I've seen several teeth that are purportedly from Cretaceous avian dinosaurs/birds, specifically Avisaurus sp. I've done some light digging, but I don't yet know how these teeth can be assigned to a bird. My thinking was that you need an identifiable jaw with these teeth, but I couldn't find any such literature on Avisaurus at least. My other guess is that they closely match the morphology of other Enantiornithine bird teeth? In any case, I'd be much obliged if someone could pass along some relevant paper(s).
These are examples of Avisaurus archibaldi posted by @Auspex: