Don't migratory birds migrate to AVOID the harsh winter cold (amongst other things)? If so, what the $#%$# are they doing climbing to an altitude where the temperature is around -45 degrees C??
Why would they bother expelling so much energy to climb so high? Soaring thermals associated with ground heating wouldn't get them that high. They'd have to make the effort themselves.
And don't birds suffer hypoxia??
With such little heads at the end of a long neck such as a duck or goose, with little feather insulation, wouldn't their brains just freeze solid at -45 degrees C??
I can believe that some birds might fly too close to the bottom of a very active thunderstorm and find themselve spat out the top. In fact I remember an article in an aviation magazine about planes striking snakes, mice, even fish that were probably sucked up via dust devils, tornadoes and water spouts. However I just find it hard to believe a bird would intentionally climb so high. And there would be that much super cooled water inside a thunderstorm, they'd be covered in ice and dead in no time.