Birds wintering in the North have to come up with a way to prevent tissue damage to their feet due to freezing.
Their solution is a specialized system of blood flow to their feet that goes through a "heat exchanger" on the way to the feet, and then again on the way back to the heart. The heat exchanger is located just below the thigh on each leg and is made of a mesh of small vessels that can take heat from the warm arterial blood flowing to the feet and use it to warm the cold venous blood that is returning.
An example of the temperature gradient looks like this: the blood heading down to the feet enters the heat exchanger at 95 degrees and by the time it circulates through the foot, the temperature has dropped to just a few degrees above freezing. The blood coming back up would shock the heart and kill the bird, so, as it goes through the heat exchanger it is rewarmed to 91 degrees by the 95 degree blood that is also circulation through the exchanger on its way down to the feet.
It's a great system for the winter, but one that could cause problems of overheating in the summer when the birds need to cool themselves.
So, on a hot summer day, the heat exchanger can be bypassed in order for the blood to be cooled as much as possible by using the feet as radiators - an ingenious solution.