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I believe this poem is about Catullus thinking of the woman he is in love with playing with her favorite bird, a sparrow. He would like to ease her pains and any grief she may have by playing with her or something of the sort. I'm not sure, but it seems as though it is his desire to do so..he wants "her heavy passion (to) become quiet".
It has become clear, from poem 51 and this one, that love and passion are key things to Catullus. It seems his life revolves around love. This makes sense considering love is something we discussed that matters greatly in our world. Love always has been vital to all of mankind and will continue to be so forever.
Catullus is heavily fixated on this woman. The people, and apparently the animals, she makes encounters with are all he wants to be. He wants to be the man who gets to listen to her and be the bird she plays with to comfort her. More specifically, her love is what matters to him.