Crag Martins apart, southern Iberian hirundines do not winter in North Africa, nor even by and large in Iberia. They have to complete their annual post-breeding moult and this must require considerable energy resources over and above the day to day requirement. I am of the opinion that those seen in southern Iberia or the Maghreb in January are early returners that have completed their moult south of the Sahara. Much the same applies to Pallid Swifts. Crag Martins may be able to overwinter in southern Europe because they are larger than the other hirundines and so are probably less sensitive to cold snaps - they still descend to low level locations, principally along coasts, where temperatures are milder and aerial insects more available.