Why the Bald Eagle has no subspecies

There are two recognized subspecies of Bald Eagle, the Northern Bald Eagle (Haliaeetus leucocephalus washingtoniensis) and the Southern Bald Eagle (H. l. leucocephalus). As many might know, I'm really into subspecies and I try to identify them whenever possible. But the taxonomy is not perfect and there's simply subspecies that do not exist. One of these is the Bald Eagle.

The problem stems from the fact that even the authors who do recognize the two populations only tentatively believe they exist. The two subspecies are differentiated purely by size and wing chord, with the largest birds, living in Alaska, having about a 244 cm wing chord, and the smallest birds in Florida, with a 168 cm wing chord. However, take these measurements with a grain of salt since there's quite a bit of sexual size dimorphism, with females possibly being 25% larger than the male. Now if you don't use the metric system, these measurements translate into 66-96 inches.

Now you might say, 30 inches is quite a size difference, surely you can identify a size difference between an eagle with a 5.5-foot wingspread from one with an 8 foot. Let's do some math here: If 66 inches is the smallest male leucoephalus and females can be as much as 25% larger, then we can see a female as large as 82 inches, which means the southern population can cover most of the size variation. Flip the coin and assume 96-inch eagles are the largest female washingtoniensis, then the smallest male can be 72 inches. That gives us quite a bit of size overlap.

Now that we've identified the root of the problem, how can you be sure that the "female" leucocephalus you're looking at is not a male washingtoniensis. And we haven't even touched clinal variation yet. We have evidence that the mass and wing chord of Bald Eagles gradually increases with the latitude. That means eagles nesting somewhere like Montana are going to be smaller than Alaskan eagles but larger than ones in Texas.

That makes making a distribution map quite difficult, because if size is gradually and consistently increasing throughout the country, then it's very difficult to draw a line and say, "eagles north of this line are washingtoniensis and those south are leucocephalus." In fact, that line I am speaking about is literally the 40th parallel. Using that logic, and using that invisible line to differentiate Bald Eagle subspecies, eagles you see in Nebraska are washingtoniensis, while those in Kansas are leucocephalus, even if there are two nests, only a few hundred yards away, separated only by a latitude and state line.

With the combination of no morphological differences, sexual size dimorphism, geographical clinal size variation, and the lack of a true distribution range all point to one thing. The Bald Eagle has no subspecies! This is why I am discouraging the use of such ids, because there is absolutely no way someone can photograph an eagle and claim it to be the northern subspecies, let alone expect an identifier to correctly confirm that sighting. That is my two cents worth.

Posted on April 17, 2022 11:57 PM by birdwhisperer birdwhisperer


Interesting and thought-provoking post. Thanks for sharing!

Posted by aprildmoore over 1 year ago

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