, B. (n.d.). Biology – Structure and Anatomy. Retrieved from The American Bald Eagle : Recovery from Near Extinction: http://www.eagles.org/vu-study/biology/structure-and-anatomy.php
- Rochelle, M. (2006, March 1). The Biogeography of Bald Eagles (Haliaeetus leucocephalus). Retrieved from San Fransisco State University: http://online.sfsu.edu/bholzman/courses/Fall02%20projects/bald%20eagle.html