and Female Eagles
In many bird species, it's easy to tell males from females because the plumage is different. Not so with eagles!
However, we can use size measurements from different parts of the eagle's body to determine the gender of the eagles we capture. Female eagles generally have larger wings, feet, talons, beak, etc.
A biologist by the name of Bortolotti discovered that 2 size measurements show the greatest separation between male and female. The two measurements are:
These measurements can be used in the following formula:
Sex = (bill depth x 0.392) + (hallux length x 0.340) -27.694
Here are the key measurements from 6 of our eagles, plus their weights. Can you determine the gender of these birds yourself? (Click here for student worksheet.)
Challenges of Using Weight to Determine Gender
We can also use weight to try to tell males and females apart--but when we talk of bald eagle weights, we need to keep two factors in mind:
This second factor, called "Bergman and Allens rule," makes things a bit more complicated: Bald eagles living farther north (in Canada or Alaska, for example) should be larger than those say, from Florida. And they are!
there is some overlap. There is not always a definitive, clear separation
of the sexes based upon size and weight. For example, the largest northern
male eagle may be as big as or even bigger than the smallest southern
By the way, when the female of a species is larger than the male it is known as "reverse sexual dimorphism." This is true in most raptors, including the bald eagle. (In most species the opposite is true. When males are larger, it's known as "sexual dimorphism.")