Once again, not a doctor/biologist/etc, but I’ll take a stab at this. I think I’ve got the hang of what is going on, but if I’m incorrect, feel free to let me know.

Body heat doesn’t just appear. As warm blooded animals, we get most of our body heat as a byproduct of metabolism. Hypothermia occurs when heat is lost faster than a body can produce new heat, as all the body systems require a certain temperature range to happen. Get too cold, and organs and bodily functions get really screwed up to the point the heart and lungs will stop working.

Heat travels from hot to cold to try to achieve equilibrium, so bodies lose heat through radiation if skin isn’t covered enough, through contact with colder matter, and convection from wind currents. You are always producing heat (if you are metabolizing!) but you are also always losing heat in a cold environment, or gaining heat if your environment is warmer than you are. Let’s focus on the cold side since that’s our original topic.

So let’s look at our graphic again. You’re on the Canada instance and seem to particpate in Canadian/EU topics rather than US, so I’ll assume you understand these values as degrees C. (For anyone else like myself, 37C is healthy human body temp, around 98 degrees F. The minimum temp in the graphic, 16C is about 60F. )

In the top circulatory system, the hot blood comes from the core at a happy and healthy standard body temp, but as that warm blood is pushed away from the core, it loses temperature to the environment. The blood eventually returns to the core to get reheated. The problem occurs the longer the exposure to cold lasts, as your body can only generate so much heat. It’s like running the heat in your house but leaving the front door and some windows open. You’re consuming more and more fuel and getting less and less efficiency. You can keep it up until your fuels runs out or you burn out your heating system. Same in the body.

In the bottom circulatory system, since the blood vessels are so close to each other, sometimes touching, they still lose heat to the environment, but since they are in close proximity to each other, the hotter part of the blood vessel is going to warm up the colder part as that heat radiates away, capturing some of it in the process. This seems to be where you’re getting lost if I’m understanding you correctly. So think if you and someone else are stuck outside in the cold together. You are both losing heat via the 3 pathways previously mentioned. Buuuuut, now you decide to cuddle up. What happens?

You both are still losing heat at the same rate as before, because you are both the same body temperature and your environment hasn’t gotten any warmer, but now due to your proximity, whatever parts of you are touching, now a good portion of the heat being pulled away from you is being radiated and conducted to the other person and vice versa where it is reabsorbed instead of draining off directly into the ether. Neither of you is generating additional heat, rather now you are both helping each other conserve heat that would have been lost, reducing you actual need to generate that heat. Neither of you are what you would call comfortable, but your ability to tolerate the temperature has increased because your circulatory systems have increased efficiency in retaining what heat you are producing.

Back to the graphic now, heat comes to the extremities at body temp, but due to the proximity of the hot parts to the colder parts, that heat differential is minimized, so the birds in this case are their own snuggle buddies, it’s just their blood vessels are what is doing the cuddling. And if they have feathered legs/feets, they are retaining even more of that heat, because that’s like if you’d cover you and your snuggle buddy under a big blanket. The body loses less heat, so the burden on the metabolism and circulatory system is lowered, and they can tolerate harsher conditions for a much longer time than we can without this type of circulatory system.

Does that make more sense?