No surprise that the one time I post about vitamin D and Callan responds! Birds have hollow bones, so presumably their bones could bend easier than ours (though they're also lighter, a turkey vulture maxes out at 32" tall and 5 lbs, while an average 32" person, or 2 year old, might weigh 25-30 lbs). And yes turkey vultures do preen (in the video they were the ones with their heads tucked into their wings). For more on the significance of vitamin d in birds: Avian Medicine and Surgery