It has been a very good autumn for divers. We always get lots of red-throated divers. You can count a hundred passing Fife Ness in a few hours on good days. This year there have been a lot of black-throated and great northern divers too. Some of it has been because there are more northerly and westerly wintering birds in the North Sea this winter. But some of it is because there has been more sea watching than usual in response to the poor showing of landbird migrants. It takes a bit of experience to reliably identify divers as they fly by at sea and I have been trying to work out the best ways to communicate what the differences are between the species to shortcut the process.
When we look at a familiar bird and identify it, we integrate a lot of features unconsciously and the name of the bird pops up in our brain. Explaining how that has happened to someone without the experience is difficult because we haven’t consciously run through a checklist. Think of the question “why is that a blackbird?” The answer can’t really be “because it looks like one”, but for most of us we have a picture in our head of what a blackbird looks like and we have pattern matched. But the key to solid identification and being able to communicate why something is a blackbird is the ability to run through a list of defining characters. Good birders should be able to articulate this list for everything they identify. To reconstruct the bird character by character in words until the unique combination adds up to the identification for everyone. So that a black bill, eyering, sooty black uniform plumage everywhere except on the wings where it is a bit browner can only equal a young male blackbird.
So what about divers? I was looking at some excellent photos of black-throated divers from John Anderson yesterday trying to articulate why they look so distinctive. I just couldn’t work it out – something to do with the relative length of the front part of the body and the back, but what? I laid out some of John’s similar angle photos of the three common diver species side by side and scaled them to their average size. Then I measured the length of their head and neck relative to their tail and feet expecting them to be very different. But the relative lengths are the same for all three species. I kept looking at the photos until it dawned on me – obvious when you see it – it is all to do with the length of the front of the body and the back of the body relative to the width of the base of the wings. Black-throated divers have both extended necks and tails and feet so they are long and skinny both sides of the wings. In the field guides this is conveyed as black-throats looking evenly balanced, with the wings in the middle of the body, but my comparisons (see below) show that isn’t the case. It’s just because black-throats are longer either side of their wings so they appear more balanced. This is then made more pronounced by the equality in size of the feet and the head, bulging in a similar way all along their length back and front: much less of a bulge for the head as you find in red-throats and much less of a bulge for the feet as you find in great northerns. This might seem very esoteric. But this is what makes birding constantly interesting and challenging to me. For every set of characters I might learn to nail a species’ identification at distance, in brief and poor views, there are hundreds (thousands on a global scale) more to learn. And occasionally to puzzle out yourself, which is really satisfying.
Wild Crail 
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