It’s pretty much all tendons and bones. So, unlike your human foot, which contains plenty of moist muscle tissue, a bird’s foot contains only very little fluid in its cells.
It seems like they do keep their feet above freezing. Which is counter to the article’s: “Given a bird’s frantic little heart, blood simply rushes by way too fast to freeze.” As the blood never actually reaches freezing temperatures.
One article I briefly looked at indicates there isn't as much study of avian ...something ....as mammalian. Maybe fattiness because I was wondering if they have any adipose tissue in their feet. Oil is harder to freeze than water.
I also googled for info about saltiness and bird feet, which helped me find some article about "antifreeze" in various species but it barely mentions birds in passing:
Salt gets used to de-ice roads in winter because it changes the freezing point of water. But I am not readily finding anything that indicates we have found a chemical antifreeze mechanism in bird feet.
It may exist but not have been studied or looked for at all.
Edit: Though upon closer examination, the article about animal "antifreeze" talks about proteins that serve that function, which is a new one on me. So someone else would need to google up info on protein structures of bird feet and speculate if those could have antifreeze properties. I don't know that much about protein and its impact on physiological characteristics (other than, you know, it's pertinent to insulin resistance or something, which most diabetics seem unfamiliar with while we blame everything on fat and overlook the role of lack of muscle).
Essentially, DNA encodes Proteins which manipulate everything else. Human cells for example have on the order of 1 to 3 billion proteins, remove water and over half of what’s left is proteins. https://en.wikipedia.org/wiki/Protein
Yeah, I'm well aware that DNA encodes proteins which, when deployed properly, become nano machines.
I have a genetic disorder that is essentially a salt-wasting condition. It also results in my body significantly misprocessing fats and oils.
So I know something about salt and about fats and about how those interrelate with biology and how those can weirdly impact things.
Beyond being aware of how those nano machines get broken when protein misfolds happen at the cellular level, I haven't taken much interest in the chemical properties of protein. It wasn't essential to my survival.
So someone else can look up the proteins of bird feet and go crazy with that if they care to. It's not my niche.
It’s pretty much all tendons and bones. So, unlike your human foot, which contains plenty of moist muscle tissue, a bird’s foot contains only very little fluid in its cells.