those of mammals
... that, amongst the kidneys of the vertebrates, only those of mammals and birds can produce concentrated urine?
... that, amongst the kidneys of the <a href="./Vertebrate" id="mwBZE" rel="mw:WikiLink" title="Vertebrate">vertebrates</a>, only <b id="mwBZI"><a href="./Mammalian_kidney" id="mwBZM" rel="mw:WikiLink" title="Mammalian kidney">those of mammals</a></b> and birds can produce concentrated urine?