Rising to the Challenge: Mounting an Acute Phase Immune Response Has No Long-Term Negative Effects on Captive Sparrow Migratory Body Composition or Migratory Restlessness.

Migratory animals may trade-off between investing energy in immune defense versus investing in energy reserves needed for seasonal migration. However, these trade-offs are often masked by other sources of variation and may not be detected through observational field studies of free-living animals. Moreover, observational studies can rarely distinguish the costs of pathogenic infection from those of mounting an immune response. To disentangle such effects, we conducted an immune challenge experiment. We captured song sparrows (Melospiza melodia) and white-throated sparrows (Zonotrichia albicollis) in autumn migratory condition, challenged the sparrows with non-infectious antigens that induce an acute-phase immune response, then monitored body composition and migratory restlessness behavior. For both species, body mass was higher the day after exposure to keyhole limpet hemocyanin (KLH) compared to controls. White-throated sparrows, but not song sparrows, increased lean mass 1 week after exposure to lipopolysaccharide (LPS), suggesting that effects of immune upregulation on body composition may be long-lasting and specific to certain combinations of hosts and antigens. White-throated sparrows exposed to KLH increased nocturnal migratory restlessness (Zugunruhe) for the week following exposure. These findings suggest that short-term activation of the acute immune response does not constrain migratory physiology in these songbirds.

Uropygial gland microbiota differ between free-living and captive songbirds.

Symbiotic microbes can affect host behavior and fitness. Gut microbiota have received the most study, with less attention to other important microbial communities like those of scent-producing glands such as mammalian anal glands and the avian uropygial gland. However, mounting evidence suggests that microbes inhabiting scent-producing glands play an important role in animal behavior by contributing to variation in chemical signals. Free-living and captive conditions typically differ in social environment, food diversity and availability, disease exposure, and other factors-all of which can translate into differences in gut microbiota. However, whether extrinsic factors such as captivity alter microbial communities in scent glands remains an open question. We compared the uropygial gland microbiota of free-living and captive song sparrows (Melospiza melodia) and tested for an effect of dietary manipulations on the gland microbiota of captive birds. As predicted, the uropygial gland microbiota was significantly different between free-living and captive birds. Surprisingly, microbial diversity was higher in captive than free-living birds, and we found no effect of dietary treatments on captive bird microbiota. Identifying the specific factors responsible for microbial differences among groups and determining whether changes in symbiotic microbiota alter behavior and fitness are important next steps in this field.

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.

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 Malaria Infection Affects Songbirds' Nocturnal Migratory Activity.

Migratory animals encounter multiple parasite communities, raising concerns that migration may aid transport of infectious disease. How migration affects disease spread depends fundamentally on how disease affects migration, specifically whether infection alters individuals' migratory physiology and behavior. We inoculated white-throated sparrows (Zonotrichia albicollis) with avian malaria parasites (Plasmodium sp.), monitored parasite loads for 5 wk as the birds reached spring migratory condition, and compared nocturnal migratory restlessness (Zugunruhe), body composition (fat, lean, and whole-body mass), and hematocrit among experimentally infected birds, sham-inoculated birds, and birds that were exposed to parasites but resisted infection. Migratory restlessness increased over time in the study, but the rate of change varied between sham (control) birds, infected birds, and birds that resisted infection. We were unable to detect any effects of malaria exposure on body condition. Our findings suggest that encountering parasites affects migratory activity, regardless of whether infection occurs or is resisted.

Alternative reproductive strategies in white-throated sparrows are associated with differences in parasite load following experimental infection.

Immune defences often trade off with other life-history components. Within species, optimal allocation to immunity may differ between the sexes or between alternative life-history strategies. White-throated sparrows (Zonotrichia albicollis) are unusual in having two discrete plumage morphs, white-striped and tan-striped. Within each sex, white-striped individuals are more aggressive and provide less parental care than tan-striped individuals. We extended immunocompetence handicap models, which predict sex differences in immunity and parasitism, to hypothesize that infection susceptibility should be greater in white-striped than tan-striped birds. We inoculated birds of both morphs with malarial parasites. Contrary to our prediction, among birds that became infected, parasite loads were higher in tan-striped than white-striped individuals and did not differ between the sexes. Circulating androgen levels did not differ between morphs but were higher in males than females. Our findings are not consistent with androgen-mediated immunosuppression. Instead, morph differences in immunity could reflect social interactions or life-history-related differences in risk of injury, and/or genetic factors. Although plumage and behavioural morphs of white-throated sparrow may differ in disease resistance, these differences do not parallel sex differences that have been reported in animals, and do not appear to be mediated by differences in androgen levels.

Birdsong signals individual diversity at the major histocompatibility complex.

The major histocompatibility complex (MHC) plays a key role in vertebrate immunity, and pathogen-mediated selection often favours certain allelic combinations. Assessing potential mates' MHC profiles may provide receivers with genetic benefits (identifying MHC-compatible mates and producing optimally diverse offspring) and/or material benefits (identifying optimally diverse mates capable of high parental investment). Oscine songbirds learn songs during early life, such that song repertoire content can reflect population of origin while song complexity can reflect early life condition. Thus birdsong may advertise the singer's genetic dissimilarity to others in the population (and, presumably, compatibility with potential mates), or individual genetic diversity (and thus condition-dependent material benefits). We tested whether song repertoire content and/or complexity signal MHC class IIß dissimilarity and/or diversity in male song sparrows (Melospiza melodia). Pairwise dissimilarity in repertoire content did not predict MHC dissimilarity between males, suggesting that locally rare songs do not signal rare MHC profiles. Thus, geographical variation in song may not facilitate MHC-mediated inbreeding or outbreeding. Larger repertoires were associated with intermediate MHC diversity, suggesting intermediate rather than maximal MHC diversity is optimal. This could reflect trade-offs between resisting infection and autoimmune disorders. Song complexity may advertise optimal MHC diversity, a trait affecting disease resistance and capacity for parental care.

Chemical composition of preen wax reflects major histocompatibility complex similarity in songbirds.

In jawed vertebrates, genes of the major histocompatibility complex (MHC) play a key role in immunity by encoding cell-surface proteins that recognize and bind non-self antigens. High variability at MHC suggests that these loci may also function in social signalling such as mate choice and kin recognition. This requires that MHC genotype covaries with some perceptible phenotypic trait. In mammals and fish, MHC is signalled chemically through volatile and non-volatile peptide odour cues, facilitating MHC-dependent mate choice and other behaviours. In birds, despite evidence for MHC-dependent mating, candidate mechanisms for MHC signalling remain largely unexplored. However, feather preen wax has recently been implicated as a potential source of odour cues. We examined whether the chemical composition of preen wax correlates with MHC class IIß genotypes of wild song sparrows (Melospiza melodia). Pairwise chemical distance reflected amino acid distance at MHC for male-female dyads, although not for same-sex dyads. Chemical diversity did not reflect MHC diversity. We used gas chromatography-mass spectrometry (GC-MS) to characterize preen wax compounds, and identified four wax esters that best reflect MHC similarity. Provided songbirds can detect variation in preen wax composition, this cue may allow individuals to assess MHC compatibility of potential mates.

Haemoproteus infected birds have increased lifetime reproductive success.

The impact of haematozoan infection on host fitness has received substantial attention since Hamilton and Zuk posited that parasites are important drivers of sexual selection. However, short-term studies testing the assumption that these parasites consistently reduce host fitness in the wild have produced contradictory results. To address this complex issue, we conducted a long-term study examining the relationship between naturally occurring infection with Haemoproteus and Plasmodium, and lifetime reproductive success and survival of Mountain White-crowned Sparrows. Specifically, we tested the hypothesis that birds infected with haematozoan parasites have reduced survival (as determined by overwinter return rates) and reproductive success. Contrary to expectation, there was no relationship between Haemoproteus and Plasmodium infection and reproduction or survival in males, nor was there a relationship between Plasmodium infection and reproduction in females. Interestingly, Haemoproteus-infected females had significantly higher overwinter return rates and these females fledged more than twice as many chicks during their lifetimes as did uninfected females. We discuss the impact of parasitic infections on host fitness in light of these findings and suggest that, in the case of less virulent pathogens, investment in excessive immune defence may decrease lifetime reproduction.

Stress, song and survival in sparrows.

The stress response--increases in circulating glucocorticoids following a stressor--is typically considered adaptive, but few studies address the fitness consequences of individual variation in stress response. Generally, due to negative consequences of prolonged elevation of glucocorticoids, animals should have a transient stress response just sufficient to cope with the stressor. In rodents, stress responsiveness is affected by early developmental experience, and hyper-responsiveness to stress is linked to morbidity and mortality. We assessed individual variation in stress responses in free-living song sparrows, Melospiza melodia, in relation to fitness-related measures including song and overwinter survival. Birds with greater increases in corticosterone 30 min following restraint stress were less likely to return to breed the following year. Stress responsiveness was also correlated with song complexity: males with fewer syllables in their song repertoires had greater stress reactivity. Our findings support the hypothesis that developmental stressors both impair song development and affect the adult stress response. Thus, individual variation in the stress response may relate to variation in fitness.

Immediate early gene response to hearing song correlates with receptive behavior and depends on dialect in a female songbird.

Stimulus-induced expression of the immediate early gene ZENK (egr-1) in the songbird's auditory forebrain presumably depends on the behavioral significance of the stimulus. Few studies, however, have quantified both the ZENK and behavioral responses to a stimulus in the same individuals. We played conspecific male song of either hatch (local) or foreign dialect to female white-crowned sparrows (Zonotrichia leucophrys oriantha) and quantified both the auditory ZENK response and their behavioral response, which is known to depend on dialect. Birds hearing hatch dialect showed greater ZENK induction in the caudomedial hyperstriatum ventrale and the dorsal portion of the caudomedial neostriatum than birds hearing foreign dialect, supporting previous work showing a relationship between ZENK and salience of the stimulus. In the dorsal portion of the caudomedial neostriatum, ZENK induction was correlated with the amount of non-vocal courtship behavior; however, in the caudomedial hyperstriatum ventrale, ZENK induction was more highly correlated with the females' own vocal behavior and thus may have been partly self-induced. Some females sang and showed a male-like pattern of ZENK induction in their song systems. This study provides the first evidence that the ZENK response in a sensory area to a social stimulus is proportional to the animal's preference for the stimulus.

Cultural and genetic evolution in mountain white-crowned sparrows: song dialects are associated with population structure.

Bird song often varies geographically within a species; when this geographic variation has distinct boundaries, the shared song types are referred to as song dialects. How dialects are produced and their adaptive significance are longstanding problems in biology, with implications for the role of culture in the evolution and ecology of diverse organisms, including humans. Here we test the hypothesis that song dialect, a culturally transmitted trait, is related to the population genetic structure of mountain white-crowned sparrows (Zonotrichia leucophrys oriantha). To address this, we compared microsatellite allele frequencies from 18 sample sites representing eight dialect regions in the Sierra Nevada. Pairwise genetic distances were not significantly correlated with geographic distances either within or between dialects, nor did dialect groups form distinct genetic groups according to neighbor-joining or UPGMA analysis, and most variation in allele frequencies occurred among individuals rather than at higher levels. However, most of the remaining variation was attributable to differences among, rather than within, dialect regions, and this among-dialect component of variance was statistically significant. Moreover, when controlling for the effect of geographic distance, song dissimilarity and genetic distance between site pairs were significantly correlated. Thus, song dialects appear to be associated with reductions in, but not strict barriers to, gene flow among dialect regions.