Feather‐degrading bacteria,uropygial gland size and feather quality in House Sparrows Passer domesticus

Feathers are dead integumentary structures that are prone to damage and thus show gradual degradation over the course of a year. This loss of quality might have negative fitness consequences. Feather‐degrading bacteria are some of the most prevalent feather‐degrading organisms, yet the relationship between feather‐degrading bacteria load and flight feather quality has rarely been assessed. We studied this relationship in free‐living House Sparrows during breeding and non‐breeding annual lifecycle stages. We also considered the size of the uropygial gland, given the antimicrobial function of its secretions, and the effect of body condition. The number of feather holes was positively associated with feather‐degrading bacteria load and was negatively related to uropygial gland size and body condition during the breeding season in both sexes. In the non‐breeding season we found the same relationships, but only in females. The degree of feather wear was unrelated to any of the variables measured during the breeding season, whereas it was negatively associated with uropygial gland size and positively with feather‐degrading bacteria load in the non‐breeding season, but only in females. Our results suggest that feather‐degrading bacteria may induce the formation of feather holes, but play only a minor role in the abrasion of flight feathers.     

The evolution of size of the uropygial gland: mutualistic feather mites and uropygial secretion reduce bacterial loads of eggshells and hatching failures of European birds

Potentially, pathogenic bacteria are one of the main infective agents against which a battery of chemical and physical barriers has evolved in animals. Among these are the secretions by the exocrine uropygial gland in birds. The antimicrobial properties of uropygial secretions may prevent colonization and growth of microorganisms on feathers, skin and eggshells. However, uropygial gland secretions also favour the proliferation of feather mites that feed on secretions and microorganisms living on feathers that would otherwise reach eggshells during incubation if not consumed by feather mites. Therefore, at the interspecific level, uropygial gland size (as an index of volume of uropygial secretion) should be positively related to eggshell bacterial load (i.e. the risk of egg infection), whereas eggshell bacterial loads may be negatively related to abundance of feather mites eating bacteria. Here, we explore these previously untested predictions in a comparative framework using information on eggshell bacterial loads, uropygial gland size, diversity and abundance of feather mites and hatching success of 22 species of birds. The size of the uropygial gland was positively related to eggshell bacterial loads (mesophilic bacteria and Enterobacteriaceae), and bird species with higher diversity and abundance of feather mites harboured lower bacterial density on their eggshells (Enterococcus and Staphylococcus), in accordance with the hypothesis. Importantly, eggshell bacterial loads of mesophilic bacteria, Enterococcus and Enterobacteriaceae were negatively associated with hatching success, allowing us to interpret these interspecific relationships in a functional scenario, where both uropygial glands and mutualistic feather mites independently reduce the negative effects of pathogenic bacteria on avian fitness.     

Uropygial gland size,feather holes and moult performance in the House Sparrow Passer domesticus

Feather holes represent damage to the plumage of birds and are correlated with delayed moult. Uropygial gland size is negatively correlated with feather holes. Consequently, it was predicted that birds with smaller uropygial glands would have more feather holes, and that this would affect moult performance. I examined this prediction in the House Sparrow Passer domesticus. Individuals with smaller uropygial glands had more feather holes, and those with more feather holes moulted later and faster. Therefore, uropygial gland size seemed to affect moult performance via its effect on feather holes. Uropygial gland size may have a positive effect on plumage quality, through a negative effect on feather holes, and therefore on moult timing and speed.     

Seasonality in the uropygial gland size and feather mite abundance in house sparrows Passer domesticus: natural covariation and an experiment

The seasonal change, i.e. the marked differences between seasons of low and high productivity, in the abundance of ectosymbionts and the defence intensity of the host against pathogens is a well defined characteristic of temperate zone organisms. Here we investigate the seasonal variation in the uropygial gland size and the abundance of Proctophyllodes feather mites on the wing feathers of house sparrows Passer domesticus in two breeding populations. The size of the uropygial gland varied significantly in male and female house sparrows over the annual cycle. The gland was small during the non‐breeding and mating season, after that it started to grow sharply, reaching its maximum size during breeding. Females had larger gland volumes than males during breeding, and the increase in gland size during breeding was more pronounced in females than in males. The number of feather mites was the lowest during breeding, followed by an increase during moult, and reaching its maximum between the wintering and mating seasons. The absence of a significant relationship between the uropygial gland size and the abundance of feather mites, after controlling for potential confounding variables, supports the view that gland oils do not regulate the number of mites. To investigate further this hypothesis, through a full factorial experimental design we tested the effect of uropygial gland and photoperiod manipulation on the population size and population dynamics of feather mites. The manipulation of uropygial gland had no effect on mites, supporting our observational results. As a result of the experimentally increased day‐length, the abundance of feather mites on wing feathers decreased significantly and more sharply than in the control group, supporting the previous anecdotal evidence about the photosensitivity of these organisms. Using photoperiodic cues, feather mites may respond to seasonal changes that affect their life‐history and population dynamics.     

Uropygial gland size correlates with feather holes,body condition and wingbar size in the house sparrow Passer domesticus

The uropygial gland is an organ exclusive of birds that secretes an oily substance, the uropygial secretion, the functions of which are still debated. One of the proposed hypothesis is its possible action against chewing lice (order Phthiraptera), a group of avian ectoparasites that feed on feathers, causing different types of harm. However, this hypothesis lacks support. The present study analyses the relationship between uropygial gland size and the number of feather holes (which is correlated with the load of chewing lice) in the house sparrow Passer domesticus. Moreover, the relationship between the uropygial gland size and different aspects of sparrow health (body condition, immunocompetence and haematocrit), as well as sexually selected traits in males (badge and wingbar size), is tested. The results show a negative correlation between uropygial gland size and number of feather holes, a result found both years of the study. This result supports the hypothesis that uropygial secretion is used against chewing lice. Uropygial gland size also correlated positively with body condition (residuals of body mass relative to tarsus length) and immunocompetence, being therefore related to bird health. After a year in captivity, with resources provided ad libitum, no correlation was found between individual uropygial gland size and body condition or haematocrit, perhaps because the negative effect that chewing lice exert on bird health was offset by captivity conditions. Uropygial gland size was not correlated with badge size, but it was correlated with wingbar size, which furthermore supports the contention that this sexually selected signal acts as an indicator of lice resistance in the house sparrow. In summary, this study supports the idea of a positive relationship between uropygial gland and bird health in the house sparrow, the gland secretion affording resistance against chewing lice.     

Feather mite abundance increases with uropygial gland size and plumage yellowness in Great Tits Parus major

Plumicolous feather mites are ectosymbiotic organisms that live on bird feathers. Despite their abundance and prevalence among birds, the ecology of the interaction between these organisms and their hosts is poorly known. As feather mites feed on oil that birds spread from their uropygial gland, it has been hypothesized, but never tested, that the number of feather mites increases with the size of the uropygial gland of their hosts. In this study the number of feather mites is considered with respect to uropygial gland size in a breeding population of Great Tits Parus major in order to test this hypothesis. As predicted, the number of feather mites correlated positively with the uropygial gland size of their hosts, showing for the first time that uropygial gland size can explain the variance in feather mite load among conspecifics. Previous studies relating feather mite load to plumage colour have suggested that feather mites may be parasitic or neutral. To confirm this, the yellowness of breast feathers was also assessed. However, the results ran in the opposite direction to that expected, showing a positive correlation between mite load and plumage yellowness, which suggests that further work is needed to give clear evidence for a specific nature of feather mites. However, Great Tits with higher mite loads had lower hatching and breeding success, which may support the idea that feather mites are parasites, although this effect must be taken with caution because it was only found in males. Age or sex effects were not found on the number of feather mites, and it is proposed that hormonal levels may not be sufficient to explain the variation in feather mite loads. Interestingly, a positive correlation was detected between uropygial gland size and plumage brightness, which could be a novel factor to take into account in studies of plumage colour.     

Sources of variation in uropygial gland size in European birds

Defence mechanisms against parasites and pathogens are some of the most elaborate biological systems in animals. The oily secretion of the avian uropygial gland has been suggested to serve as a chemical defence against feather and eggshell bacteria. Yet, the traits associated with uropygial gland oil production are not well understood. We conducted a phylogenetic analysis comprising 132 European bird species aiming to test: (1) whether life‐history and ecological traits drive gland size evolution by potentially promoting microbial infestation and (2) how these traits affects change in the gland size throughout the annual cycle. We show that the size of the uropygial gland is dynamic (i.e. increasing from the nonbreeding to the breeding season, independent of sex). Furthermore, we found that the year‐round size of the gland was similar between sexes and was correlated with different ecological and life‐history traits promoting microbial infection throughout the annual cycle. During the breeding season, the total eggshell surface area in a clutch correlated significantly and positively with the gland size, suggesting the importance of oil in protecting eggs from microbes. Social species exhibited a larger gland size increase during the breeding season compared to nonsocials; a change that was also predicted by the total eggshell surface area. Aquatic, riparian and non‐migratory species had larger glands than terrestrials and migrants, respectively. The findings of the present study suggest that aquatic environments may promote the production of gland oil, through either the need of waterproofing the plumage and/or defending it against the intensified feather degradation in these moist conditions. Finally, we found a negative effect of the incubation period on uropygial gland size, which may suggest an energetic constraint imposed by other development‐connected costly activities. Our results show that the role of the uropygial gland dynamically varies during the annual cycle, potentially in response to seasonal variation in parasitic infection risk. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 543–563.     

Ectoparasites, uropygial glands and hatching success in birds

The uropygial gland of birds secretes wax that is applied to the plumage, where the secretions are hypothesized to eliminate fungi and bacteria, thereby potentially providing important benefits in terms of plumage maintenance. We analyzed variation in size of the uropygial gland in 212 species of birds to determine the function and the ecological correlates of variation in gland size. Bird species with larger uropygial glands had more genera of chewing lice of the sub-order Amblycera, but not of the sub-order Ischnocera, and more feather mites. There was a fitness advantage associated with relatively large uropygial glands because such species had higher hatching success. These findings are consistent with the hypothesis that the uropygial gland functions to manage the community of microorganisms, and that certain taxa of chewing lice have diverged as a consequence of these defenses.     

Chronic coccidian infestation compromises flight feather quality in house sparrows Passer domesticus

Parasites usurp indispensable resources for birds during a moult, and this is particularly relevant for those parasites residing in host intestines. This might compromise the nutritionally demanding moult and, thus, feather functionality. Although lower feather quality has profound and multifaceted adverse effects on residual fitness, surprisingly, little is known about parasites' effect on feather traits, especially over the longer term. We conducted an aviary experiment by medicating half of a group of naturally infested house sparrows Passer domesticus against intestinal coccidians for 15 months, spanning two consecutive postnuptial moults, whereas the other half was kept infested (i.e. without medication). Coccidian infestation significantly and negatively affected the size of the uropygial gland during the second moulting period compared to the medicated group. Furthermore, wing length was significantly shorter after the second moulting in the non‐medicated compared to the medicated female birds, which indicates that the negative effects of coccidians emerge only after a prolonged exposure to parasite infestation. Non‐medicated birds grew poorer quality flight feathers detected in a large number of feather traits both after the first and second moults. In the case of non‐medicated birds, the primaries were lighter and shorter, and had a smaller vane area, thinner rachis and decreased stiffness, although a higher barb and barbule density, which may have various consequences for fitness through reducing flight performance. Our findings demonstrate that, besides the well‐known immediate consequences for host breeding success, parasites might also have serious, long‐lasting effects through influencing feather quality and, ultimately, fitness of the host. © 2013 The Linnean Society of London     

From preen secretions to plumage: the chemical trajectory of blue petrels' Halobaena caerulea social scent

Petrel seabirds heavily rely on their olfactory sense and are thus appropriate models for the study of avian chemical communication. The uropygial secretions of blue petrels Halobaena caerulea, for instance, have been shown to encapsulate elaborate sociochemical information including species, gender and identity. Yet, it is the plumage, and not preening secretions per se, which acts as the final substrate of avian scents. Importantly, the chemical relationship between secretions from the uropygial gland, located at the dorsal base of the tail, and plumage lipids has been considered in only a handful of studies which reported large qualitative differences. The emission process of avian scents, and the possible participation of the uropygial gland in particular, are therefore not elucidated. In the present study, we examine the early chemical trajectory of blue petrels' social chemosignals by comparing secretion and feather samples using Gas‐Chromatography Mass‐Spectrometry (GC/MS) and recently developed multivariate statistics. Our results indicate that (1) 85% of the feather lipids come from the uropygial secretions, (2) chemical differentiation between secretions and feather lipids includes qualitative and quantitative variations, which both have interesting implications for scent production, (3) the sociochemical information contained within the secretions (i.e. a sex‐specific signal and individual chemical signatures) are present in very conserved forms on the plumage. In the light of these results, it is apparent that the uropygial gland plays a critical role for chemical communication in petrels and possibly other avian groups.     

Feather mites play a role in cleaning host feathers: New insights from DNA metabarcoding and microscopy

Parasites and other symbionts are crucial components of ecosystems, regulating host populations and supporting food webs. However, most symbiont systems, especially those involving commensals and mutualists, are relatively poorly understood. In this study, we have investigated the nature of the symbiotic relationship between birds and their most abundant and diverse ectosymbionts: the vane‐dwelling feather mites. For this purpose, we studied the diet of feather mites using two complementary methods. First, we used light microscopy to examine the gut contents of 1,300 individual feather mites representing 100 mite genera (18 families) from 190 bird species belonging to 72 families and 19 orders. Second, we used high‐throughput sequencing (HTS) and DNA metabarcoding to determine gut contents from 1,833 individual mites of 18 species inhabiting 18 bird species. Results showed fungi and potentially bacteria as the main food resources for feather mites (apart from potential bird uropygial gland oil). Diatoms and plant matter appeared as rare food resources for feather mites. Importantly, we did not find any evidence of feather mites feeding upon bird resources (e.g., blood, skin) other than potentially uropygial gland oil. In addition, we found a high prevalence of both keratinophilic and pathogenic fungal taxa in the feather mite species examined. Altogether, our results shed light on the long‐standing question of the nature of the relationship between birds and their vane‐dwelling feather mites, supporting previous evidence for a commensalistic–mutualistic role of feather mites, which are revealed as likely fungivore–microbivore–detritivore symbionts of bird feathers.     

Of 11 candidate steroids,corticosterone concentration standardized for mass is the most reliable steroid biomarker of nutritional stress across different feather types

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Uropygial gland volume and malaria infection are related to survival in migratory house martins

Pathogens such as bacteria, fungi and malaria and related haemosporidians provoke negative effects on the fitness of their hosts. Animals have developed a range of defensive mechanisms to resist or eliminate these parasitic infections and their negative fitness costs. The uropygial gland secretion has been proposed to act as defensive barrier of skin and plumage in the fight against bacteria and fungi, and may prevent birds from acquiring haemosporidian infections. Thus, the secretion of uropygial glands of birds may favour survival of individuals. However, whether uropygial gland secretion influence survival remains unknown. Here we explore if the size of the uropygial gland and malaria infection influence survival of house martins Delichon urbica. We showed, for the first time, that the volume of the uropygial gland positively predicted survival prospects of malaria infected house martins. Malaria infected birds had the lowest probability of survival, with the effect of gland size on survival prospects depending on infection: infected house martins with larger uropygial glands were better able to survive to the next breeding season, while infected birds with small uropygial glands were not. These results highlight the importance of uropygial gland secretion in the life history of wild birds.     

Body‐mass‐dependent trade‐off between immune response and uropygial gland size in house sparrows Passer domesticus

Parasites greatly impact host fitness. The immune system is fundamental to combat endoparasites, and survival increases with greater investment in immunity. Some ectoparasites, by contrast, are reportedly combated by the use of the uropygial gland, an organ exclusive to birds, which secretes an oily substance (preen oil) that is spread on plumage. However, both mounting an immune response against a parasite and producing uropygial gland secretion depend on the same resources, a situation which may induce trade‐offs between the two antiparasitic functions. In this study, I experimentally test whether immune response is traded off against uropygial gland size in the house sparrow Passer domesticus. In the experiment, a group of sparrows were injected with an antigen (lipopolysaccharide, LPS), which stimulates the immune system, while the other group received a sham injection. The uropygial gland of LPS‐treated birds decreased significantly more than that of the control birds after treatment. Nevertheless, the effect of the treatment was limited to house sparrows with low body mass, suggesting that heavy house sparrows were able to produce an immune response while maintaining a relatively large uropygial gland. Given that uropygial gland size is strongly related to production of preen oil, these results suggest that preen oil production declines in birds in poor body condition when resources are preferentially diverted to other demanding functions, such as the immune system. Considering that the uropygial gland is involved in several fitness‐related processes in birds, the trade‐off between immune response and uropygial gland size may have important consequences for bird life histories.     

Feather mite abundance varies but symbiotic nature of mite‐host relationship does not differ between two ecologically dissimilar warblers

Feather mites are obligatory ectosymbionts of birds that primarily feed on the oily secretions from the uropygial gland. Feather mite abundance varies within and among host species and has various effects on host condition and fitness, but there is little consensus on factors that drive variation of this symbiotic system. We tested hypotheses regarding how within‐species and among‐species traits explain variation in both (1) mite abundance and (2) relationships between mite abundance and host body condition and components of host fitness (reproductive performance and apparent annual survival). We focused on two closely related (Parulidae), but ecologically distinct, species: Setophaga cerulea (Cerulean Warbler), a canopy dwelling open‐cup nester, and Protonotaria citrea (Prothonotary Warbler), an understory dwelling, cavity nester. We predicted that feather mites would be more abundant on and have a more parasitic relationship with P. citrea, and within P. citrea, females and older individuals would harbor greater mite abundances. We captured, took body measurements, quantified feather mite abundance on individuals’ primaries and rectrices, and monitored individuals and their nests to estimate fitness. Feather mite abundance differed by species, but in the opposite direction of our prediction. There was no relationship between mite abundance and any measure of body condition or fitness for either species or sex (also contrary to our predictions). Our results suggest that species biology and ecological context may influence mite abundance on hosts. However, this pattern does not extend to differential effects of mites on measures of host body condition or fitness.     

Feather mites and birds: an interaction mediated by uropygial gland size?

Feather mites (Arachnida: Acari: Astigmata) feed mainly on secretions of the uropygial gland of birds. Here, we use analyses corrected for phylogeny and body size to show that there is a positive correlation between the size of this gland and mite abundance in passerine birds at an interspecific level during the breeding season, suggesting that the gland mediates interactions between mites and birds. As predicted on the basis of hypothesized waterproofing and antibiotic functions of uropygial gland secretions, riparian/marsh bird species had larger glands and higher mite loads than birds living in less mesic terrestrial environments. An unexpected pattern was a steeper relationship between mite load and gland size in migratory birds than in residents. If moderate mite loads are beneficial to a host but high loads detrimental, this could create complex selection regimes in which gland size influences mite load and vice versa. Mites may exert selective pressures on gland size of their hosts that has resulted in smaller glands among migratory bird species, suggesting that smaller glands may have evolved in these birds to attenuate a possible detrimental effect of feather mites when present in large numbers.     

Chemical regulation of body feather microbiota in a wild bird

The microbiota has a broad range of impacts on host physiology and behaviour, pointing out the need to improve our comprehension of the drivers of host–microbiota composition. Of particular interest is whether the microbiota is acquired passively, or whether and to what extent hosts themselves shape the acquisition and maintenance of their microbiota. In birds, the uropygial gland produces oily secretions used to coat feathers that have been suggested to act as an antimicrobial defence mechanism regulating body feather microbiota. However, our comprehension of this process is still limited. In this study, we for the first time coupled high‐throughput sequencing of the microbiota of both body feathers and the direct environment (i.e., the nest) in great tits with chemical analyses of the composition of uropygial gland secretions to examine whether host chemicals have either specific effects on some bacteria or nonspecific broad‐spectrum effects on the body feather microbiota. Using a network approach investigating the patterns of co‐occurrence or co‐exclusions between chemicals and bacteria within the body feather microbiota, we found no evidence for specific promicrobial or antimicrobial effects of uropygial gland chemicals. However, we found that one group of chemicals was negatively correlated to bacterial richness on body feathers, and a higher production of these chemicals was associated with a poorer body feather bacterial richness compared to the nest microbiota. Our study provides evidence that chemicals produced by the host might function as a nonspecific broad‐spectrum antimicrobial defence mechanism limiting colonization and/or maintenance of bacteria on body feathers, providing new insight about the drivers of the host's microbiota composition in wild organisms.     

Histological and histochemical study of the uropygial gland of chimango caracara (Milvago chimango vieillot, 1816)

The uropygial glands of birds are sebaceous organs that contribute to the water-repellent properties of the feather coat. We studied the histological and histochemical characteristics of the uropygial gland of chimango caracara using hematoxylin and eosin (H & E), Gomori´s trichrome, orcein, Gomori´s reticulin, periodic acid-Schiff (PAS), Alcian blue (AB) and a variety of lectins. The gland is composed of two lobes and a papilla with 20 downy feathers. It is surrounded by a capsule of dense connective tissue that contains elastic, reticular and smooth muscle fibers. The papilla is delicate and has two excretory ducts. The gland mass relative to body mass was 0.143%. Both adenomer cells and their secretions were stained with Sudan IV, PAS and AB, and were positive for numerous lectins that indicated the presence of lipids and carbohydrates. Immunohistochemical techniques to detect PCNA confirmed cell proliferation in the basal stratum of the adenomer cells. The lipids and glycoconjugates secreted by the uropygial gland serve numerous functions including protection against microorganisms.     

Exploring the adjustment to parasite pressure hypothesis: differences in uropygial gland volume and haemosporidian infection in palearctic and neotropical birds

Parasites are globally widespread pathogenic organisms, which impose important selective forces upon their hosts. Thus, in accordance with the Adjustment to parasite pressure hypothesis, it is expected that defenses among hosts vary relative to the selective pressure imposed by parasites. According to the latitudinal gradient in diversity, species richness and abundance of parasites peak near the equator. The uropygial gland is an important defensive exocrine gland against pathogens in birds. Size of the uropygial gland has been proposed to vary among species of birds because of divergent selection by pathogens on their hosts. Therefore, we should expect that bird species from the tropics should have relatively larger uropygial glands for their body size than species from higher latitudes. However, this hypothesis has not yet been explored. Here, we analyze the size of the uropygial gland of 1719 individual birds belonging to 36 bird species from 3 Neotropical (Peru) and 3 temperate areas (Spain). Relative uropygial gland volume was 12.52% larger in bird species from the tropics than from temperate areas. This finding is consistent with the relative size of this defensive organ being driven by selective pressures imposed by parasites. We also explored the potential role of this gland as a means of avoiding haemosporidian infection, showing that species with large uropygial glands for their body size tend to have lower mean prevalence of haemosporidian infection, regardless of their geographical origin. This result provides additional support for the assumption that secretions from the uropygial gland reduce the likelihood of becoming infected with haemosporidians.