Some years ago, in springtime, I was crewing on a yacht delivery from Caernarfon to Falmouth. Crossing St David's Bay (between Miford Haven and Lands End) as the sun was setting two swallows dived across the cockpit, through the open hatch into the cabin and disappeared. When I turned in a couple of hours later going off watch, the swallows had settled themselves in the forecabin, on my berth. I found somewhere else to kip so as not to disturb them.
They left at first light, having left me some souvenirs on my sleeping bag.
Apparently, this is not unusual - they hitch lifts on passing ships at bed-time too.