I would imagine that the areas to avoid for bird nesting would be far bigger for army manoeuvres than for letterboxing or geocaching. The remote geocaches get barely 20 visits a year whereas just one army training day would cause more impact than 10 years geocaching.
I would think the biggest threat to bird nesting (and other creatures) is fire. That huge fire on the north moor a few weeks ago must have caused immense damage to all wild life and coming during the nesting season would have destroyed a very large number of ground nesting birds nests.
