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: కొన్ని జాతులు తమ తిరిగి వలసవెళ్ళడాన్ని ఎందుకు క్రమంగా తగ్గించాయి?
answers: to stay in the northern hemisphere