Can someone explain me why, please? I'm reading a book, The life of buzzards, because that argument interesting me and to improve my English. In a page there's this period: Adult buzzards in western Europe moult and replace their feathers each year, a process spread over 5-6 month period during the warm months from spring to early autumn. Although no specific studies have been made of British birds, one may assume that our birds differ little in timing from those breeding at similar latitudes in Southern Sweden that were studied by Magnus Sylven. In the second part of this period has been used the present perfect tense (no specific studies have been made of British birds), then the past simple ( that were studied by), and I don't understand the reason, because I would have used the present perfect in the second part too: … in Southern Sweden that have been studied by… Why? Giuseppe