8th February

Still plenty of temptation to get out in the continuing lovely weather but no evidence of any changes afoot. A nice selection of winterers in the Church Ope Cove area included the first sighting of 2 Firecrests for a while along with the regular Black Redstart. The 11 Long-tailed Tits reappeared again at the Obs with the otherwise samey selection from elsewhere around the Bill including 11 more passing Red-throated Divers and the regular 30 or so Common Scoter offshore and 7 Turnstones, 2 Purple Sandpipers and single Black Redstart on the land.

And now back a couple of weeks to a rather gruesome discovery that's led us on an interesting little voyage of discovery. On 23rd January Pete and Debby Saunders came across this dead diver on the shore of Portland Harbour and gave us a call to see if we'd be interested in having a closer look at it, particularly because although they'd initially taken it to be a Great Northern Diver on close looks it seemed to have a quite pale bill with a marked gonys angle © Debby Saunders...

The paleness of the bill was perhaps an artefact created by the strong sunlight and the dark background as on close inspection it was actually steely-blue, as it should be on a Great Northern Diver. The bill shape was indeed striking but the features we thought were the ones to check both backed up an ID as Great Northern Diver: the culmen was dark (should be pale on a White-billed Diver)...

...and the primary shafts were dark (should be pale on a White-billed Diver)

These flight-shots - Great Northern on the left, White-billed on the right - show the real-world difference in primary shaft colouration (we hope Marlin Harms and Markus Varesvuo won't mind us lifting these portions of their photos off the web for our education)

Having got that far with our poking around at the bird we thought we may as well go the whole hog and check out what other in-hand differences there are. The best source of information relating to these seems to be David Burn and John Mather's 1974 clarification of the British status of White-billed Diver in BB from which we took the liberty of copying this diagram:
Our bird looked to have the extent of the feathering above the nostril - the maxillary feathering - spot on for Great Northern...

...but the extent of the gonys fusion - the groove on the underside of the bill - looked a lot more like it should be on a White-billed Diver; this puzzled us until we measured the distance from the end of the groove to the end of the lower mandible (32mm) and discovered that was bang in the middle of the range for Great Northern (25-36) and way outside the range for White-billed (42-58) - it seems the diagram in the BB article gives the impression the groove is longer than it is in reality.

The final feature to check out was the one that's perhaps the oddest of all: Great Northern has 20 tail feathers, whereas White-billed has only 18 - what's all that about and what's the evolutionary advantage to one having two more or two less feathers than the other? Weirdly, this turned out to be the trickiest feature to check. We're used to handling passerines in which the tail is a doddle to check (and besides, unless you were handling a White's Thrush or a Cetti's Warbler you wouldn't bother to count the number of feathers anyway), but there was a whole mass of vaguely similar length feathers at the back end of the diver and for a while we had difficulty working out which were tail feathers and which were various upper or under tail coverts. 

Eventually, the best approach turned out to be actually pulling out the individual feathers which not only revealed there were 20 (= Great Northern) but also showed it was actively moulting this tract - we nearly overlooked two of the feathers which were only just bursting out from their sheath; the longer, well-worn feathers are juvenile ones, whereas the darker feathers with white tips are the next generation, adult-plumage feathers © Martin Cade: