I'd like to submit this photo of a Mallard that I took just over one month ago for this round. When I came upon him in the woods, he was resting on a log in the middle of a pond while his mate swam nearby, occasionally diving to eat the vegetation growing in the shallow water. I caught this photo as he took a moment to stretch his leg while sunning himself.
Thanks to a complex system of arteries and veins known as a rete mirabile, some bird species, including Mallards, use their legs to regulate their body temperatures. The closely intertwined vessels in the rete mirabile work such that warm warm blood pumped from the heart to the duck's extremities transfers some of its heat to the relatively cooler blood returning to the heart through its veins.
Image source: https://www.scienceabc.com/nature/why-do-birds-feet-not-freeze-during-winter.html
This countercurrent heat exchange in the circulatory system allows the bird to retain much of the heat that would otherwise have been lost to the environment. This system is so efficient that birds like gulls and ducks can stand in near-freezing waters while maintaining body temperatures of around 40.0 Celsius.
By tucking one of his legs in close to his body, this Mallard was further cutting back on any heat loss as he took a moment to warm himself in the sun. Why he decided to stare me down as he struck this pose, on the other hand, is anyone's guess.
A few more photos from the day:
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