A bald eagle (Haliaeetus leucocephalus) takes flight during an anomalous warm spell a few days ago. The flight sequence above is a composite of three images stitched together. After the momentary relief from the cold and a temporary re-emergence of insects, today we are back to the snowstorms…
Before living in Missouri, I had only seen bald eagles on occasion, such as a pair circling over the Boston Harbor Islands in MA or at extremely great heights that I could barely make out the white specks of their heads. This post includes an assortment of photos of these spectacular birds, together with some ramblings of the encounters. Because eagles are among my favorite birds, I will keep updating this post with new photos as I continue living in Columbia for the rest of my PhD. So stay tuned!
All eagles photographed in situ  and around Columbia, MO, unless otherwise stated
As I followed a northern harrier (Circus hudsonius), this eagle swooped over my head from behind. At close range, the eagle’s sheer size was staggering to witness— surely my closest encounter with a wild bald eagle to date. At the moment, the cold front is keeping the temperature down to the single digits, and today it will become subzero. It’s incredible that in the face of ice and wind, these birds of prey can still somehow circulate their blood properly. While I could barely manage to keep my hands functional enough to click the camera shutter, eagles can just leave their featherless feet out in open air.
One internal mechanism that many birds (but not all!) use to prevent frostbite is through the use of a venae comitantes system. In short, one or two large arteries carrying warm blood away from the heart lie adjacent to numerous veins carrying cool blood flowing in the opposite direction in the legs. Through direct contact between the two structures, blood from both counterparts is thermally equilibrated, causing the blood that reaches and circulates within the extremities to be cooler. A more commonly used term for this process is counter-current heat exchange. But isn’t this counterintuitive if the feet are to be kept warm? It turns out that supplying the extremities with excessive heat increases heat loss through those structures, putting the bird at risk of being unable to maintain a warm enough core temperature for survival. Through supplying arterial blood of a more tepid temperature, heat exchange between the feet and the cold air is reduced, while the feet are granted only an adequate amount of warmth to prevent frostbite.
This solution is all the more incredible when thinking that birds must also use their extremities to dissipate heat in warmer weather. Within the feet lie extensive connections between arteries and veins (called arteriovenous anastomoses) and a network of superficial veins that are responsible for cooling blood before it flows back to the heart. If a bird is observed with an infrared camera, in cold environmental conditions the center of the body will be warmer, while in hot conditions the face and feet will be warmest— very much the same as in humans and endotherms broadly. So, for birds tolerant of extreme cold, both competing mechanisms exist to allow for thermoregulation, depending on the external environment. Conversely, when a larger number of arteries are present within the legs (interconnected with veins in what is called a rete tibiotarsale), it provides more pathways for blood to bypass the counter-current exchange system and allow warmer blood to reach the extremities. Accordingly, it has been proposed that birds with a rete tibiotarsale may be more efficient in heat dissipation than conservation. However, upon reading more papers about these concepts I became increasingly confused, and it appears that rete tibiotarsale has likewise been proposed to function in cooling arterial blood before it reaches the extremities in cold-tolerant birds like penguins, just like the the venae comitantes system. It is likely that cold-tolerance can be achieved via both mechanisms and whether one system is more efficient than the other remains up for debate.
After watching an osprey (Pandion haliaetus) dive for fish unsuccessfully and spotting a pair of crested caracaras (Caracara cheriway) in the distance, I was thrilled to finally find my first bald eagle in my hometown, Temple, TX with my mom. You would think that growing up there, I would have seen them occasionally, but to the best of my recollection I’ve only observed them outside of the city limits. Just as the sun was setting, the bird was heavily backlit and difficult to see clearly, but its flight was unmistakeable. A brief loop-de-loop and the eagle set off for its nightly perch.
A bald eagle carries a fish back to its nest, where its mate awaits. Although no eggs or chicks reside there during the winter, the two eagles sometimes favor their inactive nest as a resting spot in between bouts of hunting. Usually I wouldn’t even have a guess for what the fish species might be, but in this case the open mouth is a giveaway— definitely a sucker, and possibly a white sucker (Catostomus commersonii).