Among raptors, or birds of prey, females are larger than males, and the female bald eagle is no exception. While size is the main difference between sexes, there is more to female bald eagles I cover in this article.
What is a female bald eagle called?
Currently, most people use the name “bald eagle” to refer to both male and female bald eagles. The word “formel” is used by some to refer to a female hawk or eagle.
The word formel was first used in Geoffrey Chaucer’s (1343–1400) poem “The Parliament of Fowl”. A formel was a female eagle whose affections were the object of three tercels (male eagles) in the love debate that dominates Chouncer’s poem. The formel eagle is a sophisticated female who speaks softly, indicating restraint and modesty, qualities coveted in England in the 14th century.
Name for young bald eagles, male or female include:
- Eaglets or Nestlings: Baby eagles that are still in the nest.
- Fledgling: Baby eagles that leave the nest and are capable of sustained flight. Eaglets usually jump from branch to branch around the nest before they are capable of sustained flight.
- Juvenile: Young eagles that are still in their first plumage. The plumage they leave the nest in is kept for about six (6) months when juvenile birds begin to molt.
- Subadult: The subadult stage follows the juvenile stage and is marked the plumage that replaces the juvenile one. The subadult stage starts roughly seven (7) months after a bird leaves its nest. The subadult stage last about 4 years during which an eagle acquire several plumage towards the adult plumage in year 5. Some eagle biologist use the term subadult only after 3 to 4 years of age.
A group or flock of eagles is called a convocation, congregation, or aerie.
Female eagles are larger than males, but it’s not as simple as that
Generally, female bald eagles are about 25% larger than males.
But bald eagle size, which is typically given as body weight, varies throughout the species range.
Female bald eagles at lower latitudes of their range, which corresponds to southern U.S. and Mexico, are smaller than females at higher latitudes, namely, mid-North America and Alaska. In fact, Alaskan and Canadian bald eagles are the largest.
Northern eagles are so big that males there are as big or bigger than southern females.
It can be challenging to distinguish eagle sexes based on size
When bald eagles from the same latitude (locality) are compared, females are consistently larger than males. Hence size differences can reliably be used to tell sexes apart.
When eagles from an unknown origin are compared, size is not a reliable way to tell sexes apart, given the variability in size across the south-north length of the eagle’s range.
In any case, weight is an unreliable metric
The bodyweight of live eagles, or any bird, varies considerably compared to other measurements. The weight of a bird can be affected by its overall condition as well as its crop content.
For instance, the weight taken from male Alaska bald eagles ranged between 8.1- 10.7 lb with an average of 9.3 lb. Weights of adult females ranged between 10.2-14.1 with an averaged of 11.7 lb.
Wing and Tail measurements are more reliable
The gradual increase in female bald eagle size is better illustrated by wing chord and tail length measures. For the simple reason that these two measurements are not subject to day-to-day variation, the chord length (from the elbow to the tip of the wing) and tail length (from the base to the tip of the tail) show less varying and more reliable differences between bald eagle sizes.
Based on wing chord and tail measures, northern female bald eagles are the largest eagles throughout the species range.
Do female bald eagles have a white head?
Some visitors to Avian Report have asked whether male and female bald eagles look the same. Others have wondered why most of the bald eagles they see are males and only a few are female.
In most, but not all, bird species, males are more colorful than females. In many bird species, males and females are very similar and cannot be distinguished by plumage or appearance alone. Ornithologists use the term sexual dimorphism to describe whether males and females look different or the same.
Male and female bald eagles look the same, as the species does not exhibit sexual dimorphism. Whether a bird species has sexual dimorphism or not depends on the species’ evolutionary path to what we see today.
The reason people ask if most or all of the bald eagles they see are males is that they are looking at adults. Brown bald eagles are assumed to be female. In reality, those brown bald eagles are juvenile or immature bald eagles that are often in the minority.
Females bald eagle give unique calls
Observations of bald eagles breeding in captivity revealed a type of vocalization that only females utter during specific circumstances.
Bald eagles begin copulating about the time the nest is nearly finished. During the mating ritual, the female signals the male about her readiness to copulate by giving soft yet high-pitched calls. This type of observation may be impossible to do on wild eagles.
A female bald eagle’s primary responsibility is to protect her eggs and young
Field observations suggest that female bald eagles are fierce defenders of their eggs and young.
Female bald eagles spend more time on the nests incubating the eggs and brooding the eaglets than males. Females incubate the eggs through the night and part of the day.
Because females are larger, they can fend off mammal predators that typically attack nests at night.
When another pair attack a territorial eagle pair, the territorial female focuses her counterattack on the attacking female, as they are roughly the same size.
The male faces the attacking male. This observation suggests that each sex faces a bird of similar size and strength.
It is usually the female bald eagle that initiates pair formation.
Although the number of pairs and nests observed is small, the female bald eagle takes the initiative about getting new mates when a male disappears.
The female bald eagle of a pair in Ohio observed for eight consecutive years picked and replaced four different males because males either died or disappeared.
Also, in Ohio, another nesting pair lost a male. However, the female picked and replaced the male in only four days.
In Arizona, a male disappeared during the pair’s incubation period. The female took only one day to pick, attract, and replace the missing male for another, which quickly assumed parenting duties.
The new male began to deliver food to the incubating female and nesting materials six days after being attracted by the resident female.
The speed at which males are replaced may be influenced by the time of the year males die or go missing and the need for help by the female to attain her nesting goals. For instance, two female bald eagles that lost their mate at the end of the breeding season took 11 and 14 weeks to return to the nesting territory with a new mate.
The female bald eagle can replace lost eggs quickly
After laying their entire clutch, eleven female bald eagles nesting in captivity had their eggs removed. After only eight days, nine out of the eleven females laid a new clutch.
Most wild female bald eagles that lose their eggs due to natural causes are likely to replace them shortly after that.
Bald eagle females select nesting territories
Although the number of observations is small, female bald eagles appear to choose and hold territories, mainly if the territory is of high quality.
In one instance, a female bald eagle that lost two mates in two separate years kept her territory and brought two new males.
Another female that lost her mate fiercely protected her territory against other bald eagles that attempted to take over her territory.
In nest building, there is a division of labor
Both males and females bring nesting material to the nest. But at one point during nest building, the female switches from bringing material to staying in the nest, arranging the male’s nesting material.
The female eagle has a well-developed brood patch
In most songbird species, only the female develops a brood patch. In other types of birds, males also develop brood patches.
A well-developed brood patch has to do with the amount of time each sex spends incubating the eggs and brooding the young birds. In American robins and hummingbirds, a brood patch is only present in the female because the male does not incubate the eggs.
What is a brood patch? It is a featherless patch of skin on a bird’s underside during the nesting season.
Male and female bald eagles incubate the eggs, but the female spends significantly more time incubating the eggs and brooding the young. This explains why female bald eagles have a more developed brood patch than males.
As mentioned above, female bald eagles spend more time incubating the eggs. Females also do most of the parental care during the first 2-3 weeks of the nestling’s life.
During the first two to three weeks of nestling development, the female is at the nest 90% of the time, while the male is only there 50% of the time.
Bald eagle females brood the young during inclement and cool weather until they are approximately four weeks old. Field observations of nesting eagles revealed that young eaglets were brooded for about 85% of daylight hours during the first week after hatching, with females brooding 65% of the time and males 35%.
Occasionally, female bald eagles assist as volunteers
On rare occasions, female bald eagles join forces with an existing breeding pair to help raise the pair’s young. While the female helper does not incubate, she helps deliver food to the growing eaglets.
Female eaglets leave the nest later than males
Observations of bald eagle nestlings indicate that males consistently leave the nest before their female siblings regardless of hatching order.
Researchers in Alaska studied a nest where a female eaglet hatched first, followed by a male sibling. The male eaglet left the nest ten (10) days earlier than the female.
Bald eagle females are often very productive
Generally, it is unknown how successful female bald eagles are at breeding. In Alaska, one marked female, who started nesting at age 6, went on to nest for 13 consecutive years and was successful in 11 years.
This female reared 23 young, which is almost two young per year. This is perhaps the largest number of fledged young produced by a female bald eagle.
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- Fraser, J. D. (1981). Breeding biology and status of Bald Eagles on Chippewa National Forest. Phd Thesis, Univ. of Minnesota, Minneapolis.
- Gerrard, J. M. and G. R. Bortolotti. (1988). The Bald Eagle: haunts and habits of a wilderness monarch. Washington, D.C: Smithsonian Inst. Press.
- Gerrard, J. M., A. R. Harmata and P. N. Gerrard. (1992b). Home range and activity of a pair of Bald Eagles breeding in northern Saskatchewan. Journal of Raptor Research 26:229-234.
- Wallin, D. O. (1982a). The influence of environmental conditions on the breeding behavior of the Bald Eagle (Haliaeetus leucocephalus) in Virginia. Master’s Thesis, College of William and Mary, Williamsburg, VA.
- Wiemeyer, S. N. (1981). Captive propagation of Bald Eagles at Patuxent Wildlife Research Center and introductions into the wild, 1976-80. Journal of Raptor Research 15:68-82.