I don't know the answer to this question, but have always thought it most unlikely. However, in the past week Juan Ruiz of the Madrid Gull Team has reported both Lesser Black-backed Gulls Black 1AS9 and Black 2AS0 at Pinto Landfill, Madrid. Often I do not know which gulls are siblings because they are running around in bracken away from the nests when we ring them, but with these two gulls there is a very high likelihood that they are from the same nest. Both were ringed on Burhou, Alderney on 14 July 2012. Is it pure coincidence that they are both now at the same landfill in central Spain in the same week, or have they travelled south together? Hopefully over the months ahead we may get more reports of these gulls and be able to shed a little more light on this little mystery of migration.
LBBGs Black 1AS9 and 2AS0 on Burhou - July 2012 (c) PKV