Journey North Bald Eagles

Bald Eagle

Comparing Male and Female Eagles
Contributed by Peter Nye

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:

beak depth and
hallux (toe claw) length

These measurements can be used in the following formula:

Sex = (bill depth x 0.392) + (hallux length x 0.340) -27.694

If the answer is POSITIVE, the eagle is a FEMALE.
If the answer is NEGATIVE, the eagle is a MALE.

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.)

Eagle

Weight (kg)

Bill Depth (mm)

Hallux Length (mm)

Gender

K58

5.5

37.9

44.5

K72

6.6

35.9

42.7

E47

6.7

35.0

42.2

E49

6.1

35.5

42.7

E63

4.5

32.6

40.5

E50

5.1

32.4

41.2

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:

#1 Female eagles are larger than the male eagles. On the average, females are about 1/3 larger than males. (However, this only holds true if the birds are in the same population. See #2.)
#2 Animals living farther north are generally larger and heavier than those living farther south.

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!

However, 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 female. Understand?

Now, with that as some background, here are some sample eagle weights:

Body weights over 7 kg are common. For example, 5 eagles captured in Alaska recently weighed an average of 7.3 kg; the largest weighed 7.5 kg.
In contrast, some southern eagles have weighed in at as little as 3 kg--less than half the weight of some larger birds.
Generally speaking, here in NY, we find male eagle weighing in from about 3.4-5.0 kg, while females are found within the 4.0-7.0 kg range.

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.")

Journaling Questions

What other animal species can you find that shows reverse sexual dimorphism?
Female eagles found in NY generally are within the 4.0-7.0 kg range. Males are in the 3.4-5.0 kg range. What kinds of things in your classroom weigh about the same as these birds? (hint:Convert these numbers to pounds if that will be easier to imagine?) Brainstorm a list and then weigh on the scale in the nurses' office.