The taking of Pipefish in Lothian is very common place, particuarly on the North Sea coast were it seems at times just about every bird has a Pipefish protruding from it's mouth, I have yet to see a yellow legged Herring Gull on that part of the coast however.
Agreed, it must be a local/regional thing. Harbour gulls probably have plenty of other options.
Regarding argentatus arrival times in Lothian, I have looked at Gulls at Musselburgh for at least 6 years now and have noticed that the really dark northern argentatus seem to arrive from late Dec-Jan, that is the really noticable ones, however clearly a movement of these birds non the less as they wern't present the previous weekend or so.
How many of the less noticable ones go unoticed? As I am sure you are aware that many argentatus have a mantle tone not too disimilar to an argenteus.
I tried to work that out at Seafield and Musselburgh one winter by focusing on their wing tip pattern and mantle tone in field conditions. On one or two days at least 10 birds could be picked out, however what if these flocks could all be captured and examined in the hand, then I would be sure that this general total would at least double!
Absolutely. When we say argentatus, or the bird report quotes numbers, I think it's assumed that these are just "obvious" argentatus, i.e. those which are outside the overlap range with argenteus. Many others must also be present and effectively invisible.
As you say you need to look at wing tip and mantle for classic individuals (also head streaking, the two in the harbour ystdy were nearly solid brown on the nape) but there are also light-mantled birds with the northern primary tip pattern, I recall one at Drem a couple of years back which was on the extreme of white in tips but mantle not noticeably dark.
You probably know it, but the essential reading on this subject is the Coulson et al paper, Scandinavian Herring Gulls wintering in Britain, fortunately available online. A bit out of date (1978-1983 study), but still lots of interesting stuff in there from a S Scotland perspective, from the arrival dates, nearly all Nov-Jan and rapid dispersal thereafter, real proportions based on careful examination of captured birds (13000 caught & rung) - 30% argentatus in mid-winter at main NE England study sites, but very few in the west and strong regional variation up the coast, much less in Scotland, and most being adult females (thus the imm argentatus may be less of a problem), and the measured correlation btwn "thayeri" primary tip pattern and mantle shade. This was generally a strong correlation, i.e. you tend to find the two features together, but there was a certain proportion had the northern wing tip but relatively pale mantle typical of argenteus, cf. above comments, and vice versa. Also, in 13k caught not one had yellow legs - may not be the same if repeated now!
Incidentally, I found 4 of these birds at Banchory btwn 1990 & 1993, i.e. those colour-ringed in their study, not sure how many were seen in Lothian.
Although I agree that a very good description is needed for YLG in Scotland as hybrids and dark argentatus do occur, I also feel that the SBRC insistance of getting the fabled all black bar on p5 rather than looking at really good photo's has put the documented spread of this species in Scotland back by at least ten years!
Agreed it is useful to know about birds which in all probability are YLG, as well as a formal record of cast iron proven ones. The bird information services archives do provide this feature to some extent (if you subscribe, they are searchable), but their databases include some dodgy reports too, one I know of for a fact at Seton!
Keep it coming as clearly this is also one of my favourite subjects, although I dont know what others might think
OK, feeling more comfortable to do so on a forum, which those who see them as avian rats can safely ignore; have probably posted too much gull stuff to LBN over the years!