Pallid Swift - illyricus??
First off..terrific shots Adam..I know how tough it is to 'nail' swifts in flight.
This is not an easy bird, especially since there's a psychological disposition to id. a late swift as a Pallid, so it's hard to break that mindset and look at them objectively when thoughts of "it's late, it's most likely a Pallid".
And, there's been enough pale swifts lately that have been educational (Filey and Shetland from several years back?) However, I can't dismiss this as a Common yet.
Common Swifts can and do show scaling on the underbody like a Pallid, but whether this is due to pale TIPS or pale bases is something I don't know - It would be nice for Ross to elucidate on his comment on feather bases and to confirm that both have scaling but it's formed by just the bases (Pallid) compared to the tips (Common)?
Birds I took in Spain in May (adult, spring...relevant??) show the scaling that is apparently made up of a dark base with a pale fringe?
Pallid's look bull-headed to me, with often a dark malar noticeable in the field formed by the pale throat and predominantly paler head sides. The throat is usually extensive and graduates into the breast, but juv. Swifts may show a somewhat similar throat pattern (aint seen any for a while!), but the darker malar is something that stands out to me, at least on spring Spanish Pallids.
Primary formula is very variable with both species showing P10 as the longest primary, AND the second longest, something I've noted on several of my images and not something that bothers me as an individual character.
Finally, the main concern for me is that we are all basing our id. on birds from the Med., since this is what we are familiar with. Although pekinensis is mentioned as a pitfall for Pallid, the eastern race of Pallid from Croatia, illyricus is very similar to Common Swift, being darker than pallidus and most likely really tough in the field. Could the Whitburn bird possibly be an illyricus Pallid? I don't see anything that wouldn't fit and Denmark's first record was a specimen taken in March. I direct you to the following illuminating paper
by Kasper Thorup.
http://www.zmuc.dk/VerWeb/staff/kthorup/Kt-pdf/DOFT95-169high.pdf
It is entirely reasonable to assume that late autumn birds may come from further east? and that if so, these Pallid's will be quite tough to id.
Be good to get some further input and a great educational thread.
JH