I Think I Have The Answer

In this article on the BBC's website, "the RSPB said it was a mystery as to why red-throated divers had done so well.

Its numbers have risen from 935 to 1,255 breeding pairs in 12 years.

However, in Shetland the population has dropped from 700 pairs to 407."

So, what you're saying is, there's about 600 less birds in the Shetlands, and about 600 more birds in the Highlands? Yep, that's some mystery isn't it? I mean it's not as if they could have just flown from the Shetlands to the Highlands is it? Being birds. In Scotland.