I don't know where the 'bird watching' idea came from. I don't think it was in the original MoD documents. 'Poaching' and 'fishing' were both suggested early on. These activities would probably be illegal, as game and fishing rights are usually owned and enforced by the lairds in those parts. This might help explain the reluctance of the 'witnesses' to come forward at the time, though less so now, after 40 years.I don't think the bird watching is confirmed and there are other suggestions as to what they were doing and why they had a camera.
Also on bird watching, don't birds usually go to roost around sunset? In summer in Scotland it may stay light until long after sunset, but that's no use if there are no birds in sight!
There was some discussion about bias here in favour of a 'hoax' explanation. The two leading theories are 'hoax' and 'secret advanced tech'. David Clarke is now very gung-ho for 'secret tech' (see my note in another thread on his Fortean Times article), so I think a bias towards 'hoax' is a useful corrective. We know that (a) UFO hoaxes in general are very common, (b) several hoaxes similar to the Calvine photograph had been perpetrated before 1990, and (c) David Clarke himself previously quoted official support within the MoD for the hoax theory, so I think 'hoax' should be given a fair crack of the whip.