In the last post I queried the ID of the Kumlien's Gull at Dernford. Why is it not a straightforward Iceland Gull given the complete lack of any spots or marks on the primaries?
The answer, so far, is that this is a returning bird, and in its juvenile plumage it had features which identify it as Kumlien's rather than Iceland. Although in its adult plumage it looks exactly like the nominate Iceland, its plumage when a juvenile marks it out as a Kumlien's.
This is a clear unsatisfactory position. We can no longer find a bird and identify it from the plumage we see at that time. We have to know the history of the plumage of that individual. This now leaves us in the position that a birder seeing an adult Iceland Gull drifting past on a sea watch cannot identify between Iceland and Kumlien's because they have no knowledge of that bird's history.
Furthermore, the decision to make the Dernford bird a Kumlien's on the basis of the juvenile plumage raises the obvious question; why is the juvenile plumage the one that determines the race classification? Why is it not the adult plumage that determines the race and we accept that individual juvenile plumages may vary?
Species is objective (nearly), but race is a matter of opinion. Two creatures are different species if they cannot mate and produce fertile offspring. Although gulls (why is it always gulls) seem to test this as I believe there are chains of gull species/sub species that can breed successfully with nearest neighbours but not with further species/sub species. But race? Humans have many 'races', but they can all interbreed successfully, and they have gradation of features not binary division of features, hence where one draws the line between race A and race B is a matter of opinion and conjecture.
And so we are back to Lumpers and Splitters. Splitters seek to divide a species into many races; /Isabelline/Daurian/Red-Tailed/Turkmenestan etc. Lumpers say there is one species, features may vary according to region, but differentiation is not sufficiently robust to assign all individuals conclusively to a given name. My personal inclination as you can probably guess is to favour the Lumpers.
I suppose my biggest bug-bear is Caspian Gull. No sooner had I learnt the essential criteria for this new race/species than people starting encountering 'hybrids' supposedly from colonies somewhere in central Europe that showed 'features of both'. What happens when the intermediates start breeding with pure birds and we get quarter-birds? It just becomes impossible to assign all individuals to a race, hence the notion of a 'race' becomes less useful.
But back to this gull at Dernford. It has to be the case that a birder seeing a bird can, on the basis of the features visible on that day, arrive at a species identification in the absence of the plumage history of that particular bird. And on that basis the bird we saw at Dernford has to be an Iceland Gull. Simple as that really.