Following hot on the heels of recent Cuckoo migration discoveries, the BTO are beginning to unravel the mysteries of Swift migration through the use of minute geolocators. Some remarkable movements across the whole of Africa have been mapped including huge sea crossings, more here:

http://www.bto.org/sites/default/files/u49/BTO_299_16-17Swifts.pdf?dm_i=IG4,U0AQ,39HD1V,2GK0S,1