Very sociable birds that love to hang out together while preforming a needed service of keeping things free of diseased carrion.
They migrate from north to south and back every winter. This is how they do it, in huge flocks like that. They will move on as soon as they are sure winter is over. We have them land for a few days like this... Hundreds of them migrating together. When they finally take off all at once to move on it looks like a black corkscrew tornado. Pretty cool to watch actually.