Calidris alba (Sanderling)
Discussion
I found this feather in the Ebro Delta (Spain) during last week. Next to it I saw some Arenaria interpres, although it might belong to another water bird.

Sergio
In my opinion, it doesn't look like the Haematopus ostralegus feathers shown here.
I think the pattern is more similar to the Charadrius hiaticula feathers, although, as Belen said, size might not fit.
However, what do you think about Arenaria interpres? The bird was exactly there when I found the feather and the pictures in this web look similar.

Belen
Im not sure,the feather you found its too big for the arenaria interpres,in the data base if you search the bird and size the feathers it is 5cm,only the big feathers are 7-11cm
(I dont speak english if u found mistakes in my comment sorry)