Slender-billed Gulls (L. genei) 2cy, harbour of Manama, end of February 2001.

Images of three different birds. In this plumage, Slender-billed Gull may resemble 2cy spring Black-headed Gull L. ridibundus, but note especially the colour of the iris. Normally, some tertials (the lower tertials) are still juvenile, dark centred and very worn at the fringes. Upper tertials may be moulted to second generation plain grey feathers. Some coverts may have been included in the partial moult last autumn and appear all grey, the typical coloration of the second generation wing-coverts or later moulted feathers. Most birds moult median coverts and feathers in the row of lower lesser coverts. However, some wing-coverts are still juvenile, especially and most noticeable in the upper rows of the lesser coverts; they are still brown and extensively worn at the fringes these juvenile feathers. In most birds, the central and outer greater coverts are juvenile as well, but less typical as these greater coverts are plain grey right from the juvenile plumage onwards. However, have a look at the very worn greater coverts in the first image: the tips are largely worn away, exposing the brown centres of the secondaries underneath. 
The juvenile tail-feathers shows a broad band.
The ear-coverts show a brownish spot, but in some individuals this spot already disappeared.
Both legs and bill are coloured dull orange, somewhat yellow. Note the pale iris.