Short-chain carboxylic acids from gray catbird (Dumetella carolinensis) uropygial secretions vary with testosterone levels and photoperiod

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.

Chemical Profiles Differ between Communal Breeding Groups in a Highly Social Bird

AbstractGregarious species must distinguish group members from nongroup members. Olfaction is important for group recognition in social insects and mammals but rarely studied in birds, despite birds using olfaction in social contexts from species discrimination to kin recognition. Olfactory group recognition requires that groups have a signature odor, so we tested for preen oil and feather chemical similarity in group-living smooth-billed anis (Crotophaga ani). Physiology affects body chemistry, so we also tested for an effect of egg-laying competition, as a proxy for reproductive status, on female chemical similarity. Finally, the fermentation hypothesis for chemical recognition posits that host-associated microbes affect host odor, so we tested for covariation between chemicals and microbiota. Group members were more chemically similar across both body regions. We found no chemical differences between sexes, but females in groups with less egg-laying competition had more similar preen oil, suggesting that preen oil contains information about reproductive status. There was no overall covariation between chemicals and microbes; instead, subsets of microbes could mediate olfactory cues in birds. Preen oil and feather chemicals showed little overlap and may contain different information. This is the first demonstration of group chemical signatures in birds, a finding of particular interest given that smooth-billed anis live in nonkin breeding groups. Behavioral experiments are needed to test whether anis can distinguish group members from nongroup members using odor cues.

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

Simple Summary

For birds, the uropygial gland is a special organ. We believe that its secretion can be used as a pheromone between the sexes to play a role in mate selection and mating. Therefore, we studied the chemical composition of duck uropygial gland secretions and the differences between males and females. After a series of screenings, 24 different volatile metabolites were obtained in our experiment. On this basis, five extremely significant volatile metabolites were screened out—significantly more males than females. The results show that these volatile substances are potential sex pheromone substances, which may be the critical olfactory clues for birds to choose mates. Our results lay the foundation for further research on whether uropygial gland secretion affects duck reproduction and production.

Abstract

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.

Individual Chemical Profiles in the Leach's Storm-Petrel

Avian chemical communication, once largely overlooked, is a growing field that has revealed the important role that olfaction plays in the social lives of some birds. Leach's storm-petrels (Oceanodroma leucorhoa) have a remarkable sense of smell and a strong, musky scent. This long-lived, monogamous seabird relies on olfaction for nest relocation and foraging, but whether they use scent for communication is less well studied. They are nocturnally active at the breeding colony and yet successfully reunite with their mate despite poor night-vision, indicating an important role for non-visual communication. We investigated the chemical profiles of Leach's storm-petrels to determine whether there is socially relevant information encoded in their plumage odor. To capture the compounds comprising their strong scent, we developed a method to study the compounds present in the air surrounding their feathers using headspace stir bar sorptive extraction coupled with gas chromatography-mass spectrometry. We collected feathers from Leach's storm-petrels breeding on Bon Portage Island in Nova Scotia, Canada in both 2015 and 2016. Our method detected 142 commonly occurring compounds. We found interannual differences in chemical profiles between the two sampling years. Males and females had similar chemical profiles, while individuals had distinct chemical signatures across the two years. These findings suggest that the scent of the Leach's storm-petrel provides sociochemical information that could facilitate olfactory recognition of individuals and may inform mate choice decisions.

Conspecific olfactory preferences and interspecific divergence in odor cues in a chickadee hybrid zone

Understanding how mating cues promote reproductive isolation upon secondary contact is important in describing the speciation process in animals. Divergent chemical cues have been shown to act in reproductive isolation across many animal taxa. However, such cues have been overlooked in avian speciation, particularly in passerines, in favor of more traditional signals such as song and plumage. Here, we aim to test the potential for odor to act as a mate choice cue, and therefore contribute to premating reproductive isolation between the black-capped (Poecile atricapillus) and Carolina chickadee (P. carolinensis) in eastern Pennsylvania hybrid zone populations. Using gas chromatography-mass spectrometry, we document significant species differences in uropygial gland oil chemistry, especially in the ratio of ester to nonester compounds. We also show significant preferences for conspecific over heterospecific odor cues in wild chickadees using a Y-maze design. Our results suggest that odor may be an overlooked but important mating cue in these chickadees, potentially promoting premating reproductive isolation. We further discuss several promising avenues for future research in songbird olfactory communication and speciation.

Pigeon odor varies with experimental exposure to trace metal pollution

Trace metals are chemical pollutants that have well-known noxious effects on wildlife and that are current major environmental issues in urban habitats. Previous studies have demonstrated their negative (e.g. lead) or positive (e.g. zinc) effects on body condition, immunity and reproductive success. Because of their effects on condition, trace metals are likely to influence the production of condition-dependent ornaments. The last decade has revealed that bird odors, like mammal odors, can convey information on individual quality and might be used as secondary sexual ornaments. Here, we used solid-phase microextraction headspace sampling with gas chromatography-mass spectrometry to investigate whether plumage scent varied with experimental supplementation in lead and/or zinc in feral pigeons. Zinc supplementation (alone or in combination with lead) changed the proportion of several volatiles, including an increase in the proportion of hydroxy-esters. The production of these esters, that most likely originate from preen gland secretions, may be costly and might thus be reduced by stress induced by zinc deficiency. Although lead is known to negatively impact pigeon condition, it did not statistically affect feather scent, despite most of the volatiles that increased with zinc exposure tended to be decreased in lead-supplemented pigeons. Further studies should evaluate the functions of plumage volatiles to predict how trace metals can impact bird fitness.

Preen oil and bird fitness: a critical review of the evidence

The uropygial gland is a holocrine complex exclusive to birds that produces an oleaginous secretion (preen oil) whose function is still debated. Herein, I examine critically the evidence for the many hypotheses of potential functions of this gland. The main conclusion is that our understanding of this gland is still in its infancy. Even for functions that are considered valid by most researchers, real evidence is scarce. Although it seems clear that preen oil contributes to plumage maintenance, we do not know whether this is due to a role in reducing mechanical abrasion or in reducing feather degradation by keratinophilic organisms. Evidence for a function against pathogenic bacteria is mixed, as preen oil has been demonstrated to act against bacteria in vitro, but not in vivo. Nor is it clear whether preen oil can combat pathogenic bacteria on eggshells to improve hatching success. Studies on the effect of preen oil against dermatophytes are very scarce and there is no evidence of a function against chewing lice. It seems clear, however, that preen oil improves waterproofing, but it is unclear whether this acts by creating a hydrophobic layer or simply by improving plumage structure. Several hypotheses proposed for the function of preen oil have been poorly studied, such as reduction of drag in flight. Similarly, we do not know whether preen oil functions as repellent against predators or parasites, makes birds unpalatable, or functions to camouflage birds with ambient odours. On the other hand, a growing body of work shows the important implications of volatiles in preen oil with regard to social communication in birds. Moreover, preen oil clearly alters plumage colouration. Finally, studies examining the impact of preen oil on fitness are lacking, and the costs or limitations of preen-oil production also remain poorly known. The uropygial gland appears to have several non-mutually exclusive functions in birds, and thus is likely to be subject to several selective pressures. Therefore, future studies should consider how the inevitable trade-offs among different functions drive the evolution of uropygial gland secretions.

Are female starlings able to recognize the scent of their offspring?

Background

Although there is growing evidence that birds may have individual chemical profiles that can function in several social contexts, offspring recognition based on olfactory cues has never been explored. This ability should be more likely evolved in colonial birds and/or species suffering brood parasitism, in which the risk of being engaged in costly misdirected parental care is high.

Methodology/Principal Findings

We performed a choice experiment to examine whether females of the spotless starling, Sturnus unicolor, a species that is colonial, and where a fraction of the population is exposed to intraspecific brood parasitism, can discriminate between the scent of their offspring and that of unrelated nestlings. We also explored whether the development of the uropygial gland secretion may play a role in such olfactory discrimination by performing the choice experiments to females rearing nestlings of two different ages, that is, without and with developed uropygial glands. Results showed that female starlings did not preferentially choose the scent of their offspring, independently of whether the gland of nestlings was developed or not.

Conclusions/Significance

Our results suggest that female starlings do not have or do not show the ability to distinguish their offspring based on olfaction, at least up to 12–14 days of nestling age. Further research is needed to examine whether odour-based discrimination may function when fledgling starlings leave the nest and the risk of costly misidentification is likely to increase.

Chemosignaling diversity in songbirds: chromatographic profiling of preen oil volatiles in different species

Large foraging seabirds are known to navigate to food sources using their excellent sense of smell, but much less is known about the use of olfaction by the songbirds (passerine birds). Some evidence of individual recognition based on the bird preen oil volatile organic compound (VOC) compositions, which is the main odor source in birds, have been reported for dark-eyed junco and house finch. In this study we have investigated preen oil VOCs in 16 different songbird species and two other small bird species in order to determine whether the VOC compositions follow phylogenetic and evolutionary relatedness. We have used the stir bar sorptive extraction (SBSE) methodology followed by gas chromatography-mass spectrometry (GC-MS) to determine preen oil VOCs during the long light summer conditions for mostly wild caught birds. Large diversity among the VOC compositions was observed, while some compound classes were found in almost all species. The divergent VOC profiles did not follow the phylogenetic family lines among the bird species. This suggests that songbirds may use VOC odors as a mate recognition cue.

Sex recognition by odour and variation in the uropygial gland secretion in starlings

1. Although a growing body of evidence supports that olfaction based on chemical compounds emitted by birds may play a role in individual recognition, the possible role of chemical cues in sexual selection of birds has been only preliminarily studied. 2. We investigated for the first time whether a passerine bird, the spotless starling Sturnus unicolor, was able to discriminate the sex of conspecifics by using olfactory cues and whether the size and secretion composition of the uropygial gland convey information on sex, age and reproductive status in this species. 3. We performed a blind choice experiment during mating, and we found that starlings were able to discriminate the sex of conspecifics by using chemical cues alone. Both male and female starlings preferred male scents. Furthermore, the analysis of the chemical composition of the uropygial gland secretion by using gas chromatography-mass spectrometry (GC-MS) revealed differences between sexes, ages and reproductive status. 4. In conclusion, our study reveals for first time that a passerine species can discriminate the sex of conspecifics by relying on chemical cues and suggests that the uropygial gland secretion may potentially function as a chemical signal used in mate choice and/or intrasexual competition in this species.

Role of testosterone in stimulating seasonal changes in a potential avian chemosignal

Songbird preen oil contains volatile and semivolatile compounds that may contain information about species, sex, individual identity, and season. We examined the relationship between testosterone (T) and the amounts of preen oil volatile and semivolatile compounds in wild and captive dark-eyed juncos (Junco hyemalis). In wild males and females, we observed an increase in volatile compound relative concentration early in the breeding season. This increase mirrored previously described seasonal elevation in T levels in wild males and females, suggesting a positive relationship between hormone levels and preen gland secretions, and a possible role for these secretions in signaling receptivity. In females, the greatest relative concentrations of most compounds were observed close to egg laying, a time when steroid hormones are high and also the only time that females respond to an injection of gonadotropin-releasing hormone with a short-term increase in T. In a study of captive juncos held on short days, we asked whether the seasonal increases observed in the wild could be induced with experimental elevation of T alone. We found that exogenous T stimulated the production of some volatile compounds in non-breeding individuals of both sexes. However, of the 15 compounds known to increase during the breeding season, only four showed an increase in relative concentration in birds that received T implants. Our results suggest that testosterone levels likely interact with other seasonally induced physiological changes to affect volatile compound amounts in preen oil.

Potential semiochemical molecules from birds: a practical and comprehensive compilation of the last 20 years studies

During the past 2 decades, considerable progress has been made in the study of bird semiochemistry, and our goal was to review and evaluate this literature with particular emphasis on the volatile organic constituents. Indeed, since the importance of social chemosignaling in birds is becoming more and more apparent, the search for molecules involved in chemical communication is of critical interest. These molecules can be found in different sources that include uropygial gland secretions, feather-surface compounds, and molecules from feces and skin. Although many studies have examined the chemical substances secreted by birds, research on bird chemical communication is still at the start, so new strategies for collecting samples and development of new methods of analysis are urgently required. As a first step, we built a database that brings together potential semiochemicals, using a unique chemical nomenclature for comparing different bird species and also for referencing the different classes of substances that can be found in order to adapt future parameters of analysis. The most important patterns of the wax fraction of preen secretions are highlighted and organized in an ordered table. We also draw up a list of various combinations of sampling and analytical techniques, so that each method can be compared at a glance.

Volatile and semivolatile compounds in gray catbird uropygial secretions vary with age and between breeding and wintering grounds

The uropygial secretions of some bird species contain volatile and semivolatile compounds that are hypothesized to serve as chemical signals. The abundance of secretion components varies with age and season, although these effects have not been investigated in many species. We used solid-phase microextraction headspace sampling and solvent extraction coupled with gas chromatography-mass spectrometry to detect and identify volatile and semivolatile chemical compounds in uropygial secretions of gray catbirds (Dumetella carolinensis). We identified linear and branched saturated carboxylic acids from acetic (C2) through hexacosanoic (C26); linear alcohols from decanol (C10) through docosanol (C22); one aromatic aldehyde; one monounsaturated carboxylic acid; two methyl ketones; and a C28 ester. We tested for the effect of age on signal strength and found that juvenile birds produced greater amounts of volatile C4 through C7 acids and semivolatile C20 through C26 acids, although the variation among individuals was large. Adult birds displayed small concentrations and minimal individual variation among volatile compounds, but produced significantly higher levels of long-chain linear alcohols than juvenile birds. We tested for the effects of season/location by sampling adult catbirds at their Ohio breeding grounds and at their Florida wintering grounds and found that the heaviest carboxylic acids are significantly more abundant in secretions from birds sampled during winter at the Florida site, whereas methyl ketones are more abundant in birds sampled during summer on the Ohio breeding grounds. We observed no effect of sex on semivolatile compounds, but we found a significant effect of sex on levels of carboxylic acids (C4 through C7) for juvenile birds only.