I thought I'd just throw in my two cents' worth, but here's what I think this is supposed to mean
From what I read, in Japan sparrows are symbols of prosperity and good luck, but they're also tied to a traditional fable called Shita-kiri Suzume (literally "tongue-cut sparrow").
In short, it's the story of an old man who saves an injured sparrow and takes cares of it. His wife can't stand the bird so she cuts its tongue and sends it flying away from their house. With the help of other sparrows, the old man eventually finds the bird. The sparrow is overjoyed to see his friend again and offers him 2 baskets to choose from as a reward. The old man chooses the smaller basket and finds a treasure inside it. His greedy wife hears about the treasure and goes after the larger basket, only to see that it's filled with poisonous snakes.
The moral of this story is that greed only leads to one's demise, but also that the purity of friendship overcomes evil.
And I think the latter fits Kazuma and Ryunosuke like a glove. You can see that line as Ryunosuke suggesting a symbol of good luck to Kazuma. However, considering that this dialogue takes place in 2-4 when he notices Kazuma's distant behaviour around him, I personally see it as Ryunosuke's witty yet roundabout way of telling Kazuma "I don't know why you're acting the way you do, but we're friends and I hope we can overcome this."