I've shown ravens allopreening on the blog before — but in those cases it's been the female preening the male (e.g., see post from 13 February 2012). And most of the time it has involved preening of the head or throat feathers. Yesterday I watched a male preening a female (males are larger than females):
She landed nearby and solicited preening by tilting her head to the side:
The male preened the side of her head first:
And then spent some time running his bill through the feathers on her left wing:
On 15 March I saw a raven (perhaps one of this pair) gathering nesting material along the bluff. I don't know where they're building a nest, but in case you're wondering what one looks like, here's an older picture of a raven's nest from 2006:
It's an impressive structure! I read that the base of a raven's nest (the portion below the bowl where the softer material is) might be ~60 cm (24 inches) high, but I think this one is even higher.