TLDR: We won’t, but read on to learn why!
When will we be able to tell the sex of the eaglets? We get asked this question every year. While most of us make private guesses, we don’t make them official – in no small part because we’ve been wrong before! Keep in mind that age is a bigger factor than sex in weight gain and size early in nest life. Sexual dimorphism begins to appear in some variables after about 20 days of life, but sex-related size difference doesn’t really start showing up until eaglets are around 35 days of age. As of this post, D34 and D35 are 23 days old and D36 is 20 days old. We’re not talking about DN12 in this post because it has no siblings to compare it to.
April 28, 2020: D34, D36, and D35 seeking shade at N2B
If sex-linked size differences start showing up around 35 days, why don’t we announce a sex at 40 or 50 days? Camera angles can make eaglets and eaglet features appear larger or smaller than they really are, which makes it hard to judge sex remotely. We also tend to assume that any given clutch contains male and female siblings. Back in 2016, a lot of us thought that we had one eaglet of each sex at N2B. But when Brett captured, measured, and banded siblings D24 and D25, we learned that both were male! Same sex runs aren’t especially common, so our brains exaggerated small differences to produce data that fit our anticipated results: one male and one female eaglet. This is a great example of confirmation bias and one of the reasons we don’t assign sex without physical data.
When Brett captures an eagle, he weighs it and measures the tail length, tarsus width (lateral and frontal), beak depth, and hallux talon length. Both Bartolotti and Garcelon agree that the most reliable measurements for eagle sexing are beak depth and hallux talon length. However, more growth in each occurs post-fledge, and fledgling ‘Tweeners’ can yield ambiguous results. Brett cited ‘Four’ as an example of a bird he was able to measure twice. He identified her as a female when he banded her, although her beak depth was 33.7mm – a little shallow for a female eagle of her age. But Four’s beak depth measured 34.3mm when she was recovered nine months later, confirming that she was a female eagle. If sex can’t be determined by physical characteristics, researchers can collect DNA and have it tested for the absence or presence of the W-chromosome, or a specific genetic sequence related to the W-chromosome. Why not XY? You can learn more about the sex determination system of birds at our old blog spot: https://raptorresource.blogspot.com/2015/02/sex-determination-in-birds.html
Why do we use the term ‘sex’ instead of gender here? The term sex is used in the biological sciences and generally refers to the differences between animals that produce sperm (males) and animals that produce eggs (females). The term gender is used in the sociological sciences and generally refers to cultural conceptions surrounding the differences between male and female modalities of living. Since we are talking about biological differences in a scientific context, we use the term sex.
While we won’t announce an official sex unless it is confirmed by a biologist like Brett or a veterinarian like Dr. Johnson, watchers are welcome to guess based on their observations! Remember:
- Female bald eagles are larger and weigh more than male bald eagles.
- If we have both male and female eaglets in Decorah, size differences will become more pronounced after 35 days. Now would be a good time to start keeping notes. Do any of the eaglets appear to change size relative to their siblings around or after about the 35-day mark?
- While adult bald eagles appear to show a very slight differentiation in plumage (think of a male eagle’s snowy white head, or a female eagle’s ‘grey eyeshadow’ and ‘smoky eye’, plumage features are not useful in sexing juvenile bald eagles.
Did you know?
- Unlike most birds, female raptors or birds of prey are larger than male birds of prey. Despite many scientific studies, there’s still no consensus on why this disparity exists in birds of prey: https://www.audubon.org/news/most-female-raptors-are-bigger-and-stronger-males-why.
- Have you heard of the bird with four sexes? Read on to learn more about the white-throated sparrow – a bird that has two sexes and could be argued to have, in human terms, two genders. But in this case, gender correlates with eyebrow color (and supergenes), not sex! https://www.audubon.org/news/the-fascinating-and-complicated-sex-lives-white-throated-sparrows.
- We class reproductive cells into male/sperm or female/egg based on physical characteristics like size and motility. But some sexual reproducers produce gametes that look alike! https://en.wikipedia.org/wiki/Isogamy
- Followers were very fond of using the term “Diva” to describe D1 and D27, who were eventually identified as female eagles. The term was never applied to female eagle Four, or to any of the known males in the nest that I can find. Perhaps we’ll have to start polling watchers and comparing majority guesses with Brett’s data, assuming he enrolls any of our eagles in his study again.
Things that helped me learn and write about this topic
The Raptor Resource Project