Chemical Communication in Songbirds
Abstract
Avian chemical communication has been understudied due to the misconception that olfaction is unimportant or even lacking in birds. Early work focused on the olfactory foraging capabilities of seabirds because of their ecology (open ocean foraging) and large olfactory bulbs. In contrast, olfaction in passerine birds, comprising over half of all extant avian taxa, was long overlooked due to their relatively small olfactory bulbs. It is now well established that passerines can smell, and their olfactory acuity is comparable to that of macrosmatic mammals such as rats. Much of our theory on communication and mate choice has involved studying visual and acoustic signals in birds, especially passerines. However, there is mounting evidence that chemical cues are a previously overlooked but important element of avian communication and mate choice. I used gas chromatography to explore sources of variation in song sparrow (Melospiza melodia) preen oil. I then performed behavioural experiments to test whether song sparrows are capable of discriminating among preen oil odour cues. Finally, I explored the hypothesis that major histocompatibility complex (MHC) genotype underlies variation in preen gland microbiota and that this contributes to variation in preen oil chemical composition, providing a potential mechanism for MHC-based mate assessment. Preen oil differed between birds experimentally infected with haemosporidian malaria parasites (Plasmodium sp.) and sham-inoculated controls; between populations, ages, sexes, and breeding versus postbreeding seasons; and with MHC genotype. Song sparrows used preen oil odour to discriminate between the sexes, and to discriminate the MHC similarity and diversity of potential mates. Preen gland microbes differed between populations and sexes, and covaried with MHC genotype but not with preen oil composition. Collectively, my thesis establishes that preen oil is information-rich and that birds use preen oil odour cues in ecologically relevant contexts. I provide some of the first evidence that pathogen exposure alters chemical cues in birds, that birds use odour cues to discriminate the MHC genotype of potential mates, and that MHC genotype is positively correlated with both preen gland microbes and preen oil chemical composition.