[NJ] Thanks. It was the last question I was most interested in. What, in the idealised bird-spotting world, is a sighting supposed to represent? I think you are saying that, ideally, one sighting should equal a particular individual bird -- so if two different individuals spot the same bird on two different days (assuming you could tell, somehow), then that would count as one sighting, not two. Is that right?
Basically, yes. But, of course these records relate to areas; so, if my hoopoe had flown 10 miles East then it might have then also entered the records in Nottinghamshire, even if a series of sightings along its path showed that it was almost certainly the same bird. It's certainly very common for one person to sight a rarity and the next day dozens of twitchers (not birders) will turn up because they want it on their personal tick list. However they wouldn't regard that as a new sighting. It's not clear-cut.