The role of preen oil and body odour in birds

Gilles M. <em>The role of preen oil and body odour in birds</em>. Bielefeld: Universität Bielefeld; 2024.

While the roles of auditory and visual cues in avian communication have been studied extensively, the importance of olfactory cues has been largely overlooked. During preening, birds extract a waxy secretion called preen oil from their uropygial gland (or preen gland) and smear it over their plumage. Preen oil is an important source of avian body odour and may have perfuming (or odour-related) roles, in addition to other roles like plumage maintenance and waterproofing. In this thesis, I explored the odour-based roles of preen oil, notably olfactory crypsis against predators and olfactory intraspecific communication, including mate choice and parent-offspring recognition. To do so, I first investigated what information (e.g. season, sex) is encoded in avian olfactory cues by analysing the variation in the chemical composition of preen oil. In a systematic review (Chapter 2), I found that almost all bird species studied exhibit seasonal changes in preen oil composition, whereas only half of bird species studied show sex differences. Why would seasonal changes be common yet sex differences vary between species? To answer this question, I conducted a comparative analysis on 59 species and showed that both seasonal and sex differences can be predicted by the nesting ecology of the species (uni- vs biparental incubation, ground vs non-ground nesting) (Chapter 2). These results suggest that preen oil odours could be used to increase olfactory crypsis at the nest, especially in ground-nesting species, but also as a sex semiochemical for olfactory mate choice. Because the role of preen oil odours is probably species-specific, I sampled and analysed the preen oil of two specific species, the Kentish plover (Anarhynchus alexandrinus) and the European pied flycatcher (Ficedula hypoleuca). In Kentish plovers, a ground-nesting shorebird with biparental incubation, I found no sex difference during incubation (Chapter 5), corroborating results from the comparative analysis. In pied flycatchers, a hole-nesting songbird with uniparental incubation, I also found no sex differences during breeding (Chapter 4), which was contrary to the predictions from the comparative analysis. Furthermore, in pied flycatchers, preen oil changed rapidly across breeding stages (between incubation and nestling-rearing) in females and changed across ontogeny (from nestling to adulthood) (Chapters 3 & 4). Interestingly, I did not find individual chemical signatures, but breeding pair and family signatures, which suggest that preen oil is influenced by the nest environment (Chapters 3 & 4). To make sure that results from such studies are robust and reliable, I conducted an almost exact replication (Chapter 4) of the study on the preen oil of pied flycatchers (Chapter 3). Importantly, part of the results were not reproducible (e.g. the subtle sex differences detected in the first study were not found in the replication), highlighting the value of replication studies, which are still very scarce in chemical ecology. Based on the results from my descriptive studies, I speculate that preen oil may have a role in olfactory crypsis in Kentish plovers and in olfactory mate choice in pied flycatchers, although this remains to be experimentally tested. A few studies have shown that birds can use olfactory cues for parent-offspring recognition, but these studies did not use solely preen oil odour. I performed behavioural trials to test if chicks of white-fronted plovers (Anarhynchus marginatus), which are precocial and can thus lose contact with their parents, can use preen oil odours to recognise their parents (Chapter 6). In a Y-maze, chicks showed no preference for the preen oil odour of parents or of unfamiliar adults. This result is inconclusive: it could be that chicks were not able to perceive the odours in the experimental setup, were not able to discriminate between these odours, or simply had no preference. In summary, I combined chemical analyses and behavioural experiments on multiple wild bird species to investigate the roles of preen oil and body odour in birds. Although I could not find any clear evidence for a specific role, my thesis provides valuable information and ideas to promote further study on the ecological significance of preen oil and body odour in birds. Notably, I recommend future studies to better assess the relative contribution of preen oil (and other sources) in whole-body odour, to measure nest odour rather than preen oil or body odour to study olfactory crypsis, and to test for olfactory parent-offspring recognition in colonially-nesting precocial species or in species with intraspecific brood parasitism. The importance of odours in the ecology of birds has long been neglected, but the field of avian chemical ecology is now growing rapidly and promises important discoveries.