In reply to toad:
Slightly more complex than that, as most things in life are. Dont work in England or Wales, but my understanding was that the "reckless" bit, at least in Scotland comes from the Nature Conservation(Scotland) Act 2004 ie. only applies in Scotland. I am not aware of any equivalent legislation in E&W, but happy to be corrected.
In Scotland its also abit more complicated than that statement. There are Schedule 1 and 1A species that have different level of protection than other species, and in terms of climbing, only eagles and peregrines are really the Scheduled species that climbers may come across. For them, definitely disturbing the birds "at, on, or near" the nest is an offence. For them and non-Schedule species it is an offence to interefere with the nest, kill, take birds or eggs, obstructing or preventing any wild bird from using its nest. Strictly speaking although not desirable, it is not an offence in law to disturb a non-Schedule species eevn if on a nest although if that gets to the point where it is preventing it from using its nest, then you are on dodgy ground. This latter point has never been defined through case law. Have a look at the Scottish link in the news article and there is a full explanation of the laws in Scotland. Im guessing the BMC probably have an extensive explanation on their website too for E&W.