Context: Short-distance passerine migrants have two evolutionary origins. Those that have long-distance migrants in the same family, such as the common chiffchaff Phylloscopus collybita, are species of southern hemisphere origins that have progressively shortened their return migration to stay in the northern hemisphere.

Question: How many evolutionary origins do short distance passerine migrants have?
Answer: two

Question: Which species have long-distance migrants in the same family?
Answer: the common chiffchaff Phylloscopus collybita

Question: Why have some specied progressively shortened their return migration?
Answer: to stay in the northern hemisphere