Immune gene expression and epigenetic potential affect the consumption of risky food by female house sparrows.

When organisms move into new areas, they are likely to encounter novel food resources. Even if they are nutritious, these foods can also be risky, as they might be contaminated by parasites. The behavioural immune system of animals could help them avoid the negative effects of contaminated resources, but our understanding of behavioural immunity is limited, particularly whether and how behavioural immunity interacts with physiological immunity. Here, we asked about the potential for interplay between these two traits, specifically how the propensity of an individual house sparrow (Passer domesticus) to take foraging risks was related to its ability to regulate a key facet of its immune response to bacterial pathogens. Previously, we found that sparrows at expanding geographic range edges were more exploratory and less risk-averse to novel foods; in those same populations, birds tended to over-express Toll-like receptor 4 (TLR4), a pattern-recognition receptor that distinguishes cell-wall components of Gram-negative bacteria, making it the major sensor of potentially lethal gut microbial infections including salmonellosis. When we investigated how birds would respond to a typical diet (i.e., mixed seeds) spiked with domesticated chicken faeces, birds that expressed more TLR4 or had higher epigenetic potential for TLR4 (more CpG dinucleotides in the putative gene promoter) ate more food, spiked or not. Females expressing abundant TLR4 were also willing to take more foraging risks and ate more spiked food. In males, TLR4 expression was not associated with risk-taking. Altogether, our results indicate that behaviour and immunity covary among individual house sparrows, particularly in females where those birds that maintain more immune surveillance also are more disposed to take foraging risks.

"Submergence" of Western equine encephalitis virus: Evidence of positive selection argues against genetic drift and fitness reductions.

Understanding the circumstances under which arboviruses emerge is critical for the development of targeted control and prevention strategies. This is highlighted by the emergence of chikungunya and Zika viruses in the New World. However, to comprehensively understand the ways in which viruses emerge and persist, factors influencing reductions in virus activity must also be understood. Western equine encephalitis virus (WEEV), which declined during the late 20th century in apparent enzootic circulation as well as equine and human disease incidence, provides a unique case study on how reductions in virus activity can be understood by studying evolutionary trends and mechanisms. Previously, we showed using phylogenetics that during this period of decline, six amino acid residues appeared to be positively selected. To assess more directly the effect of these mutations, we utilized reverse genetics and competition fitness assays in the enzootic host and vector (house sparrows and Culex tarsalis mosquitoes). We observed that the mutations contemporary with reductions in WEEV circulation and disease that were non-conserved with respect to amino acid properties had a positive effect on enzootic fitness. We also assessed the effects of these mutations on virulence in the Syrian-Golden hamster model in relation to a general trend of increased virulence in older isolates. However, no change effect on virulence was observed based on these mutations. Thus, while WEEV apparently underwent positive selection for infection of enzootic hosts, residues associated with mammalian virulence were likely eliminated from the population by genetic drift or negative selection. These findings suggest that ecologic factors rather than fitness for natural transmission likely caused decreased levels of enzootic WEEV circulation during the late 20th century.

The causative effects of corticosterone on innate immunity during the stress response in the House Sparrow, Passer domesticus.

Stress-induced inhibition of innate immune activity has been observed in a variety of wild birds and may increase chances of infection because this activity constitutes the first line of defense against pathogens. We previously reported that the transient elevation of plasma corticosterone (CORT; the primary avian glucocorticoid) that occurs during stress is necessary for stress-induced suppression of natural antibody-mediated, complement-mediated, and bactericidal activity. Here, we further investigated the regulatory role of CORT during this suppression. To this end, we treated House Sparrows (Passer domesticus) with mitotane to block endogenous CORT production, administered CORT at one of three doses (HI: 1.34 mg/kg; LO: 1.00 mg/kg; CON: vehicle), and assessed natural antibody-mediated, complement-mediated, and bactericidal activity during acute stress induced by handling and restraint. Mitotane administration eliminated the endogenous plasma CORT increase that normally takes place during stress, and corticosterone treatment increased plasma CORT to levels similar to those measured in intact birds during acute stress. As predicted, mitotane-treated birds receiving CON injections did not exhibit stress-induced suppression of complement-mediated and bactericidal activity, and CORT administration at both LO and HI doses restored this suppression. Contrary to expectations, mitotane-treated birds receiving CON injections demonstrated stress-induced suppression of natural antibody-mediated activity. Furthermore, CORT administration did not influence this parameter. These results suggest that stress inhibits innate immune activity through both CORT-dependent and CORT-independent mechanisms, but the contribution of these mechanisms can vary. This variation may result from effects of environmental factors, the identity and role of which warrant further research.

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.

Glucocorticoid receptor selectively mediates stress-induced suppression of innate immunity in the House Sparrow, Passer domesticus.

Stress-induced inhibition of innate immune activity is widespread in free-ranging birds, but the mechanisms that are responsible for this inhibition are poorly understood. We previously demonstrated that an increase in plasma corticosterone (CORT), the primary avian glucocorticoid, is necessary for the inhibition of natural antibody- and complement-mediated as well as bactericidal activities to occur during stress. Here we investigated the role of glucocorticoid receptors in stress-induced inhibition of natural antibody- and complement-mediated activities and bactericidal activity within non-genomic (<10 min) and genomic (<120 min) time frames in male House Sparrows, Passer domesticus. Treatment with the selective glucocorticoid receptor antagonist mifepristone (RU486) attenuated stress-induced suppression of natural antibody-mediated activity within 10 min and 120 min of experimental stress. By contrast, this treatment did not influence stress-induced suppression of complement-mediated or bactericidal activity. These results suggest that stress-induced elevated plasma CORT inhibits natural antibody-mediated activity, but not complement-mediated or bactericidal activity, by activating glucocorticoid receptors, and that both non-genomic and genomic mechanisms underlie this activation. Additional research is needed to identify the receptors that regulate inhibitory effects of elevated plasma CORT on complement-mediated and bactericidal activity.

Parasite Tolerance and Host Competence in Avian Host Defense to West Nile Virus.

Competence, or the propensity of a host to transmit parasites, is partly underlain by host strategies to cope with infection (e.g., resistance and tolerance). Resistance represents the ability of hosts to prevent or clear infections, whereas tolerance captures the ability of individuals to cope with a given parasite burden. Here, we investigated (1) whether one easy-to-measure form of tolerance described well the dynamic relationships between host health and parasite burden, and (2) whether individual resistance and tolerance to West Nile virus (WNV) were predictable from single cytokine measures. We exposed house sparrows (HOSP) to WNV and measured subsequent changes in host performance, viral burden, and cytokine expression. We then used two novel approaches (one complex, one simpler) to estimate tolerance within-individual HOSP using four separate host performance traits. We lastly investigated changes in the expression of pro-inflammatory cytokine interferon-γ (IFN-γ) and anti-inflammatory cytokine interleukin-10 (IL-10). Both approaches to estimating tolerance were equivalent among WNV-infected HOSP; thus, an easy-to-measure tolerance estimation may be successfully applied in field studies. Constitutive expression of IFN-γ and IL-10 were predictive of resistance and tolerance to WNV, implicating these cytokines as viable biomarkers of host competence to WNV.

Experimental increase in baseline corticosterone level reduces oxidative damage and enhances innate immune response.

Glucocorticoid (GC) hormones are significant regulators of homeostasis. The physiological effects of GCs critically depend on the time of exposure (short vs. long) as well as on their circulating levels (baseline vs. stress-induced). Previous experiments, in which chronic and high elevation of GC levels was induced, indicate that GCs impair both the activity of the immune system and the oxidative balance. Nonetheless, our knowledge on how mildly elevated GC levels, a situation much more common in nature, might influence homeostasis is limited. Therefore, we studied whether an increase in GC level within the baseline range suppresses or enhances condition (body mass, hematocrit and coccidian infestation) and physiological state (humoral innate immune system activity and oxidative balance). We implanted captive house sparrows Passer domesticus with either 60 days release corticosterone (CORT) or control pellets. CORT-treated birds had elevated baseline CORT levels one week after the implantation, but following this CORT returned to its pre-treatment level and the experimental groups had similar CORT levels one and two months following the implantation. The mass of tail feathers grown during the initial phase of treatment was smaller in treated than in control birds. CORT implantation had a transient negative effect on body mass and hematocrit, but both of these traits resumed the pre-treatment values by one month post-treatment. CORT treatment lowered oxidative damage to lipids (malondialdehyde) and enhanced constitutive innate immunity at one week and one month post-implantation. Our findings suggest that a relatively short-term (i.e. few days) elevation of baseline CORT might have a positive and stimulatory effect on animal physiology.

Measuring short-term stress in birds: Comparing different endpoints of the endocrine-immune interface.

Stress is a collective term for certain conditions and sequences of physiological events enabling living organisms to overcome unpredictable and uncontrollable situations. The context-dependent nature, multidimensional course and large individual variability make stress responses difficult to measure. In avian species, a plethora of studies on short-term stress responses have been conducted by measuring the corticosteroid response to a standardized stress protocol. Here we aimed to test the viability of the leukocyte coping capacity (LCC), measuring oxygen radical production by leukocytes, to assess short-term stress in birds. We collected blood samples from captive house sparrows (Passer domesticus) in the two seasons of winter and spring, right after capture and 30min thereafter. In order to assess the overall physiological stress response to a standardized stressor, i.e. handling and temporary constraint, we measured LCC and additionally combined it with measures of total circulating glucocorticoids (GCs) and oxidative stress. All three methodologies detected significant changes due to the stressor albeit they were not correlated with each other and revealed different information. There was no significant change in oxidative stress levels between the two time points although the amount of relative oxidative damage as well as the anti-oxidative capacity changed significantly. We observed a significant seasonal difference in GC stress response with no difference between sexes. On the contrary, LCC measures revealed with a high individual consistency, that individuals experienced a similar magnitude of stress in both seasons with a significant difference between sexes. Total GC-levels have to be interpreted with caution regarding the assessment of short-term stress reactions. We therefore suggest to supplementary combine classical approaches for measuring stress with the immunological tool of LCC. Our results reveal LCC as a strong and reliable tool to assess short-term stress in captive house sparrows and as promising for other bird species. Collectively the study highlights the necessity to incorporate a range of physiological systems and their endpoints to measure and to assess stress reactions effectively.

Captivity influences immune responses, stress endocrinology, and organ size in house sparrows (Passer domesticus).

Studies using wild animals in laboratory-based research require bringing wild-captured organisms into a novel setting, which can have long-lasting impacts on physiology and behavior. In several species, captivity stimulates stress hormone production and can alter immune function. Despite this, there is little consensus on how captivity influences stress hormone regulation, or if captivity-induced changes in stress hormone production and regulation mediate changes in immune function. In this study, we investigate the influence of captivity on the physiology of a wild bird commonly-used in laboratory-based research, the house sparrow (Passer domesticus). We tested how captivity influences stress endocrinology, immune responses, and organ mass, and also investigated if the production or regulation of corticosterone, the main stress hormone in birds, correlated with changes in immunity. We found that baseline corticosterone concentrations and maximum capacity of the adrenals to secrete corticosterone increase following captivity and remain elevated after 9weeks of captivity. A measure of innate immune function, the bactericidal ability of plasma, also increased with time spent in captivity. Wound healing was also influenced by time spent in captivity, with birds taking almost 2days longer to heal if they were wounded after 3weeks in captivity when compared with birds that were wounded immediately upon capture. Additionally, captivity caused notable reductions in spleen and liver mass. Together, these results imply that captivity can have long-lasting effects on house sparrow corticosterone release and immune function, and suggest that even after 9weeks house sparrows do not acclimate physiologically to life in captivity.

Life-history dependent relationships between body condition and immunity, between immunity indices in male Eurasian tree sparrows.

In free-living animals, recent evidence indicates that innate, and acquired, immunity varies with annual variation in the demand for, and availability of, food resources. However, little is known about how animals adjust the relationships between immunity and body condition, and between innate and acquired immunity to optimize survival over winter and reproductive success during the breeding stage. Here, we measured indices of body condition (size-corrected mass [SCM], and hematocrit [Hct]), constitutive innate immunity (plasma total complement hemolysis activity [CH50]) and acquired immunity (plasma immunoglobulin A [IgA]), plus heterophil/lymphocyte (H/L) ratios, in male Eurasian tree sparrows (Passer montanus) during the wintering and the breeding stages. We found that birds during the wintering stage had higher IgA levels than those from the breeding stage. Two indices of body condition were both negatively correlated with plasma CH50 activities, and positively with IgA levels in wintering birds, but this was not the case in the breeding birds. However, there was no correlation between CH50 activities and IgA levels in both stages. These results suggest that the relationships between body condition and immunity can vary across life-history stage, and there are no correlations between innate and acquired immunity independent of life-history stage, in male Eurasian tree sparrows. Therefore, body condition indices predict immunological state, especially during the non-breeding stage, which can be useful indicators of individual immunocompetences for understanding the variations in innate and acquired immunity in free-living animals.

Costs of immunity and their role in the range expansion of the house sparrow in Kenya.

There are at least two reasons to study traits that mediate successful range expansions. First, dispersers will found new populations and thus impact the distribution and evolution of species. Second, organisms moving into new areas will influence the fate of resident communities, directly competing with or indirectly affecting residents by spreading non-native or spilling-back native parasites. The success of invaders in new areas is likely mediated by a counterbalancing of costly traits. In new areas where threats are comparatively rare, individuals that grow rapidly and breed prolifically should be at an advantage. High investment in defenses should thus be disfavored. In the present study, we compared the energetic, nutritional and collateral damage costs of an inflammatory response among Kenyan house sparrow (Passer domesticus) populations of different ages, asking whether costs were related to traits of individuals from three different capture sites. Kenya is among the world's most recent range expansions for this species, and we recently found that the expression of Toll-like receptors (TLRs), leukocyte receptors that instigate inflammatory responses when bound to microbial elements, was related to the range expansion across the country. Here, we found (contrary to our expectations) that energetic and nutritional costs of inflammation were higher, but damage costs were lower, in range-edge compared with core birds. Moreover, at the individual level, TLR-4 expression was negatively related to commodity costs (energy and a critical amino acid) of inflammation. Our data thus suggest that costs of inflammation, perhaps mediated by TLR expression, might mitigate successful range expansions.

Repeated immune challenges affect testosterone but not sperm quality.

Mounting an immunological response is energetically demanding and necessarily redirects allocation of resources toward immune system activation and away from other energetically expensive processes, such as reproduction. Lipopolysaccharide (LPS), a major component of the outer membrane of the cell wall of Gram-negative bacteria Escherichia coli, mimics a bacterial infection without producing the cost of replicating the pathogen and is one of the most commonly used agents to induce an acute phase immune response. Here, we ask if a trade-off can be induced between activation of the acute phase immune response and sperm function, a key indicator of sperm competitive ability. Further, we ask whether repeated exposure to this endotoxin in a social species such as the house sparrow (Passer domesticus), where repeated pathogen exposure may be common, may have a more pronounced effect. To address our questions, we exposed individuals to two rounds of LPS treatment or control, to mimic a repeated pathogen exposure in the wild. We predicted that repeated pathogen exposure would have detrimental effects on sperm quality, and therefore, reproductive success. We compared a measure of sperm quality (straight-line velocity) in captive male house sparrows between LPS-treated and control individuals. We found that although LPS treatment impaired circulating testosterone and induced a hypothermic state when compared with controls, it did not affect sperm quality within days or weeks following a single or repeated LPS exposure.

Immune profiles vary seasonally, but are not significantly related to migration distance or natal dispersal, in a migratory songbird.

A central tenet of ecoimmunology is that an organism's environment shapes its optimal investment in immunity. For example, the benefits of acquired (relatively pathogen specific) versus innate (nonspecific) immune defenses are thought to vary with the risk of encountering familiar versus unfamiliar pathogens. Because pathogen communities vary geographically, individuals that travel farther during seasonal migration or natal dispersal are predicted to have higher exposure to novel pathogens, and lower exposure to familiar pathogens, potentially favoring investment in innate immunity. During the breeding season, migratory animals' exposure to familiar pathogens should increase, potentially favoring investment in acquired immunity. We hypothesized that song sparrows Melospiza melodia adjust their constitutive immune profiles in response to risk of encountering novel versus familiar pathogens. We predicted that individuals migrating longer distances (inferred from stable hydrogen isotope analysis of claws) and less philopatric individuals (inferred from microsatellite assignment testing) would rely more heavily on acquired than innate defenses. We also predicted that reliance on acquired defenses would increase throughout the early breeding season. Consistent with trade-offs between acquired and innate defenses, levels of immunoglobulin Y (acquired) varied negatively with macrophage phagocytosis activity (innate). Levels of acquired relative to innate immunity did not vary significantly with migration distance or philopatry, but increased throughout the early breeding season. Macrophage phagocytosis was not significantly repeatable between years. Song sparrows appear to shift from innate defenses immediately after migration to acquired defenses with increasing time at the breeding grounds. These patterns highlight the plasticity of constitutive immune defenses in migratory animals.

Corticosterone rapidly suppresses innate immune activity in the house sparrow (Passer domesticus).

Stress-induced effects on innate immune activity in wild birds have been difficult to predict. These difficulties may arise from the frequent assumptions that (1) the stress response influences different components of the immune response similarly, (2) stress-induced effects do not change over the course of the stress response and (3) glucocorticoids are the primary regulators of stress-induced changes of immune activity. We tested the first two assumptions by measuring three components of innate immunity at two times during the stress response in captive adult male house sparrows, Passer domesticus Acute stress resulting from handling and restraint suppressed plasma lytic and microbicidal activity within 10 min and reduced plasma agglutination ability within 120 min. We tested the third assumption by measuring stress-induced effects in sparrows that were pharmacologically adrenalectomized by mitotane administration. Confirming the effectiveness of this treatment, mitotane-treated birds had lower pre-stress plasma CORT than control birds and showed no increase in plasma CORT during acute stress. The innate immune activity of mitotane-treated birds did not decrease during the stress response, but the pre-stress immune activity of these birds did not differ from that of vehicle-treated birds. These results suggest that elevated plasma CORT during stress is primarily responsible for mediating stress-induced suppression of innate immune activity.

Population Differences at MHC Do Not Explain Enhanced Resistance of Song Sparrows to Local Parasites.

Infectious disease represents an emerging threat to natural populations, particularly when hosts are more susceptible to novel parasites (allopatric) than to parasites from the local area (sympatric). This pattern could arise through evolutionary processes (host populations become adapted to their local parasites and genetically differentiated from other populations at immune-related loci) and/or through ecological interactions (host individuals develop resistance to local parasites through previous exposure). The relative importance of these candidate mechanisms remains unclear. In jawed vertebrates, genes of the major histocompatibility complex (MHC) play a fundamental role in immunity and are compelling candidates for spatially varying selection. We recently showed that song sparrows (Melospiza melodia) are more susceptible to allopatric than to sympatric strains of malaria (Plasmodium). In the current study, to determine whether population differences at MHC explain this pattern, we characterized the peptide-binding regions of MHC (classes I and II) of birds that did or did not become infected in the previous experiment. We recovered up to 4 alleles per individual at class I, implying at least 2 loci, and up to 26 alleles per individual at class II, implying at least 13 loci. Individuals with more class I alleles were less likely to become infected by Plasmodium, consistent with parasite-mediated balancing selection. However, we found no evidence for population genetic differentiation at either class of MHC, based on 36 individuals sequenced. Resistance to sympatric parasites previously described for this system likely stems from individuals' prior immune experience, not from population differentiation and locally protective alleles at MHC.

Seasonal Patterns and Relationships among Coccidian Infestations, Measures of Oxidative Physiology, and Immune Function in Free-Living House Sparrows over an Annual Cycle.

Temporal variation in oxidative physiology and its associated immune function may occur as a result of changes in parasite infection over the year. Evidence from field and laboratory studies suggests links between infection risk, oxidative stress, and the ability of animals to mount an immune response; however, the importance of parasites in mediating seasonal change in physiological makeup is still debated. Also, little is known about the temporal consistency of relationships among parasite infestation, markers of oxidative status and immune function in wild animals, and whether variation in oxidative measures can be viewed as a single integrated system. To address these questions, we sampled free-living house sparrows (Passer domesticus) every 2 mo over a complete year and measured infestation with coccidian parasites as well as nine traits that reflect condition, oxidative physiology, and immune function. We found significant seasonal variation in coccidian infestation and in seven out of nine condition and physiological variables over the year. However, we found little support for parasite-mediated change in condition, oxidative physiology, and immune functions in house sparrows. In accordance with this, we found no temporal consistency in relationships between the intensity of infestation and physiology. Among measures of oxidative physiology, antioxidants (measured as the total antioxidant capacity and the concentration of uric acid in the plasma) and oxidative damage (measured through the level of malondialdehyde in plasma) positively and consistently covaried over the year, while no such associations were found for the rest of traits (body mass, total glutathione, and leukocyte numbers). Our results show that natural levels of chronic coccidian infection have a limited effect on the seasonal change of physiological traits, suggesting that the variation of the latter is probably more affected by short-term disturbances, such as acute infection and/or season-specific stress stimuli.

Impacts of short-term food restriction on immune development in altricial house sparrow nestlings.

Food limitation is a common ecological scenario for nestling altricial birds, and reductions in growth and maintenance have been observed in resource-limited nestlings. Substantial development of the immune system occurs during the nestling period, yet the resource dependence of this immune development is understudied. We examined constitutive immune system development as well as acute-phase responses to lipopolysaccharide (LPS) injection after 48 h of food restriction in house sparrows at 7 and 13 d posthatch. We also examined nestlings that were restricted early (5-7 d) but refed and tested at 13 d posthatch to determine whether altered immune function and growth early in the nestling period were recovered upon return to adequate resource supply. Induced acute-phase protein response was reduced in food-restricted birds, yet no lasting reductions in acute-phase protein levels were observed in previously restricted nestlings that were challenged with LPS after refeeding. Food restriction did not significantly impact constitutive levels of complement-mediated lysis or circulating IgY antibodies. As a comparator to immune measures, we found that organ and tarsus size, as well as muscle size and citrate synthase enzyme activity (an index of muscle cellular aerobic capacity), were significantly reduced in food-restricted nestlings. Reductions in flight muscle mass and function persisted in birds refed after early food restriction, which may have contributed to persistent body temperature reductions observed in refed birds.

Early-life stress has sex-specific effects on immune function in adult song sparrows.

Multiple components of the immune system are modulated by environmental factors, including exposure to stressors. In particular, chronic stressors can impair development of the immune system, leading to alterations in immune function in adulthood. While these effects have been well established in mammals, less is known about how developmental stress modulates immunity in nonmammalian species. We determined the long-term effects of exposure to early-life stressors on immunity in song sparrows including the swelling response to phytohemagglutinin (PHA) and several measures of constitutive innate immunity. Song sparrows were reared in captivity from 3 d of age and exposed to control conditions, food restriction, or corticosterone (CORT) treatment. Males exposed to food restriction or CORT treatment had less swelling of the wing web in response to PHA than control males; however, neither treatment affected the swelling response to PHA in females. The treatments also had sex-specific effects on constitutive innate immune function. Specifically, CORT-treated males had lower antimicrobial capacity toward a strain of the bacterium E. coli but higher antimicrobial activity toward a strain of the fungus Candida albicans compared to food-restricted or control males. In contrast, neither treatment affected constitutive innate immunity in females. These results suggest that male and female song sparrows may differ in how they allocate resources to development of the immune system when reared in stressful or food-limited conditions.

A blurring of life-history lines: Immune function, molt and reproduction in a highly stable environment.

Rufous-collared sparrows (Zonotrichia capensis peruviensis) from valleys in the Atacama Desert of Chile, live in an extremely stable environment, and exhibit overlap in molt and reproduction, with valley-specific differences in the proportion of birds engaged in both. To better understand the mechanistic pathways underlying the timing of life-history transitions, we examined the relationships among baseline and stress-induced levels of corticosterone (CORT), testosterone, and bacteria-killing ability of the blood plasma (BKA), as well as haemosporidian parasite infections and the genetic structure of two groups of sparrows from separate valleys over the course of a year. Birds neither molting nor breeding had the lowest BKA, but there were no differences among the other three categories of molt-reproductive stage. BKA varied over the year, with birds in May/June exhibiting significantly lower levels of BKA than the rest of the year. We also documented differences in the direction of the relationship between CORT and BKA at different times during the year. The direction of these relationships coincides with some trends in molt and reproductive stage, but differs enough to indicate that these birds exhibit individual-level plasticity, or population-level variability, in coordinating hypothalamo-pituitary-adrenal axis activity with life-history stage. We found weak preliminary evidence for genetic differentiation between the two populations, but not enough to indicate genetic isolation. No birds were infected with haemosporidia, which may be indicative of reduced parasite pressure in deserts. The data suggest that these birds may not trade off among different life-history components, but rather are able to invest in multiple life-history components based on their condition.

Covariation in stress and immune gene expression in a range expanding bird.

The enemy release hypothesis (ERH) posits that hosts encounter fewer infectious parasites when they arrive in new areas, so individuals that adjust their immune defenses most effectively should thrive and even expand the range of that species. An important aspect of vertebrate immune defense is inflammation, as it provides rapid defense against diverse parasites. Glucocorticoids (GCs) are integral to the regulation of inflammation, so here we investigated whether and how covariation in the expression of genes affecting the regulation of inflammation and GCs might have impacted the house sparrow (Passer domesticus) invasion of Kenya. Toll-like receptors 2 and 4 (TLRs) detect microbial threats and instigate inflammatory responses, whereas the glucocorticoid receptor (GR) is integral to resolving inflammation via both local and systemic pathways. As with a previous study on circulating leukocytes, we found that splenic TLR-4 and TLR-2 (the latter marginally non-significant) expression was higher in younger than older populations but only when differences in spleen size were considered; birds at the range edge had larger spleens. In regards to covariation, we found that TLR-2, TLR-4 and GR expression were closely inter-related within individuals, but covariation did not differ among populations. Subsequently, our data suggest that house sparrows are using variants of a common stress-immune regulatory mechanism to expand their Kenyan range.