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Birds have two legs but some species commonly stand on just one leg, usually to rest or warm the other one.  

Recently, when an immature Eagle flew to an Elm tree, it perched and then raised its right leg and tucked the foot under the lower breast feathers for a time.  At that moment, the air temperature was hovering near zero. 

The Eagle had no problem repositioning itself on the small limb and leaned a bit to the left to adjust its center of gravity and balance, as Canada Geese are seen doing while standing on ice.

Most bird legs have little feathering to protect the underlying tissues from cold.  One interesting way some birds also conserve heat is having their arteries and veins close to one another.  This cools the arterial blood flowing down the leg, which heats the blood in the vein moving up the leg.  

Overall, this makes the temperature in the legs somewhat lower than the bird's body temperature and they lose less heat to the environment on cold days.  Every little bit helps.

An Eagle's average body temperature is 106 degrees.