I think the similarities in perspective between poems 2 and 51 is really remarkable, with the love triangle Catullus-Sappho('s poetry)-Lesbia in 51 mirroring the love triangle Catullus-Lesbia-the sparrow in 2 (If I read you correctly). So Catullus' desire to play with the sparrow as Lesbia does is also a desire to play with love and love poetry as Sappho does. It's plausible to me that poem 2 is a translation of a lost Sappho poem. I'm wondering now what other evidence might exist to support this assertion. If there were another poem by a Roman lyricist that also plausibly alludes to this lost poem of Sappho, for example. I don't have anything in mind, but I may come back to this.