This probable 3rd-Basic western Caspian
Gull (Larus cachinnans ponticus) was photographed
by Martin Reid at Milton Landfill near Cambridge, England on March
10, 1999. Dick Newell and I independantly picked out this bird
but had some reservations because of the shortish bill (lacking
any black subterminally) and apparent brownish marks on the underbody
- especially the "throat-patch". Note that female cachinnans
look very different than males - more like a Mew/Common Gull in
structure. Looking at the photos of the underwing and leading
edge of the upperwing, I wonder if this a retarded 3rd-basic or
even an advanced 2nd-basic? this would account for the sparse
underbody marks; the "throat-patch" was seen to have
disappeared a hour later - thus it was staining; maybe some of
the other marks are also stains? Dick studied the bird's behaviour
and warmed to it being Caspian Gull, as they can be picked out
among other large gulls by their actions: always doing something,
never standing still for long - instead wandering around among
the other resting gulls and pecking at things on the ground all
the time - especially at things on the bottom in shallow water;
also often aggressing to other gulls (usually dominant over all
forms except perhaps marinus). We finally agreed that this
was very probably a female Caspian Gull - it certainly did not
fit any other gull normally found at Milton, where they get fair
numbers of michahellis (usually nominate, but sometimes
probable Portuguese or atlantis) and a few cachinnans
every winter, in with lots of argenteus, graellsii,
marinus, and a scattering of argentatus and intermedius:
- aggressing at a dark 1B marinus
