The bald eagle, a prominent North American raptor, is one of the largest birds on the continent, commanding attention with its imposing physical presence. Understanding the size of this species involves looking at its specific measurements and the factors that influence them. This article details the physical dimensions of a mature bald eagle, covering its length, weight, and impressive wingspan.
Key Measurements of the Bald Eagle
An adult bald eagle’s body length, measured from beak to tail, typically ranges from 70 to 102 centimeters (28 to 40 inches). This measurement, combined with its robust mass, contributes to its powerful appearance. The average body weight for the species falls between 3 and 6.3 kilograms (6.6 and 13.9 pounds).
The wingspan is arguably the most impressive dimension, providing the bird with its characteristic soaring silhouette. A mature eagle’s wingspan generally measures between 1.8 and 2.3 meters (5 feet 11 inches and 7 feet 7 inches). This wide span allows the eagle to ride thermal currents effortlessly, covering vast distances.
How Size Varies Across the Species
The size of an individual bald eagle is not uniform and is primarily influenced by two biological factors. The first is a phenomenon known as reverse sexual dimorphism, where the female is noticeably larger and heavier than the male. Females are often about 25% larger than their male counterparts, with an average female weighing around 5.6 kilograms compared to the male’s average of 4.1 kilograms.
This size difference is thought to be an adaptation related to the division of labor. The larger female can better defend the nest and eggs, while the smaller, more agile male is a more efficient hunter. The second factor is geographic location, which follows Bergmann’s rule: birds in colder, northern latitudes tend to be larger and heavier than those found closer to the equator.
Eagles living in Alaska, for example, are notably larger than those in the southern United States like Florida. The largest females in the northern range can weigh over 7 kilograms and boast a wingspan of up to 2.44 meters (8 feet). An eagle’s final size is a product of both its sex and its native climate.
When Bald Eagles Reach Full Adult Size
Bald eagles achieve their full physical size, including maximum length and wingspan, relatively quickly after leaving the nest. By the time a young eagle fledges, or takes its first flight, around 12 weeks of age, it is already near adult size. However, the eagle’s weight and overall bulk continue to increase as the bird matures and develops muscle mass over the next several months.
While the bird’s body reaches its final size in the first year, it takes much longer to attain full sexual maturity and the distinct adult plumage. The iconic white head and tail feathers, as well as the bright yellow beak, begin to appear over a progressive series of molts. Most bald eagles do not reach the full adult coloration and are not ready to breed until they are four to five years old.