Sentence:  Though they breed in the High Arctic summer, Arctic terns 'winter' south in the austral summer, thus living a life of perpetual summer and almost endless daylight in the classic example of 'globe-spanning'. ... <WSD>Longhauling</WSD> 22,000 miles a year they are the only northern breeding species which migrates regularly to Antarctic waters. 
Meaning of the word marked with <WSD> tags:  To travel a long distance.