Possibly due to the small size of the olfactory bulb (OB) as compared to rodents, it was generally believed that songbirds lack a well-developed sense of smell. This belief was recently revised by several studies showing that various bird species, including passerines, use olfaction in many respects of life. During courtship and nest building, male European starlings (Sturnus vulgaris) incorporate aromatic herbs that are rich in volatile compounds (e.g., milfoil, Achillea millefolium) into the nests and they use olfactory cues to identify these plants. Interestingly, European starlings show seasonal differences in their ability to respond to odour cues: odour sensitivity peaks during nest-building in the spring, but is almost non-existent during the non-breeding season.
de Groof, G., Gwinner, H., Steiger, S., Kempenaers, B., & van der Linden, A. (2010). Neural correlates of behavioural olfactory sensitivity changes seasonally in european starlings. PLoS ONE, 5(12). https://doi.org/10.1371/journal.pone.0014337