I think its buried in the olfactory nerves:
The olfactory nerve has been ascertained in the mammalia to be the instrument of smell; but in the class of birds, experiments and observations are wanting to determine its precise function, although analogy would lead us to suppose it to be the same in them. So inaccurate have observers been in this matter, that some of them have mistaken the large branch of the fifth pair, which traverses the nasal cavity, for the olfactory nerve. The experiments instituted upon Vultures shew that not only are they not led to their prey by the sense of smell, but also that they are not made sensible by it of the presence of food when in their immediate proximity. Yet, if the olfactory nerve be really the nerve of smell, and if a large expansion of the nasal membrane be indicative of an extension of the faculty, one would necessarily infer that Vultures must possess it in a high degree. On the other hand, however, the organ and the nerves being found to be equally developed in birds, such as Geese and Gallinaceous species, which have never been suspected of being guided by smell when searching for food, it would seem to follow that the precise function of this nerve, and the nasal cavities, has not yet been determined in birds. That the nasal passages must be subservient to some other purpose than that of respiration merely, is evident from their complexity, but what that purpose is, remains to be determined by accurate observations and experiments.
