With so much annual erosion of the cliffs here it is a wonder that the Sand Martins return from Africa to breed again in exactly the same place, as these nesting holes can penetrate up to 3' in depth this could also make the cliff unstable, as I took these photos last week [Mid April 2011] I would think that these holes are from a previous season, if that is the case then these are the only nesting holes existing between Benacre and Covehithe Broads [note the single Sand Martin in flight on the second photo]