Context: Reverse migration, where the genetic programming of young birds fails to work properly, can lead to rarities turning up as vagrants thousands of kilometres out of range.

Question: What is it called when the genetic programing of young birds fail?
Answer: Reverse migration

Question: What is reverse migration?
Answer: where the genetic programming of young birds fails to work properly

Question: What does reverse migration lead to?
Answer: rarities turning up as vagrants thousands of kilometres out of range

Question: How far out of range can rarities get?
Answer: thousands of kilometres