This is the weekly poem at the Oxford Brookes University Poetry Centre website , and it also appears in Sidekick Books' 2012 anthology Birdbook II: Freshwater Habitats.
The website's an excellent resource, and the four Birdbooks are packed full of brilliant poetry and illustrations, whether or not you have an interest in birds.
As for Red-throated Divers, well, this is a good time of year to see them offshore all around the UK, and diverse divers (Black-throated and Great Northern too) turn up inland, too (there are two or three Great Northerns at Draycote Water, near home, at the moment). But the best time to see any of them is in spring and summer, when they're in their glorious breeding plumage. In the UK, you'd need to be in Scotland to find any.
The pictures above were taken in Iceland. Apologies for the generally poor quality, but both were taken close to midnight at the end of the longest day of birdwatching I've ever experienced. The one below, of Great Northern Divers, was probably on the same day.