Are these 'crabs' destroying my bird's nest?

[newreefer_59]

The pics are hard to make out, but I have just discovered these 2 crabs that are nesting and seem to be eating at my bird's nest. They are the size of a quarter I would say, and look more like a hairy spider than a crab. Don't know where they came from! Do they need to go...if so, can they be caught easy like?

Thanks!