Unusual tail pattern in first winter common gull

This first winter (1CY) Common Gull was another bird that I photographed on 28 December at Nimmo's Pier, Galway. One of its pair of central tail feathers (T1) lacks the normal wide black subterminal bar. Instead this pattern is replaced by a much reduced area of pigmentation. The reason for this pattern is unknown but it is possible that this feather has been lost at some point and then regrown.