I think its wild birds that build nests in sheltered places,notice robin and wrens carrying at the allotment,plenty places there to shelter tho a new pair of magpies have built in a tall leafless tree 6 yards behind the loft so now iv 2 pairs 1 infront 30yards and 1 behind. going to leave them because without tempting fate them things that take pigeons have left me alone(not saying the name...fatal).