Although a few gulls were seen earlier in the trip, most of the gulls and terns were seen on the Ebro Delta on the last day.
Yellow-Legged Gulls Larus micahellis are the typical large white gull of the western Mediterranean and Atlantic islands (these last are sometimes split as Atlantic Gull L. atlantis). Holiday makers usually just call them Herring Gulls, but these are a different species altogether. The old picture of a ring species around the northern hemisphere of Herring Gulls eventually intergrading with Lesser Black-Backed Gulls has been discarded and instead a patchwork of large gulls with distinct habitats, ranges, calls, and behaviour is found around the northern hemisphere. As with almost all gulls, the best place to find them is the nearest rubbish tip or any source of waste food.
By contrast, Audouins’ Gull Icthyaetus audouinii is much more of a traditionalist in matters of diet. A fish specialist, it avoids rubbish tips but is more than willing to hang around fishing trawlers for discards. As a result of this additional food source its population has expanded considerably in recent years to over 20,000 pairs in the Mediterranean, although more than half the population lives in a single colony around the Ebro Delta. In the winter it mostly leaves the Mediterranean for the North African coast. We found a few individuals on the last day at the Ebro Delta on the coast.
While Audouins’ Gulls are sea-going birds, although they prefer to fish along the coast rather than out to sea, Slender-billed Gulls Chroicocephalus genei are inland and coastal birds, visiting the coasts in winter but breeding around inland lakes and tidal lagoons around the Mediterranean and into central Asia. They are often found in saline lagoons in the company of Avocets and Greater Flamingos.
A true freshwater “seabird” the Whiskered Tern Chlidonias hybrida is one of the “marsh” terns, breeding in reedbeds and on islands in inland lakes, often in the company of Black-Headed Gulls. They have a much wider range than the other species in this post, reaching South Africa and across to Australia. It is more of a warm climate species, replaced further north by Black Tern and in Asia and China by White-winged Black Tern.
(images from Wikipedia)