Manipulation of a social signal affects DNA methylation of a stress-related gene in a free-living bird.

Social status directly affects the health of humans and other animals. Low status individuals receive more antagonistic encounters, have fewer supportive relationships and have worse health outcomes. However, the physiological and cellular processes that mediate the relationship between the social environment and health are incompletely known. Epigenetic regulation of the hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine pathway that activates in response to stressors, may be one process that is sensitive to the social environment. Here, we experimentally manipulated plumage, a key social signal in female tree swallows (Tachycineta bicolor) and quantified methylation of four genes in the HPA axis before and after treatment. We found that dulling the white breast plumage affected methylation in one gene, CRHR1; however, the effect depended on the original brightness of the bird. Methylation in this gene was correlated with baseline corticosterone levels, suggesting that DNA methylation of CRHR1 helps regulate glucocorticoid production in this species. Methylation in two other genes, FKBP5 and GR, changed over the course of the experiment, independent of treatment. These results show that methylation of these genes is labile into adulthood and suggest that epigenetic regulation of the HPA axis could help birds respond to current environmental conditions.

Corticosterone exposure is associated with long-term changes in DNA methylation, physiology and breeding decisions in a wild bird.

When facing challenges, vertebrates activate a hormonal stress response that can dramatically alter behaviour and physiology. Although this response can be costly, conceptual models suggest that it can also recalibrate the stress response system, priming more effective responses to future challenges. Little is known about whether this process occurs in wild animals, particularly in adulthood, and if so, how information about prior experience with stressors is encoded. One potential mechanism is hormonally mediated changes in DNA methylation. We simulated the spikes in corticosterone that accompany a stress response using non-invasive dosing in tree swallows (Tachycineta bicolor) and monitored the phenotypic effects 1 year later. In a subset of individuals, we characterized DNA methylation using reduced representation bisulfite sequencing shortly after treatment and a year later. The year after treatment, experimental females had stronger negative feedback and initiated breeding earlier-traits that are associated with stress resilience and reproductive performance in our population-and higher baseline corticosterone. We also found that natural variation in corticosterone predicted patterns of DNA methylation. Finally, corticosterone treatment influenced methylation on short (1-2 weeks) and long (1 year) time scales; however, these changes did not have clear links to functional regulation of the stress response. Taken together, our results are consistent with corticosterone-induced priming of future stress resilience and support DNA methylation as a potential mechanism, but more work is needed to demonstrate functional consequences. Uncovering the mechanisms linking experience with the response to future challenges has implications for understanding the drivers of stress resilience.

The effects of humidity on thermoregulatory physiology of a small songbird.

Scholander-Irving curves describe the relationship between ambient temperature and metabolic rate and are fundamental to understanding the energetic demands of homeothermy. However, Scholander-Irving curves are typically measured in dry air, which is not representative of the humidity many organisms experience in nature. Consequently, it is unclear (1) whether Scholander-Irving curves (especially below thermoneutrality) are altered by humidity, given the effects of humidity on thermal properties of air, and (2) whether physiological responses associated with Scholander-Irving curves in the lab reflect organismal performance in humid field conditions. We used laboratory experiments and biophysical models to test the effects of humidity on the thermoregulatory physiology of tree swallows (Tachycineta bicolor). We also tested whether physiological responses measured under lab conditions were correlated with field body temperatures and nestling provisioning rates. We found that humidity reduced rates of evaporative water loss but did not have large effects on body temperature or metabolic rate, suggesting that swallows can decouple evaporative cooling, body temperature and metabolic rate. Although the effect of humidity on metabolic rate in the lab was small, our biophysical models indicated that energetic costs of thermoregulation were ∼8% greater in simulations that used metabolic rates from birds in humid compared with dry conditions. Finally, we found mixed evidence that physiological responses measured in the lab under humid or dry conditions were associated with body temperature and nest provisioning rates in the field. Our results help clarify the effect of humidity on endotherm thermoregulation, which may help forecast organismal responses to environmental change.

Maternal social environment shapes yolk testosterone allocation and embryonic neural gene expression in tree swallows.

Offspring from females breeding in competitive social environments are often exposed to more testosterone (T) during embryonic development, which can affect traits from growth to behavior in potentially adaptive ways. Despite the important role of maternally derived steroids in shaping offspring development, the molecular mechanisms driving these processes are currently unclear. Here, we use tree swallows (Tachycineta bicolor) to explore the effects of the maternal social environment on yolk T concentrations and genome-wide patterns of neural gene expression in embryos. We measured aggressive interactions among females breeding at variable densities and collected their eggs at two timepoints, including the day laid to measure yolk T concentrations and on embryonic day 11 to measure gene expression in whole brain samples. We found that females breeding in high-density sites experienced elevated rates of physical aggression and their eggs had higher yolk T concentrations. A differential gene expression and weighted gene co-expression network analysis indicated that embryos from high-density sites experienced an upregulation of genes involved in hormone, circulatory, and immune processes, and these gene expression patterns were correlated with yolk T levels and aggression. Genes implicated in neural development were additionally downregulated in embryos from high-density sites. These data highlight how early neurogenomic processes may be affected by the maternal social environment, giving rise to phenotypic plasticity in offspring.

Developmental stage-dependent effects of perceived predation risk on nestling tree swallows (Tachycineta bicolor).

The risk of predation directly affects the physiology, behavior, and fitness of wild birds. Strong social connections with conspecifics could help individuals recover from a stressful experience such as a predation event; however, competitive interactions also have the potential to exacerbate stress. Few studies have investigated the interaction between environmental stressors and the social landscape in wild bird populations. In 2 years of field studies, we experimentally simulated predation attempts on breeding female tree swallows (Tachicyneta bicolor). At the same time, we manipulated female breast plumage color, a key social signal. Simulated predation events on tree swallows early in the nestling period reduced young nestlings' mass by approximately 20% and shortened telomere lengths. Ultimately, only 31% of nestlings in the predation group fledged compared with 70% of control nestlings. However, the effects of experimental manipulations were timing dependent: the following year when we swapped the order of the experimental manipulations and simulated predation during incubation, there were no significant effects of predation on nestling condition or fledging success. Contrary to our expectations, manipulation of the social environment did not affect the response of tree swallows to simulated predation. However, manipulating female plumage during the nestling period did reduce nestling skeletal size and mass, although the effects depended on original plumage brightness. Our data demonstrate that transient stressors on female birds can have carry-over effects on their nestlings if they occur during critical periods in the breeding season.

Phylogeny and historical biogeography of the swallow family (Hirundinidae) inferred from comparisons of thousands of UCE loci.

Swallows (Hirundinidae) are a globally distributed family of passerine birds that exhibit remarkable similarity in body shape but tremendous variation in plumage, sociality, nesting behavior, and migratory strategies. As a result, swallow species have become models for empirical behavioral ecology and evolutionary studies, and variation across the Hirundinidae presents an excellent opportunity for comparative analyses of trait evolution. Exploiting this potential requires a comprehensive and well-resolved phylogenetic tree of the family. To address this need, we estimated swallow phylogeny using genetic data from thousands of ultraconserved element (UCE) loci sampled from nearly all recognized swallow species. Maximum likelihood, coalescent-based, and Bayesian approaches yielded a well-resolved phylogenetic tree to the generic level, with minor disagreement among inferences at the species level, which likely reflect ongoing population genetic processes. The UCE data were particularly useful in helping to resolve deep nodes, which previously confounded phylogenetic reconstruction efforts. Divergence time estimates from the improved swallow tree support a Miocene origin of the family, roughly 13 million years ago, with subsequent diversification of major groups in the late Miocene and Pliocene. Our estimates of historical biogeography support the hypothesis that swallows originated in the Afrotropics and have subsequently expanded across the globe, with major in situ diversification in Africa and a secondary major radiation following colonization of the Neotropics. Initial examination of nesting and sociality indicates that the origin of mud nesting - a relatively rare nest construction phenotype in birds - was a major innovation coincident with the origin of a clade giving rise to over 40% of extant swallow diversity. In contrast, transitions between social and solitary nesting appear less important for explaining patterns of diversification among swallows.

Pathogen Surveillance in Swallows (family Hirundinidae): Investigation into Role as Avian Influenza Vector in Eastern Canada Agricultural Landscapes.

First detected in Atlantic Canada in December 2021, highly pathogenic avian influenza virus (HPAIV) subtype H5N1 clade 2.3.4.4b, A/Goose/Guangdong/1/96 lineage, has caused massive mortality in wild birds and domestic poultry in North America. Swallows (Hirundinidae), abundant in North American agricultural ecosystems, have been proposed as possible (bridge) species for HPAIV transmission between wild and domestic birds. We aimed to seek evidence of the potential role of swallows in bridging AIV infection between wild bird reservoirs and poultry flocks in eastern Canada. During a wide-scale outbreak of HPAIV in wild birds and poultry farms across eastern Canada, 200 samples were collected from swallow breeding sites in the Canadian provinces of New Brunswick, Nova Scotia, Ontario, and Quebec, June-August 2022. Samples came from Barn Swallow (Hirundo rustica; n=142), Tree Swallow (Tachycineta bicolor; n=56), and Cliff Swallow (Petrochelidon pyrrhonota; n=2) nests. All samples tested negative for AIV, suggesting that HPAIV and low pathogenic AIV (LPAIV) strains were probably not circulating widely in swallows during the 2022 breeding season in eastern Canada; thus swallows may present a low risk of transmitting AIV. Within a management context, these findings suggest that removing nests of Barn Swallows, a species at risk in Canada, from the exterior of biosecure domestic poultry facilities may not significantly reduce risks of HPAI transmission to poultry.

Colonial breeding impacts potentially fitness-relevant cognitive processes in barn swallows.

Many animals breed colonially, often in dense clusters, representing a complex social environment with cognitive demands that could ultimately impact individual fitness. However, the effects of social breeding on the evolution of cognitive processes remain largely unknown. We tested the hypothesis that facultative colonial breeding influences attention and decision-making. Barn swallows (Hirundo rustica) breed in solitary pairs or in a range of colony sizes, up to dozens of pairs. We tested for selective attention to social information with playbacks of conspecific alarm calls and for decision-making with simulated predator intrusions, across a range of colony sizes from 1 to 33 pairs. We also evaluated the adaptive value of both processes by measuring seasonal reproductive success. Swallows breeding in larger colonies were more selective in their attention to social information. Birds breeding in larger colonies were also less risk averse, deciding to return more quickly to their nests after a predator approach paradigm. Finally, birds that showed higher selective attention hatched more eggs and birds that returned to their nests more quickly after a predator intrusion had more nestlings. Although we cannot fully attribute these fitness outcomes to the cognitive measures considered in this study, our results suggest that social breeding plays a role in adaptively shaping both the acquisition of social information and decision-making.

Experimentally elevated corticosterone does not affect bacteria killing ability of breeding female tree swallows (Tachycineta bicolor).

The immune system can be modulated when organisms are exposed to acute or chronic stressors. Glucocorticoids (GCs), the primary hormonal mediators of the physiological stress response, are suspected to play a crucial role in immune modulation. However, most evidence of stress-associated immunomodulation does not separate the effects of glucocorticoid-dependent pathways from those of glucocorticoid-independent mechanisms on immune function. In this study, we experimentally elevated circulating corticosterone, the main avian glucocorticoid, in free-living female tree swallows (Tachycineta bicolor) for one to two weeks to test its effects on immune modulation. Natural variation in bacteria killing ability (BKA), a measure of innate constitutive immunity, was predicted by the interaction between timing of breeding and corticosterone levels. However, experimental elevation of corticosterone had no effect on BKA. Therefore, even when BKA is correlated with natural variation in glucocorticoid levels, this relationship may not be causal. Experiments are necessary to uncover the causal mechanisms of immunomodulation and the consequences of acute and chronic stress on disease vulnerability. Findings in other species indicate that acute increases in GCs can suppress BKA; but our results support the hypothesis that this effect does not persist over longer timescales, during chronic elevations in GCs. Direct comparisons of the effects of acute vs. chronic elevation of GCs on BKA will be important for testing this hypothesis.

Exposure to and Biomarker Responses From Legacy and Emerging Contaminants Along Three Drainages in the Milwaukee Estuary, Wisconsin, USA.

Legacy contaminants and contaminants of emerging concern (CECs) were assessed in tree swallow (Tachycineta bicolor) tissue and diet samples from three drainages in the Milwaukee estuary, Wisconsin, USA, to understand exposures and possible biomarker responses. Two remote Wisconsin lakes were assessed for comparative purposes. Bioaccumulative classes of contaminants, such as polybrominated diphenyl ethers and per- and polyfluoroalkyl substances, while at higher concentrations than the reference lakes, did not vary significantly among sites or among the three drainages. Polycyclic aromatic hydrocarbons were assessed in diet and sediment and were from primarily pyrogenic sources. Ten biomarkers were assessed relative to contaminant exposure. Polychlorinated biphenyls (PCBs) were elevated above reference conditions at all Milwaukee sites but did not correlate with any measured biomarker responses. Only one site, Cedarburg, just downstream from a Superfund site, had elevated PCBs compared to other sites in the Milwaukee estuary. Few non-organochlorine insecticides or herbicides were detected in tree swallow liver tissue, except for the atrazine metabolite desethylatrazine. Few pharmaceuticals and personal care products were detected in liver tissue except for N,N-diethyl-meta-toluamide, iopamidol, and two antibiotics. The present study is one of the most comprehensive assessments to date, along with the previously published Maumee River data, on the exposure and effects of a wide variety of CECs in birds. Environ Toxicol Chem 2024;43:856-877. © 2024 SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Nightly reductions in body temperature and effect of transmitter attachment method in free-living welcome swallows (Hirundoneoxena).

Small birds and mammals face similar energetic challenges, yet use of torpor to conserve energy while resting is considered less common among birds, especially within the most specious order Passeriformes. We conducted the first study to record the natural thermoregulatory physiology of any species from the family Hirundinidae, which we predicted would use torpor because of their specialised foraging by aerial pursuit of flying insects, that are less active during cold or windy weather. We used temperature telemetry on wild-living welcome swallows (Hirundo neoxena, 13 to 17 g) and found that skin temperature declined during nightly resting by an average by 5 °C, from daytime minima of 41.0 ± 0.8 °C to nightly minima of 36.3 ± 0.8 °C, and by a maximum of 8 °C to a minimum recorded skin temperature of 32.0 °C. The extent of reduction in skin temperature was greater on cold nights and following windy daytime (foraging) periods. Further, we found that transmitters glued directly to the skin between feather tracts (i.e., an apterium) provided a less variable and probably also more accurate reflection of body temperature than transmitters applied over closely trimmed feathers. A moderate decrease in skin temperature, equivalent to shallow torpor, would provide energy savings during rest. Yet, deeper torpor was not observed, despite a period of extreme rainfall that presumedly decreased foraging success. Further studies are needed to understand the resting thermoregulatory energetics of swallows under different environmental conditions. We advocate the importance of measuring thermal biology in wild-living birds to increase our knowledge of the physiology and ecological importance of torpor among passerine birds.

Digest: Islands promote population differentiation in dispersive swallows.

How can highly dispersive species give rise to genetically distinct populations? This seemingly paradoxical pattern is common among insular birds, but not in those with continental distributions. Broyles and Myers sequence the genomes of almost 150 individuals from the island-dwelling Pacific swallow (Hirundo tahitica) and its continental counterpart, the welcome swallow (H. neoxena). They find strong population structure only among island populations and attribute this to a behavioral reduction in dispersal propensity following island colonization. However, wing shape remains consistent across populations, suggesting it might not accurately reflect dispersal propensity in this group. This study illustrates the interplay between dispersal, isolation, and divergence, offering insights into how geographic factors affect speciation and population differentiation on islands.

The relationship between nest location selection of Barn swallows (Hirundo rustica) and human activity and residence.

We found that barn swallow (Hirundo rustica) breeding occurs within close proximity to humans. An evaluation of barn swallow breeding frequency and the breeding success rate of swallows at research sites, which were buildings inhabited by humans and buildings where humans had previously resided, was conducted in order to establish a relationship between the location of barn swallow nests and human habitation and activity frequency. The results demonstrated that barn swallows often breed in human-inhabited buildings. No significant relationship was observed between the wall material and the direction of the wall and the type of building, whereas a much higher proportion of the nests were located near doors with a high level of human movement. In addition, no significant correlation was observed between the location of the nest and the distance from potential resources (food, water etc.), however, a statistically significant relationship was observed between the frequency of human activity measured through the video camera and the number of nests located at a certain distance. The average number of offspring and the reproductive success rate were higher in nests located within close proximity to human activity compared to nests not located within close proximity to human activity, suggesting that the presence of humans had a positive effect on reproduction. This study show that barn swallow nesting occurs in locations where there is a human influence and humans provide implicit protection of swallows from predation, which has a significant impact on breeding.

Evolutionarily labile dispersal behavior and discontinuous habitats enhance population differentiation in island versus continentally distributed swallows.

The causes of population divergence in vagile groups remain a paradox in evolutionary biology: dispersive species should be able to colonize new areas, a prerequisite for allopatric speciation, but dispersal also facilitates gene flow, which erodes population differentiation. Strong dispersal ability has been suggested to enhance divergence in patchy habitats and inhibit divergence in continuous landscapes, but empirical support for this hypothesis is lacking. Here we compared patterns of population divergence in a dispersive clade of swallows distributed across both patchy and continuous habitats. The Pacific Swallow (Hirundo tahitica) has an insular distribution throughout Southeast Asia and the Pacific, while its sister species, the Welcome Swallow (H. neoxena), has a continental distribution in Australia. We used whole-genome data to demonstrate strong genetic structure and limited introgression among insular populations, but not among continental populations. Demographic models show that historic changes in habitat connectivity have contributed to population structure within the clade. Swallows appear to exhibit evolutionarily labile dispersal behavior in which they reduce dispersal propensity after island colonization despite retaining strong flight ability. Our data support the hypothesis that fragmented habitats enhance population differentiation in vagile groups, and suggest that labile dispersal behavior is a key mechanism underlying this pattern.

Defensive tolerance to parasitism is correlated with sexual selection in swallows.

Parasite-mediated sexual selection has been the topic of extensive research and enthusiastic debate for more than three decades. Here, we suggest that secondary sexual characters may not only signal parasite resistance but also defensive tolerance. We exemplify this possibility by analysing information on two sexually selected traits, annual reproductive success, and ectoparasitism in a barn swallow Hirundo rustica population followed for more than 30 years. For each individual, we estimated the slope of the association between reproductive success and parasitism as an index of tolerance and subsequently explored the association with the expression of the sexually selected traits. In accordance with expectations of parasites playing a role in sexual selection, tail length was negatively related to load of chewing lice and nest size was positively related to tolerance to chewing lice. We discuss the importance of considering defensive tolerance for understanding the role of parasite-mediated sexual selection.

Social signal manipulation and environmental challenges have independent effects on physiology, internal microbiome, and reproductive performance in tree swallows (Tachycineta bicolor).

The social environment that individuals experience appears to be a particularly salient mediator of stress resilience, as the nature and valence of social interactions are often related to subsequent health, physiology, microbiota, and overall stress resilience. Relatively few studies have simultaneously manipulated the social environment and ecological challenges under natural conditions. Here, we report the results of experiments in wild tree swallows (Tachycineta bicolor) in which we manipulated both ecological challenges (predator encounters and flight efficiency reduction) and social interactions (by experimental dulling of a social signal). In two experiments conducted in separate years, we reversed the order of these treatments so that females experienced either an altered social signal followed by a challenge or vice-versa. Before, during, and after treatments were applied, we tracked breeding success, morphology and physiology (mass, corticosterone, and glucose), nest box visits via an RFID sensor network, cloacal microbiome diversity, and fledging success. Overall, we found that predator exposure during the nestling period reduced the likelihood of fledging and that signal manipulation sometimes altered nest box visitation patterns, but little evidence that the two categories of treatment interacted with each other. We discuss the implications of our results for understanding what types of challenges and what conditions are most likely to result in interactions between the social environment and ecological challenges.

A novel method for extraction and quantification of feather triiodothyronine (T3) and application to ecotoxicology of Purple Martin (Progne subis).

Seventy-three percent of aerial insectivore species of birds breeding in North America have declined in the past five years. This decline is even greater in migratory insectivorous species, which face stressors in both their breeding and non-breeding ranges. The Purple Martin (Progne subis) is an aerial insectivore swallow that overwinters in South America and migrates to North America to breed. Purple Martin populations have declined by an estimated 25% since 1966. The eastern subspecies (P. subis subis) has declined the most and overwinters in the Amazon Basin, a region rich in environmental mercury (Hg) contamination. Previous studies reported elevated levels of Hg in feathers of this subspecies, which correlated negatively with body mass and fat reserves. Given the propensity of Hg to disrupt the endocrine system, and the role of thyroid hormones in regulating fat metabolism, this study quantifies concentrations of Hg and the thyroid hormone triiodothyronine (T3) in the feathers of P. subis subis. To our knowledge, this is the first study to extract and quantify T3 in feathers; thus, we developed, tested, and optimized a method for extracting T3 from feather tissue and validated an enzyme immunoassay (EIA) to quantify T3 in Purple Martin feathers. The developed method yielded acceptable results for both parallelism and accuracy. The observed T3 concentrations were statistically modeled along with total Hg (THg) concentrations, but these variables were not significantly correlated. This suggests that the observed variation in THg concentration may be insufficient to cause a discernible change in T3 concentration. Furthermore, the observed effect of breeding location on feather T3 concentration might have obscured any effect of Hg.

Females with Increased Costs Maintain Reproductive Output: A Field Experiment in a Common Songbird.

Reproduction and self-maintenance are energetically costly activities involved in classic life history trade-offs. However, few studies have measured the responses of wild organisms to simultaneous changes in reproductive and self-maintenance costs, which may have interactive effects. In free-living female Barn Swallows (Hirundo rustica), we simultaneously manipulated reproductive costs (by adding or removing two nestlings) and self-maintenance costs (by attaching a ∼1 g weight in the form of a GPS tag to half of our study birds) and measured mass, immune status, blood glucose, feather growth, and reproductive output (likelihood of a second clutch, number of eggs, and time between clutches). GPS tags allowed us to analyze how movement range size affected response to brood size manipulation. Tagging altered females' immune function as evidenced by an elevated heterophil to lymphocyte (H:L) ratio, but all females were equally likely to lay more eggs. There was no evidence of interactive effects of the tagging and brood size treatment. Range size was highly variable, and birds with large ranges grew feathers more slowly, but analyzing the effect of brood size manipulation while accounting for variation in range size did not result in any physiological response. Our results support the theoretical prediction that short-lived vertebrates do face a trade-off between reproduction and self-maintenance and, when faced with increased costs, tend to preserve investment in reproduction at the expense of parental condition. This experiment also helps us to understand how movement patterns may be relevant to life history trade-offs in wild birds.

Candidatus Rickettsia Vini DNA in Ticks Collected from Nest Burrows of the European Sand Martin (Riparia riparia) in Sweden.

Background: Birds can cross geographical and environmental barriers and thereby facilitate dispersal of tick-borne pathogens both as carriers of infected ticks and as reservoirs of pathogenic microorganisms. Ixodes lividus (Ixodida: Ixodidae) is an endophilic tick in the Palearctic region that is highly specialized on its host, the European sand martin Riparia riparia. The purpose of this study was to determine whether I. lividus ticks sampled from sand martin nests in Sweden carry vector-borne pathogens. Materials and Methods: Fed ticks were collected in the autumns of 2017 and 2019 from the nests of a European sand martin colony in southern Sweden. Ticks were identified morphologically to developmental stage and species and were tested for tick-borne pathogens using PCR-based methods. Results: None of the 41 ticks tested positive for five tick-borne pathogens including Borrelia spp., tick-borne encephalitis virus (TBEV), Neoehrlichia mikurensis, Anaplasma phagocytophilum, and Babesia spp. Thirty-seven (13 females, 23 nymphs and 1 larva) of the 41 ticks tested positive for the gltA gene of Rickettsia spp. The sequences of the 17 kDa and gltA genes were most closely related to Candidatus Rickettsia vini. Conclusion: Our study confirms other reports that I. lividus ticks associated with the European sand martin have high infection prevalence of Ca. R. vini.

Elevated nest temperature has opposing effects on host species infested with parasitic nest flies.

Environmental factors, such as elevated temperature, can have varying effects on hosts and their parasites, which can have consequences for the net outcome of this relationship. The individual direct effects of temperature must be disentangled to determine the net-effect in host-parasite relationships, yet few studies have determined the net-effects in a multi-host system. To address this gap, we experimentally manipulated temperature and parasite presence in the nests of two host species infested by parasitic blowflies (Protocalliphora sialia). We conducted a factorial experiment by increasing temperature (or not) and removing all parasites (or not) in the nests of eastern bluebirds (Sialia sialis) and tree swallows (Tachycineta bicolor). We then measured nestling morphometrics, blood loss, and survival and quantified parasite abundance. We predicted that if temperature had a direct effect on parasite abundance, then elevated temperature would cause similar directional effects on parasite abundance across host species. If temperature had a direct effect on hosts, and therefore an indirect effect on the parasite, parasite abundance would differ across host species. Swallow nests with elevated temperature had fewer parasites compared to nests without temperature manipulation. In contrast, bluebird nests with elevated temperatures had more parasites compared to nests without temperature manipulation. The results of our study demonstrate that elevated temperature can have differential effects on host species, which can impact infestation susceptibility. Furthermore, changing climates could have complex net-effects on parasite fitness and host health across multi-host-parasite interactions.