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Slender-billed Gulls are medium-sized white-headed gulls. They take two years to reach maturity, so there are three phases of plumage (the differences between breeding and eclipse plumage are small). Adult Slender-billed Gulls have a white head, with subtle grey markings around the eyes, neck and front. The front can have a pinkish tinge. The upperwings are light-grey, with black-tipped outer primaries, of which the outer 4 feathers are white. The outermost primary has a narrow black leading edge. The tail is white. The irises are grey. As their name suggests, they have an untypically slender bill. During the breeding season that bill is black. In eclipse the bill is blackish-tinged orange to dark-red. First-winter birds roughly resemble adults, but with a scalloped pattern on the wing coverts and with grey spots behing the ears. The bill is pinkish-red with a small black tip. The legs and feet are pale-orange. Juveniles resemble first-winter Slender-billed Gulls, but with a bolder scalloped pattern on the upperwings.

The overall distribution of this species can be assessed, and specific locations where birds have been spotted can be found, based on individual sighting reports submitted by birdwatchers to ebird.org .

The global distribution of the Slender-billed Gull is available HERE .

Slender-billed Gulls populate mostly the northern part of the western Indian Ocean, from the Indian coastline via Pakistan and Iran around the Persian Gulf and the Red Sea. There is also a population in the Mediterranean. Their movements are regionally migratory. In Europe they appear to be expanding their range northwards from the Mediterranean into central Europe.

In Europe Slender-billed Gulls are found along the northern Mediterranean coastline. They breed in a few selected (near-)coastal locations in southern Spain, the Cote d'Azure, Sardinia, near Venice and one location in Greece. There are larger breeding populations along the North coast of the Black Sea and the Caspian Sea. Some Slender-billed Gulls spend the winters along the southern Spanish coastline, in Sicily and along the western coastline of Greece. Recently they appear to be colonizing the European Atlantic coastline and the Baltic Sea.

Slender-billed Gulls breed on the edges of wetlands, not only in coastal areas, but also around inland lakes and marshes. Outside the breeding season they are found primarily in coastal, i.e. salt water, habitats, such as beaches and estuaries, including mudflats.

H. Dahlem-Senger reports spotting a single Slender-billed Gull on the German Baltic Sea coast at Fehmarn in April 2016.

D. Wilczynska spotted Slender-billed Gulls near Santa Giusta, western Sardinia, Italy, in June 2016.

J. Pires found a Slender-billed Gull at RN Sapal Castro Marim e VRSA-salinas Cerro do Bufo, Castro Marim, Faro, Portugal, in October 2019. Others were found in various locations in the Algarve, Portugal, in September/October 2020. Another was spotted at Marismas de Isla Cristina, Huelva, Andalucia, Spain, in October 2024.

We have previously found Slender-billed Gulls in Oman.