as i understand it, much like bird colonies across the world, there is a hierachy among rooks and the higher up you are, the better your nesting position within the roost, this may not be the most impressive nest, but it's probably the safest one. the nest may have belonged to someone else before but will be subjected to extensive renovations every year. "how do they know which one is theirs" is one of those questions, how do swallows know the way from here to africa n back, how to turtles find that same beach every time they lay eggs, how do sheep know which lamb is theirs and penguins...well the mind boggles. :-)