It wont have been birds, probably as its the end ones it could be wind damage, they wont recover, could you not dig them out and plant a couple of nice bird friendly trees, something with berries on?
It's nothing to do with birds' nests. Unfortunately it's a virus of some sort which has appeared in the UK in the past few years and seems now to have spread everywhere. I've noticed that some varieties of conifer hedging seem to be more affected than others, but there's no really effective way of preventing it or dealing with its presence, so far as I know
After the nesting season, you might be able to deal with the brown/dead patches by removing them and tying the remaining branches across the resulting gap. That's what I'm going to try!