Hmmmmm, if it's fledged already, it should be able to forage on it's own.
I know the adults love thistle seeds (especially in late summer) but I'm not sure about the taste cravings of a Juvi
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Even though they are a more seed than insect based diet, I'd still try offering some small mealworms & tiny crickets as well.
If all else fails, then you could try one of the commercial formulas for unweaned baby pet birds