I want to know the answer to the following question:
Why have some specied progressively shortened their return migration?
All the background I'm provided with is that "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.". Is that enough to answer the question?
Answer: yes