Aelian says that when the singers would sing hymns to Apollo, the swans would join the chant in unison.
ˈeɪliən sˈɛz ðæt wɛn ðə sˈɪŋɚz wʊd sˈɪŋ hˈɪmz tʊ ɐpˈɑːloʊ, ðə swˈɑːnz wʊd dʒˈɔɪn ðə tʃˈænt ɪn jˈuːnɪsən.