The Impact of Avian Haemosporidian Infection on Feather Quality and Feather Growth Rate of Migratory Passerines.

Bird feathers have several functions, including flight, insulation, communication, and camouflage. Since feathers degrade over time, birds need to moult regularly to maintain these functions. However, environmental factors like food scarcity, stress, and parasite infections can affect feather quality and moult speed. This study examined the impact of avian haemosporidian infection and uropygial gland volume, as well as feather quality and feather growth rate in two migratory hirundine species captured in southwestern Spain-the house martin (Delichon urbicum) and sand martin (Riparia riparia). Our findings showed that the prevalence of infection varied among species, with house martins having the highest rates, possibly due to their larger colony size. Moreover, haemosporidian infection had a different impact on each species; infected house martins exhibited lower feather quality than healthy individuals, although this outcome was not observed in sand martins. Furthermore, no effect of infection on feather growth rate was observed in both hirundinids. Additionally, feather growth rate only correlated positively with feather quality in house martins. Finally, no link was observed between uropygial gland volume and feather quality or feather growth rate in any of the species in this study. These findings highlight the effect of haemosporidian infections on the plumage of migratory birds, marking, for the first time, how avian haemosporidian infection is shown to adversely impact feather quality. Even so, further research is needed to explore these relationships more deeply.

Morphometric, macroscopic and microscopic investigation of glandulae uropygiale in turkeys (Meleagris gallopavo).

This study was carried out to investigate the macroanatomical, morphometric, histological, and electron microscopic characteristics of the uropygial gland in adult male and female turkeys (Meleagris gallapovo).

No sex difference in preen oil chemical composition during incubation in Kentish plovers.

Preen oil, the secretion from the uropygial gland of birds, may have a specific function in incubation. Consistent with this, during incubation, the chemical composition of preen oil is more likely to differ between sexes in species where only one sex incubates than in species where both sexes incubate. In this study, we tested the generality of this apparent difference, by investigating sex differences in the preen oil composition of a shorebird species, the Kentish plover (Anarhynchus, formerly Charadrius, alexandrinus). As both sexes incubate in this species, we predicted the absence of sex differences in preen oil composition during incubation. In the field, we sampled preen oil from nine females and 11 males during incubation, which we analysed with gas chromatography-mass spectrometry (GC-MS). Consistent with predictions, we found no sex difference in preen oil composition, neither in beta diversity (Bray-Curtis dissimilarities) nor in alpha diversity (Shannon index and number of substances). Based on these results, we cannot conclude whether preen oil has a function during incubation in Kentish plovers. Still, we discuss hypothetical roles, such as olfactory crypsis, protection against ectoparasites or olfactory intraspecific communication, which remain to be tested.

Volatile organic compounds in preen oil and feathers - a review.

For a long time birds were assumed to be anosmic or at best microsmatic, with olfaction a poorly understood and seldom investigated part of avian physiology. The full viability of avian olfaction was first discovered through its functions in navigation and foraging. Subsequently, researchers have investigated the role of olfaction in different social and non-social contexts, including reproduction, kin recognition, predator avoidance, navigation and foraging. In parallel to the recognition of the importance of olfaction for avian social behaviour, there have been advances in the techniques and methods available for the sampling and analysis of trace volatiles and odourants, leading to insights into the chemistry underlying chemical communication in birds. This review provides (i) an overview of the current state of knowledge regarding the volatile chemical composition of preen oil and feathers, its phylogenetic coverage, chemical signatures and their potential functions, and (ii) a discussion of current methods used for the isolation and detection of volatiles. Finally, lines for future research are proposed.

Host space, not energy or symbiont size, constrains feather mite abundance across passerine bird species.

Comprehending symbiont abundance among host species is a major ecological endeavour, and the metabolic theory of ecology has been proposed to understand what constrains symbiont populations. We parameterized metabolic theory equations to investigate how bird species' body size and the body size of their feather mites relate to mite abundance according to four potential energy (uropygial gland size) and space constraints (wing area, total length of barbs and number of feather barbs). Predictions were compared with the empirical scaling of feather mite abundance across 106 passerine bird species (26,604 individual birds sampled), using phylogenetic modelling and quantile regression. Feather mite abundance was strongly constrained by host space (number of feather barbs) but not by energy. Moreover, feather mite species' body size was unrelated to the body size of their host species. We discuss the implications of our results for our understanding of the bird-feather mite system and for symbiont abundance in general.

Effects of genotype, sex, and feed restriction on the biochemical composition of chicken preen gland secretions and their implications for commercial poultry production.

Preen gland secretions spread on the feathers contain various chemical compounds dominated by fatty acids (FAs) and volatile organic compounds (VOCs). These chemicals may significantly affect plumage condition, microbial and ectoparasitic load on feathers, and chemical communication of birds. However, how chemical composition of preen secretions varies in commercially produced chickens with respect to their genotype, sex, and feeding regime remain largely unknown, as well as the welfare implications for farmed poultry. We found that while polyunsaturated fatty acids in chicken preen secretions differed significantly with genotype (P << 0.001), saturated fatty acids and monounsaturated fatty acids varied with genotype-dependent preen gland volume (P < 0.01). Chickens of meat-type fast-growing Ross 308 genotype had reduced preen gland volume and lower proportions of all FA categories in their preen secretions compared with dual-purpose slow-growing ISA Dual chickens. A total of 34 FAs and 77 VOCs with tens of unique FAs were detected in preen secretions of both genotypes. While differences in the relative proportion of 6 of the 10 most dominant VOCs in chicken preen gland secretions were related to genotype (P < 0.001), only 1 of the 10 most dominant VOCs showed a sex effect (P < 0.01), and only 2 of the 10 most dominant VOCs showed a genotype-dependent effect of feed restriction (P < 0.05). Feed restriction had no effect on the relative proportion of any of the FAs in chicken preen gland secretions. Moreover, we found that meat-type Ross 308 preen secretions were dominated by VOCs, which are proven attractants for poultry red mite and may also increase infestation with other ectoparasites and negatively influence overall odor-mediated intraspecific communication and welfare. This study shows that no feeding management, but long-term genetic selection in commercial breeding may be the main cause of the differences in the biochemistry and function of chicken preen secretions. This might have negative consequences for chemosignaling, antiparasitic, and antimicrobial potential of preen secretions and can lead to increased susceptibility to ectoparasites, plumage care disorders, and can affect the overall condition, welfare, and productivity of commercially bred chickens. Selection-induced preen gland impairments must therefore be considered and compensated by proper management of the chicken farm and increased care about animal well-being.

The preen gland is the largest sebaceous gland in birds, which produces a secretion that is spread on the feathers during comfort behavior. The secretion of the preen gland contains various chemical compounds that are responsible for mechanical, antimicrobial, and antiparasitic protection of the plumage and probably also for chemical communication between birds. However, there are only a limited number of studies on the composition and function of preen secretions in wild birds and only limited evidence in poultry. In this study, we compared the chemical composition of preen secretions in fast-growing meat-type and slow-growing dual-purpose chickens and evaluated the effect of sex, body condition, and feeding regime on preen secretion composition. Fast-growing meat-type chickens had smaller preen glands and lower proportions of all analyzed compounds in preen secretions compared to slow-growing dual-purpose chickens. In addition, compounds that are proven attractants for a poultry-threatening ectoparasite, poultry red mite, were predominant in the secretions of meat-type chickens. This study is the first to show that genetically distinct breeds of chickens can differ significantly in the biochemistry of preen secretions, which can influence susceptibility to ectoparasites, plumage care disorders, and can affect the overall condition, well-being, and productivity of commercially raised chickens.

Microbial infection risk predicts antimicrobial potential of avian symbionts.

Symbiotic bacteria on animal hosts can prevent pathogenic bacterial infections by several mechanisms. Among them, symbiotic bacteria can indirectly enhance host's immune responses or, directly, produce antimicrobial substances against pathogens. Due to differences in life-style, different host species are under different risks of microbial infections. Consequently, if symbiotic bacteria are somewhat selected by genetically determined host characteristics, we would expect the antimicrobial properties of bacterial symbionts to vary among host species and to be distributed according to risk of infection. Here we have tested this hypothesis by measuring the antimicrobial ability of the bacterial strains isolated from the uropygial-gland skin of 19 bird species differing in nesting habits, and, therefore, in risk of microbial infection. In accordance with our predictions, intensity and range of antimicrobial effects against the indicator strains assayed varied among bird species, with hole-and open-nesters showing the highest and the lowest values, respectively. Since it is broadly accepted that hole-nesters have higher risks of microbial infection than open nesters, our results suggest that the risk of infection is a strong driver of natural selection to enhance immunocompetence of animals through selecting for antibiotic-producing symbionts. Future research should focus on characterizing symbiotic bacterial communities and detecting coevolutionary processes with particular antibiotic-producing bacteria within-host species.

Metabolic Profiling Reveals That the Olfactory Cues in the Duck Uropygial Gland Potentially Act as Sex Pheromones.

The exchange of information between animals is crucial for maintaining social relations, individual survival, and reproduction, etc. The uropygial gland is a particular secretion gland found in birds. We speculated that uropygial gland secretions might act as a chemical signal responsible for sexual communication. We employed non-targeted metabolomic technology through liquid chromatography and mass spectrometry (LC-MS) to identifying duck uropygial gland secretions. We observed 11,311 and 14,321 chemical substances in the uropygial gland secretion for positive and negative ion modes, respectively. Based on their relative contents, principal component analysis (PCA) showed that gender significantly affects the metabolite composition of the duck uropygial gland. A total of 3831 and 4510 differential metabolites were further identified between the two sexes at the positive and negative ion modes, respectively. Of them, 139 differential metabolites were finally annotated. Among the 80 differential metabolites that reached an extremely significant difference (p < 0.01), we identified 24 volatile substances. Moreover, we further demonstrated that five kinds of volatile substances are highly repeatable in all testing ducks, including picolinic acid, 3-Hydroxypicolinic acid, indoleacetaldehyde, 3-hydroxymethylglutaric acid, and 3-methyl-2-oxovaleric acid. All these substances are significantly higher in males than in females, and their functions are involved in the reproduction processes of birds. Our data implied that these volatile substances act as sex pheromones and may be crucial olfactory clues for mate selection between birds. Our findings laid the foundation for future research on whether uropygial gland secretion can affect ducks' reproduction and production.

Olfactory camouflage and communication in birds.

Smell is a sensory modality that is rarely considered in birds, but evidence is mounting that olfaction is an important aspect of avian behaviour and ecology. The uropygial gland produces an odoriferous secretion (preen oil) that can differ seasonally and between the sexes. These differences are hypothesized to function in olfactory camouflage, i.e. minimizing detection by nest predators (olfactory crypsis hypothesis), and/or intraspecific olfactory communication, particularly during breeding (sex semiochemical hypothesis). However, evidence for seasonal and sex differences in preen oil is mixed, with some studies finding differences and others not, and direct evidence for the putative function(s) of seasonal variation and sex differences in preen oil remains limited. We conducted a systematic review of the evidence for such changes in preen oil chemical composition, finding seasonal differences in 95% of species (57/60 species in 35 studies) and sex differences in 47% of species (28/59 species in 46 studies). We then conducted phylogenetic comparative analyses using data from 59 bird species to evaluate evidence for both the olfactory crypsis and sex semiochemical hypotheses. Seasonal differences were more likely in the incubating than non-incubating sex in ground-nesting species, but were equally likely regardless of incubation strategy in non-ground-nesting species. This result supports the olfactory crypsis hypothesis, if ground nesters are more vulnerable to olfactorily searching predators than non-ground nesters. Sex differences were more likely in species with uniparental than biparental incubation and during breeding than non-breeding, consistent with both the olfactory crypsis and sex semiochemical hypotheses. At present, the data do not allow us to disentangle these two hypotheses, but we provide recommendations that will enable researchers to do so.

Preen gland microbiota covary with major histocompatibility complex genotype in a songbird.

Pathogen-mediated selection at the major histocompatibility complex (MHC) is thought to promote MHC-based mate choice in vertebrates. Mounting evidence implicates odour in conveying MHC genotype, but the underlying mechanisms remain uncertain. MHC effects on odour may be mediated by odour-producing symbiotic microbes whose community structure is shaped by MHC genotype. In birds, preen oil is a primary source of body odour and similarity at MHC predicts similarity in preen oil composition. Hypothesizing that this relationship is mediated by symbiotic microbes, we characterized MHC genotype, preen gland microbial communities and preen oil chemistry of song sparrows (Melospiza melodia). Consistent with the microbial mediation hypothesis, pairwise similarity at MHC predicted similarity in preen gland microbiota. Counter to this hypothesis, overall microbial similarity did not predict chemical similarity of preen oil. However, permutation testing identified a maximally predictive set of microbial taxa that best reflect MHC genotype, and another set of taxa that best predict preen oil chemical composition. The relative strengths of relationships between MHC and microbes, microbes and preen oil, and MHC and preen oil suggest that MHC may affect host odour both directly and indirectly. Thus, birds may assess MHC genotypes based on both host-associated and microbially mediated odours.

Preen gland microbiota of songbirds differ across populations but not sexes.

Metabolites produced by symbiotic microbes can affect the odour of their hosts, providing olfactory cues of identity, sex or other salient features. In birds, preen oil is a major source of body odour that differs between populations and sexes. We hypothesized that population and sex differences in preen oil chemistry reflect underlying differences in preen gland microbiota, predicting that these microbes also differ among populations and between the sexes. We further predicted that pairwise similarity in the community composition of preen gland microbiota would covary with that of preen oil chemical composition, consistent with the fermentation hypothesis for chemical recognition. We analysed preen oil chemistry and preen gland bacterial communities of song sparrows Melospiza melodia. Birds were sampled at sites for which population and sex differences in preen oil have been reported, and at a third site that has been less studied. Consistent with prior work in this system, we found population and sex differences in preen oil chemistry. By contrast, we found population differences but not sex differences in the community composition of preen gland microbes. Overall similarity in the community composition of preen gland microbiota did not significantly covary with that of preen oil chemistry. However, we identified a subset of six microbial genera that maximally correlated with preen oil composition. Although both preen gland microbiota and preen oil composition differ across populations, we did not observe an overall association between them that would implicate symbiotic microbes in mediating variation in olfactory cues associated with preen oil. Instead, certain subsets of microbes may be involved in mediating olfactory cues in birds, but experiments are required to test this.

Female-Based Patterns and Social Function in Avian Chemical Communication.

Much of the growing interest in avian chemical signals has focused on the role of kin recognition or mate attraction, often with an emphasis on males, with uropygial gland secretions perhaps providing information about an individual's identity and quality. Yet, data collected to date suggest sexual dimorphism in uropygial glands and secretions are often emphasized in female, rather than in male birds. That is, when a sexual difference occurs (often during the breeding season only), it is the female that typically exhibits one of three patterns: (1) a larger uropygial gland, (2) a greater abundance of volatile or semi-volatile preen oil compounds and/or (3) greater diversity of preen oil compounds or associated microbes. These patterns fit a majority of birds studied to date (23 of 30 chemically dimorphic species exhibit a female emphasis). Multiple species that do not fit are confounded by a lack of data for seasonal effects or proper quantitative measures of chemical compounds. We propose several social functions for these secretions in female-based patterns, similar to those reported in mammals, but which are largely unstudied in birds. These include: (1) intersexual advertisement of female receptivity or quality, including priming effects on male physiology, (2) intrasexual competition, including scent marking and reproductive suppression or (3) parental behaviors, such as parent-offspring recognition and chemical protection of eggs and nestlings. Revisiting the gaps of chemical studies to quantify the existence of female social chemosignals and any fitness benefit(s) during breeding are potentially fruitful but overlooked areas of future research.

Micro- and macroanatomical features of the uropygial gland of duck (Anas platyrhynchos) and pigeon (Columba livia).

We describe the morphological, histological and histochemical characteristics of the uropygial gland of the duck (Anas platyrhynchos) and pigeon (Columba livia). The weight and dimensions of the glands were recorded. The gland openings onto the papilla in ducks were surrounded by double tufts of downy feathers; the tufts were held together by an oily secretion. In the pigeon, the gland opening was onto naked skin. Samples for light and transmission electron microscopy were obtained from adult birds. Gland morphometry of the duck showed greater dimensions and relative gland weight than the pigeon. In both species the gland was lined by stratified epithelium, packed with secretary tubules and filled with oil droplets that were discharged into a central cavity. In the pigeon, the gland was branched and alveolar with a wide central lumen, whereas in the duck it was simple and branched tubular. Ultrastructural analysis of both species revealed that the fat globules tended to condense in the peripheral layers of the secretory and degenerative cells, and the nuclei exhibited pyknosis, which suggest the holocrine nature of the gland. Morphometric and histological analysis of the gland showed that the architecture is similar among birds; however, some species-specific differences suggest a functional correlation with the habitat.

Autoclaving Nest-Material Remains Influences the Probability of Ectoparasitism of Nestling Hoopoes (Upupa epops).

Nest bacterial environment influences avian reproduction directly because it might include pathogenic- or antibiotic-producing bacteria or indirectly because predators or ectoparasites can use volatile compounds from nest bacterial metabolism to detect nests of their avian hosts. Hoopoes (Upupa epops) do not build nests. They rather reuse holes or nest-boxes that contain remains of nest-materials from previous breeding seasons. Interestingly, it has been recently described that the nest's bacterial environment partly affects the uropygial gland microbiota of hoopoe females and eggshells. Blood-sucking ectoparasites use chemical cues to find host nests, so we experimentally tested the hypothetical effects of microorganisms inhabiting nest-material remains before reproduction regarding the intensity of ectoparasitism suffered by 8-day-old nestling hoopoes. In accordance with the hypothesis, nestlings hatched in nest-boxes with autoclaved nest-material remains from the previous reproductive seasons suffered less from ectoparasites than those hatched in the control nest-boxes with nonautoclaved nest-material. Moreover, we found a positive association between the bacterial density of nest-material during the nestling phase and ectoparasitism intensity that was only apparent in nest-boxes with autoclaved nest-material. However, contrary to our expectations, nest bacterial load was positively associated with fledgling success. These results suggest a link between the community of microorganisms of nest-material remains and the intensity of ectoparasitism, and, on the other hand, that the nest bacterial environment during reproduction is related to fledging success. Here, we discuss possible mechanisms explaining the experimental and correlative results, including the possibility that the experimental autoclaving of nest material affected the microbiota of females and nestlings' secretion and/or nest volatiles that attracted ectoparasites, therefore indirectly affecting both the nest bacterial environment at the nestling stage and fledging success.

Modified Tail Amputation Technique in a Blue and Gold Macaw (Ara ararauna) With Uropygial Gland Adenocarcinoma.

A 33-year-old male blue and gold macaw (Ara ararauna) presented with a 5-month history of an ulcerated lesion and feather loss at the tail base. Two 4-mm biopsies obtained by the primary care veterinarian were consistent with uropygial gland adenocarcinoma. The bird was examined at the Veterinary Medical Teaching Hospital, University of California, and on physical evaluation, the dorsal and ventral surface of the tail base were devoid of feathers, ulcerated and crusted without an identifiable uropygial gland. Complete blood count, plasma biochemistry panel, whole-body radiographs, and an echocardiogram were performed before surgery. The bird was anesthetized, and a complete amputation of the tail was performed. The skin was incised with a radiofrequency electrosurgical system approximately 2 mm circumferentially cranial to the diseased tissue. The musculature was transected to the level of the vertebral column, disarticulating between the second and third caudal vertebrae and transecting the spinal cord with a no. 15 blade. Lateral vertebral processes of the second vertebra were removed with a rongeur. Coccygeus lateralis muscles and tensor fasciae latae muscles and skin were closed laterolaterally with 2 layers and 3-0 polydioxanone suture. The bird recovered uneventfully and was discharged after 6 days of hospitalization. The histopathological diagnosis was adenocarcinoma with squamous differentiation, marked scirrhous response, and superficial epithelial ulceration. It was determined that narrow margins of unaffected tissue were achieved from the pathological examination of submitted material. The bird was evaluated 24 days after surgery and again 3.5 months after surgery, without evidence of complications or recurrence. Approximately 10 days after the last reexamination, the bird was euthanatized after being found minimally responsive at home. A postmortem examination was not performed.

Food stress, but not experimental exposure to mercury, affects songbird preen oil composition.

Mercury is a global pollutant and potent neurotoxic metal. Its most toxic and bioavailable form, methylmercury, can have both lethal and sublethal effects on wildlife. In birds, methylmercury exposure can disrupt behavior, hormones, the neuroendocrine system, and feather integrity. Lipid-rich tissues and secretions may be particularly susceptible to disruption by lipophilic contaminants such as methylmercury. One such substance is feather preen oil, a waxy secretion of the uropygial gland that serves multiple functions including feather maintenance, anti-parasitic defense, and chemical signaling. If methylmercury exposure alters preen oil composition, it could have cascading effects on feather quality, susceptibility to ectoparasites, and mate choice and other social behaviors. We investigated whether exposure to methylmercury, either alone or in association with other stressors, affects preen oil chemical composition. We used a two-factor design to expose adult song sparrows (Melospiza melodia) to an environmentally relevant dietary dose of methylmercury and/or to another stressor (unpredictable food supply) for eight weeks. The wax ester composition of preen oil changed significantly over the 8-week experimental period. This change was more pronounced in the unpredictable food treatment, regardless of dietary methylmercury. Contrary to our prediction, we found no main effect of methylmercury exposure on preen oil composition, nor did methylmercury interact with unpredictable food supply in predicting the magnitude of chemical shifts in preen oil. While it remains critical to study sublethal effects of methylmercury on wildlife, our findings suggest that the wax ester composition of preen oil is robust to environmentally relevant doses of this contaminant.

Experimental study of the effect of preen oil against feather bacteria in passerine birds.

Avian plumage harbors various pathogens such as feather-degrading bacteria, which have the potential to reduce host fitness. A growing body of evidence suggests that the secretion of the uropygial gland of birds-preen oil-acts as one of the first lines of defence against harmful bacteria. However, previous studies on the antimicrobial impact of preen oil have yielded controversial results. The impact of preen oil on bacterial densities of feathers was experimentally investigated in two passerine species: great tits Parus major and pied flycatchers Ficedula hypoleuca. More specifically, we tested whether the antibacterial effect of the preen oil secreted by the same individual differs from that of the preen oil originating from the gland of other species. In the laboratory, ventral feathers were treated with preen oil from (1) an individual's own gland, (2) from the gland of another passerine species, or (3) from the gland of a phylogenetically distant bird species. We detected a significant antibacterial effect of preen oil on bacteria that were attached to feathers, though the effect did not depend on whether the oil originated from the individual's own gland or from the gland of another bird species. However, treatment with preen oil suppressed the density of bacteria more in the pied flycatcher than in the great tit. This is the first study providing experimental evidence that preen oil represents an important antimicrobial mechanism against those plumage bacteria that are attached to feathers.

Large feet are beneficial for eiders Somateria mollissima.

Many waterbirds have fully (totipalmate) or partially webbed (palmate) feet that are used for locomotion in aquatic environments.If webbed feet and wings both contribute to efficient diving, we predicted a positive association between the area of webbed feet and the size of the frontal locomotor apparatus (wing area, heart mass, and breast muscle, after adjusting for any partial effects of body size). We predicted that individuals able to acquire more and better quality food due to larger webbed feet should have larger livers with higher concentrations of fat-soluble antioxidants such as vitamin E, and invest more in immune function as reflected by the relative size of the uropygial gland than individuals with small webbed feet.Here, we examine if the area of webbed feet is correlated with locomotion, diet, and body condition in a sea-duck, the eider (Somateria mollissima). We analyzed an extensive database of 233 eiders shot in Danish waters and at Åland, Finland during winter and early spring.Eiders with larger webbed feet had a larger locomotor apparatus, but did not have larger body size, they had larger uropygial glands that waterproof the plumage, they had larger beak volume and larger gizzards, and they had higher body condition.These findings imply that eiders with large webbed feet benefitted in terms of locomotion, feeding, and reproduction.

Plumage brightness and uropygial gland secretions in barn swallows.

The uropygial gland has been hypothesized to play a role in sexual signaling through a "make-up" function derived from the effects of secretions from the gland on the appearance of the plumage and bare parts of the body. Here we show that plumage brightness of dorsal feathers of individual barn swallows Hirundo rustica was greater in mated than in unmated individuals. In addition, plumage brightness increased with colony size. Furthermore, plumage brightness was positively correlated with the amount of wax in the uropygial gland, negatively correlated with time of sampling of uropygial wax (perhaps because more wax is present early in the morning after an entire night of wax production without any preening), and negatively correlated with the number of chewing lice that degrade the plumage. Experimentally preventing barn swallows from access to the uropygial gland reduced plumage brightness, showing a causal link between secretions from the uropygial gland and plumage brightness. These findings provide evidence consistent with a role of uropygial secretions in signaling plumage brightness.

Wax Ester Composition of Songbird Preen Oil Varies Seasonally and Differs between Sexes, Ages, and Populations.

Chemical signaling has been well studied in invertebrates and mammals but less so in birds, due to the longstanding misconception that olfaction is unimportant or even non-existent in this taxon. However, recent findings suggest that olfaction plays an important role in avian mate choice and reproductive behavior, similar to other taxa. The leading candidate source for compounds involved in avian chemical communication is preen oil, a complex mixture secreted from the uropygial gland. Preen oil contains volatile compounds and their potential wax ester precursors, and may act as a reproductive chemosignal. Reproductive signals are generally sexually dimorphic, age-specific, seasonally variable, and may also vary geographically. We tested whether preen oil meets these expectations by using gas chromatography to examine the wax ester composition of preen oil in song sparrows (Melospiza melodia). We found that the wax ester composition of preen oil was significantly different between sexes, age classes, seasons, and populations. Collectively, our results suggest that song sparrow preen oil meets the criteria of a chemical cue that may influence mate choice and reproduction. Our findings in song sparrows, which are sexually monomorphic in plumage, also parallel patterns described for dark-eyed juncos (Junco hyemalis), a closely related songbird with sexually dimorphic plumage. Behavioral tests are needed to confirm that song sparrows attend to the cues present in preen oil, but our findings support the increasingly accepted idea that chemical communication is common and widespread in birds as it is in other taxa.