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Female bald eagles are bigger than males!

Males of most species tend to be bigger than females (think lions). But in most birds of prey and for all of North America’s eagles, the females are bigger. This is called “reversed sexual size dimorphism.” The logical question then, is why?
The answer is a little complicated. To date, no single study has conclusively proven why females are the larger sex in some species. But some strong theories exist. The most accepted one says that the females are better positioned to be in charge of key nesting and rearing behaviors. If they’re going to be in charge, there’s an advantage to being the bigger bird. Here are a few examples where this size advantage could come in handy...
  • Both sexes bring nesting materials to the nest, but most of the arranging is done by the female. It’s not unusual to see the female take charge over the male during nest building.
  • Incubation duties are shared, but not equally. The female tends to spend much more time incubating eggs than the male. As hatching begins, the female takes even greater control over the nest. For about two weeks after hatching, the young eaglets cannot thermo-regulate. Without body heat from the parent, they could quickly perish. Logically, the maternal instinct is powerful and probably ensures that, come what may, the adult female is dedicated to keeping the nestlings warm. It also makes sense that during this time, when the chicks are at their most vulnerable, being the bigger, more powerful bird would come in handy to keep the nest safe from predators.
  • In the first weeks after hatching, the male will do most of the hunting, and bring food to the female and to the young. I’ve witnessed males, upon delivering food, try and settle onto the nest – only to have the female stand her ground and not allow him access. It’s not unusual to see a male attempt to feed a fish he’s just brought in to the young, only to have the female take the fish away and push the male to the side of the nest. In cases like this, size would seem to matter.

At the same time, the leaner the male, the more advantage he might have chasing down and trying to outmaneuver prey.

Once the young are old enough to regulate their internal temperature, and don’t need to be kept warm by the adults, nest duties become more evenly shared.

Did you miss the last issue of Island Naturalist? Click HERE.

Issue #11 / Eagle News You Can Use


Catalina’s Bald Eagles Are Back
New Eggs, New Chicks!
Fact or Fiction: Still Endangered?
Did You Know … Females Outsize Males

Photo by Frank Hein

Steffani Jijon returns a juvenile bald eagle to its nest after a successful banding operation. People often confuse juvenile bald eagles with golden eagles since the young balds lack a white head and tail until they become older. Click HERE for a quick Identification review.


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