Bald eagles are one of the most recognizable bird species in North America. The dark body profile, white head, and strong yellow beak are a large part of our national culture. How do we know if we are looking at a male or a female? Are the two sexes completely the same, as some assume, or are there differences between them?
Size is the major difference between males and females. Unlike most animals, female bald eagles are larger than males, with a larger back-facing talon and deeper beak. A female’s wingspan can be over 2 feet wider, although this is hard to see unless looking at a pair. Females also have a lower-pitched call than males.
In this article, we look at the differences and find out why females are larger than males.
Size
The most significant difference between males and females is their size. Many large birds of prey show signs of reverse sexual dimorphism concerning their size. It is an expectation in the natural world that females would be the smaller of the species as males have to be the more dominant and aggressive.
However, this doesn’t work for many birds of prey as it makes more sense for the males to be smaller than the females. This is true for many hawks and falcons, where males are much smaller. The size difference helps them when flying as males are more agile hunters and, in turn, better providers. This is especially true for the bald eagle.
The male bald eagle is around two-thirds of the size of the female. This is apparent if you ever get the chance to see them side-by-side, but it isn’t always easy to tell if you only have one individual at a distance. The female also has a larger wingspan, sometimes as much as 2 feet wider, and bigger beak and talons. Again, this is only obvious when side-by-side and with experience. It is much easier to notice if you can see the pair in flight together
The U.S. has no standard size for a male or female bald eagle. There is always be a size difference between the males and females in a pair, but there is the possibility of a male eagle from the colder north being the same size or larger than a female from the south. Similarly, a female from the north could be even larger than usual compared to a male from the south.
This is all part of Bergmann’s rule, which states that animals that live further north in colder climates should be larger to conserve heat and live easier in those extremes. This ties into Allen’s rule, which states that animals in the north tend to have shorter limbs. Although this doesn’t relate to the northern or southern bald eagles’ legs, females do have a longer lower leg than males. Females also have an extended back-facing talon than males and a deeper beak.
Do you know why birds of prey are great hunters? Find out in this article I wrote.
Why Are Female Bald Eagles Bigger?
Female bald eagles are larger than males to help them look after the eggs and chicks in the nest. Incubation is a tough job for any mother, and they need to ensure that they can provide enough body heat and coverage for each chick. Eggs that don’t feel heat could fail to develop and hatch. Birds with larger bodies that can easily conserve their heat are able to transfer this to their young easier.
Females also need to provide warmth for newborn chicks at night and during bad weather. This is another reason why females are larger in colder regions where freezing temperatures and snowfall is a factor.
Larger female birds also have a more dominating presence in the nest. The heavier build, larger beak, and extended talons can deter any other creature that may try and raid the nest. Adult eagles may not have any natural predators, but their chicks can be an excellent meal for other birds of prey or opportunistic arboreal mammals like raccoons or even bears. Hawks are more likely to think twice about facing a larger female than a smaller male for a meal.
Click here to find out about the 11 largest birds of North America.
Why Are Male Bald Eagles Smaller?
While the female does the hard work of incubating and protecting the nest’s chicks, the male becomes their provider. He must head out to find enough food for both the mother and the young – not to mention his own needs.
Smaller raptors are much more agile and capable hunters. They can fly with greater ease over longer distances without the need to exert as much energy. This means more chances to find food and head out on more hunting trips.
A lighter weight also means it is easier for eagles to get airborne with their prey, especially if they hunt around water. The last thing they need is to struggle with their body weight and the weight of the fish.
There is the risk that smaller birds won’t carry as much, leading to smaller prey, but the regularity of trips and lower amounts of energy used can balance this out. Eagles will often lower the fish’s weight by eating the head and maybe the guts. This is highly nutritious for them and leaves the meat of the fish for the female and young.
Vocal Differences
The sounds of these two birds are also different, with the female’s calls lower in pitch than the males. Whether this is down to their size is not clear, but helps them respond to calls and attract mates. The difference is almost indistinguishable for our ears until we use specialist equipment to spot the subtle differences.
- Calls: The calls are typically used for communication between nesting pairs, during courtship, and while defending their territory. Bald eagles have a distinctive high-pitched, shrill call that sounds like a series of whistling or chirping notes. This call is often described as a “kek-kek-kek” or “kree-kree-kree” sound. Both males and females can make this call.
- Chatter: This vocalization is often associated with both male and female eagles during courtship and territorial disputes. It’s a rapid series of sharp, staccato notes and can sound quite aggressive.
- Whistles and Chirps: Bald eagles also produce various whistles and chirps that may not be easily distinguishable between genders. These sounds can be heard during interactions with other eagles, especially during territorial disputes or when defending their nest.
The vocalizations of male and female bald eagles are similar, and it can be challenging to differentiate them based on their calls alone. Behavioral cues, such as nesting and courtship behaviors, are often more reliable indicators of gender in bald eagles.
Behavior Differences
Male and female bald eagles exhibit some differences in behavior, although these differences are not always easily discernible, and individual variations can be significant. Here are some general behavioral differences between male and female bald eagles:
Nesting and Incubation
Both male and female bald eagles share responsibilities for nesting and raising their young. However, the female often takes the lead in incubating the eggs, especially during the initial stages of incubation. Males may take over incubation duties for short periods, allowing the female to hunt and eat.
Hunting and Foraging
While both male and female eagles are skilled hunters, females may tend to catch slightly larger prey due to their larger size. Females are often responsible for providing food for themselves and their offspring during nesting season. Males may assist in hunting but may also rely on the female’s efforts.
Territorial Defense
Both male and female eagles play an active role in defending their nesting territory, but females can be particularly protective during the nesting season. They are known to engage in aggressive behaviors when protecting the nest and their young.
Courtship Displays
During courtship, both males and females engage in aerial displays and vocalizations to strengthen their pair bond. These displays can include synchronized soaring, locking talons in mid-air, and other acrobatic behaviors. While both sexes participate, the male may initiate some of these displays.
Migratory Behavior
Female bald eagles may be more likely to migrate than males, especially in northern regions where winter weather conditions can be harsh. However, this behavior can vary depending on the specific population and local conditions.
References And Further Reading
The Bald Eagle: Haunts and Habits of a Wilderness Monarch by Jon M. Gerrard: This book provides a comprehensive overview of bald eagles, their behaviors, habitats, and conservation efforts.
Bald Eagles in the Wild: A Visual Essay of America’s National Bird by Mark Stalmaster: This visually stunning book offers a photographic journey into the world of bald eagles and their natural history.
The Bald Eagle: The Improbable Journey of America’s Bird by Jack E. Davis: This book delves into the cultural and ecological significance of the bald eagle in American history.
Bald Eagles: Their Life and Behavior in North America by Art Wolfe and Donald M. Bentz: This book combines remarkable photographs with detailed information on bald eagle behavior and ecology.
Return of the Eagle: How America Saved Its National Symbol by Greg Breining: This book focuses on the conservation efforts that helped bring the bald eagle back from the brink of extinction and its status as America’s national symbol.