How are two of these cranes staying warm?

Photo Joe Duff, Operation Migration

Cranes normally fly with legs outstretched behind them. What have two of these cranes done to keep their legs warm on this cold day at high altitude? They've pulled their legs up into their tummy feathers. (Do you see the aerial from the VHF transmitter they wear on their leg?) Their wings are doing all the work, so it's no problem. Operation Migration pilot Joe Duff took the photo from his ultralight plane during the young cranes' journey south. Which crane probably isn't as cold?