Bald eagles are one of the most recognizable bird species in North America. The dark body profile, white head, and strong yellow beak is a large part of our national culture. But, are we looking at a male or a female bird in those images? Are the two sexes completely the same, as some assume, or are there differences between them?
Size is the biggest difference between the male and female. 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 these differences and find out why females are larger than males.
The biggest difference comes down to size. Many large birds of prey show signs of reverse sexual dimorphism in relation to their size. It is an expectation in the natural world that females would be the smaller of the species as the males have to be the more dominant and aggressive. However, this doesn’t work for a lot of birds of prey as it makes more sense for the males to be smaller than the females. This is true for a lot of hawks and falcons, where the males are much smaller. This helps them when flying as they are more agile hunters and, in turn, better providers. This is 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. It isn’t so easy to tell which you are looking at if you have one individual at a distance. The female will also have a larger wingspan, possibly as much as 2 feet wider, and a bigger beak and talons. Again, this is only obvious side-by-side and with experience. It is much easier to notice if you can see the pair in flight together
There is no definitive size of a male bald eagle or a female bald eagle in the USA. There will 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 normal 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 more heat and handle living in those extremes. There is also a related rule called Allen’s rule, where 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 a longer back-facing talon than males and a deeper beak.
Why Are Female Bald Eagles Bigger?
Female bald eagles are larger as this helps when it comes to looking after the eggs and chicks in the nest. Incubation is a tough job for any mother and they need to be sure that they can provide enough body heat and coverage for each chick. Eggs that don’t feel the heat could fail to develop and hatch. Birds with larger bodies that can conserve their heat more easily can transfer this to their young.
Females also need to provide warmth for newborn chicks at night and during bad weather. This is another reason why it makes sense for the females to be even larger in colder regions where freezing temperatures and snowfall could be a bigger issue.
The larger size of female birds also means that they have a more dominating presence on the nest. The heavier build, larger beak, and more dangerous talons can deter any other creature that may try and raid the nest. Adult eagles may not have any natural predators, but their chicks could be a great meal for other birds of prey or opportunistic arboreal mammals like raccoons or even bears. For example, Hawks are more likely to think twice facing a larger female than the smaller male.
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 needs to 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 to head out on more hunting trips.
A lighter weight also means that 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 under their own body weight and the weight of the fish. There is the risk that smaller birds won’t carry as much in one go, leading to smaller prey, but the regularity of trips and lower amounts of energy used can balance this out. Also, many eagles will 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.
The voices of these two birds are also different. It is noted that the female’s calls tend to be lower in pitch than those of the male. Whether this is down to their size is not clear. It makes sense for male and female birds to sound different, as this can help when responding to calls and attracting mates. For humans, the difference is pretty indistinguishable until we use specialist equipment to spot the subtle differences.
There are important dimorphic differences between male and female eagles, even if they aren’t immediately obvious.
We might not be able to tell the difference between a male and a female eagle at first glance, but there are important differences in their size. Males need to be two-thirds the size of the female so that they can be better able to hunt and provide and the female can handle her duties on the nest. As a pair, this works perfectly to help them create a strong partnership where they can provide the best start in life for their chicks.