Past research and future directions in understanding how birds use their sense of smell

Our understanding of the functional importance of olfaction to birds has improved over the past 60 years, largely as the result of experimental studies testing how birds use their sense of smell in different contexts. As it is impossible to measure directly which odours birds can detect, we rely on measuring behavioural responses to scent cues or manipulations which diminish their olfactory acuity. Here, we review the findings of these studies structured by when and how birds used their sense of smell: during foraging, navigation, social interactions with con- or hetero-specifics, nesting or for predator detection. Most studies investigated how birds use their olfactory sense during foraging (n = 52) and nesting (n = 42), but fewer examined how birds use olfaction during social interactions (n = 38), navigation (n = 20) or predator recognition (n = 21). Knowledge is mainly restricted to certain Orders (e.g. Procellariiformes). Although the diversity of neuroanatomical and molecular structures underlying olfaction suggests that olfactory acuity varies considerably among extant bird species, an understanding of which ecological factors drive selection for olfactory acuity in birds is mostly lacking. How experiences expand a bird's chemosensory knowledge over time and whether birds can learn to recognize odours associated with foraging opportunities or danger from predators is still poorly understood. Finally, there is a lack of knowledge about how long some volatiles remain useful to birds before they have decayed, and which volatiles, or combinations of volatiles, contained within some olfactory cues are being detected by birds. We encourage ornithologists to tackle these broader questions to better understand the functional importance of olfaction to birds.