The barn swallow (Hirundo rustica) is one of the most widely distributed bird species. Its breeding range includes North America, Europe, Asia, northern Africa, the Middle East and Japan. Their characteristic nests, usually seen in clusters, are common sights on the outside walls of buildings in rural areas of Turkey.
I photographed these swallows and their nests in the town of Gömbe in southwestern Turkey last July. I am assuming the shorter tailed bird in the center is a female.
As you can see in the pictures, these nests appear to have been constructed from mud pellets.
More info on barn swallows is available on the Bird Web and the Animal Diversity Web.