Scoter scoping

Monday morning - sea flat calm and those strange optical conditions where distant birds can seem magnified - scanning from Seton harbour was able to clearly see the seals on Craigielaw Point about 4 miles north-east (black dots in mobile phone photo above, only x27 zoom), the expanse of Gosford Bay nearly nearly devoid of birds save for the odd Razorbill and a female Sparrowhawk coming over the Forth from at least 2 miles out, a single imm Black-throated Diver initially straight out from the harbour but soon well west towards Inchkeith, and finally right round in the west 420+ scoter, apparently mainly Common Scoter, attended by many gulls on sea off Portobello beach about 5 miles west.

Postscript - 1330 scoter reported there later on Monday (IJA), so presumably the full bunch of Common Scoter resident off Port Seton since start of May. This causes me to wonder, further to recent Surf's up speculation, why do scoter in general apparently congregate off Musselburgh in May - one might have thought as weather becomes more settled they could go further out to sea, but clearly something drives them into more sheltered waters at this time of year - perhaps food supply? An old, but sufficient, map shows that sea depth is very similar in both locations, not exceeding 4 fathoms (c. 8m) to some distance offshore. Equally many Velvet Scoter fly west up the Forth at dusk, presumably to roost on calmer waters, though this is also baseless speculation!

Another afterthought - checking my own records for Gosford I have noted a Common Scoter peak late spring every year except 09 - c. 310 5/6/05, 410 4/6/06, 550 6/5/07, 360 24/5/08, 1070 2/5/10; another explanation for the apparent progress into the Forth might be more simple - these are mainly migrants coming up the east coast (trektellen graph shows a massive late April peak in this species at UK sites, dominated by Dungeness) and they simply settle in the Forth at the most accessible location, i.e. initially Gosford Bay. After spending some time there, they simply head further in, to Musselburgh which is a more natural end point to their incursion into the Forth, and remain there until their eventual departure to their far northern breeding grounds. Of course, many immatures may stay all summer, and given breeding males are known to arrive back by late June there is a very narrow window to determine whether they are truly gone (indeed this is completely masked by bird report totals, which report monthly maxima, any early June minimum thus being hidden by any early return). Worth keeping an eye on them to see if this can be detected.

A further look in local bird reports shows a consistent trend of high numbers for Gullane throughout the year with exception of May/June when consistently high numbers off Musselburgh; the inevitable conclusion that actually these are basically one and the same, notwithstanding being enhanced by a few migrants, so I'm back to square one and at a loss to explain the trend. Any comments?

Anyway, even if the migrants incursion had been a better theory for Common Scoter, it does not greatly help with the others, e.g. why does the Surfie behave the same way, progressing to Musselburgh, if indeed this is the Fife bird which has been resident in the Forth over winter? Plenty more to discover...