Perhaps the story that these nests are made with blood derives from well-intentioned nature lovers, in hopes that they might win a little sympathy for the birds. Or maybe it's a reflection of how people would feel if their hand-built home had been stolen twice without a trace so that they had to build it yet a third time.
血燕之說，也許是心存上天有好生之德者，希望如此能為燕子博取一些同情；也可能是人們想像自己辛苦經營的家，被三番兩次、連偷帶拆得無影無蹤時的反應。