Kelp Gull at Costa del Este, a report by David Uit de Weerd & Lennaert Steen

Today David and I (two birders from the Netherlands) found a 3cy or 4cy Kelp Gull at Costa del Este in Panama City. The gull was on the mudflats between the bridge over the creek (end of
the river mouth) and the end of Costa del Este on the most western point of Costa del Este.
Description of the gull: Big gull with dark upperparts. Gull was nearly twice as big as the surrounding Laughing Gulls. (Also much bigger then the two present Ring-billed Gulls). The head of the gull was entirely white with some dark striping in the neck. The bill was also big/heavy with dark tip at the end. Base of the bill seems to be yellowish. (but distance was faraway, certainly the bill has two colours). On the breast the bird had some streaking.
The gull had whitish ends of the secondaries. We saw the bird one time from the back and had two patches of white in the secondaries. Colour of the legs not noticed. Too faraway and too muddy.