Task: Question Answering
Input (English): Passage: 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: Why have some specied progressively shortened their return migration?
Output (English): to stay in the northern hemisphere