@Arella Mae,
Only the males have colorful tushes. And the color brightens up when the males get worked up.
I just read that the tushes may also be useful in keeping the group together as it makes its way through tall grass. Easy to spot a blue butt in the green grass.
When I first saw the video, I gasped. Why? Because the eagle didn't flap. It just spread its wings and took off. I wanted desperately for that bird to understand what it was doing. I wanted it to have a human consciousness. To know and understand that it was floating on air.