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.

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.

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.

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.

Brood size is associated with apparent telomere lengthening in nestling barn swallows.

Early life for animals is often a time of rapid growth and development. In a resource-limited environment, life history theory predicts that there must be trade-offs between resource sinks in ways that optimize future survival and reproductive success. Telomeres have emerged as putative indicators of these early life trade-offs, but there are conflicting accounts as to how developmental traits and conditions impact telomere length and dynamics. For 2 years, we studied the nestlings of a breeding population of barn swallows from day 6 to day 12 of life, measuring various ontogenetic factors to understand to what extent they explain variation in telomere length and dynamics. We unexpectedly found that telomeres lengthened between the two sampling points. Nestlings in large broods had shorter telomeres, but surprisingly, individuals that grew faster from day 6 to day 12 had longer telomeres and more telomere lengthening. Nestlings with higher mass relative to their nestmates on d6 had shorter telomeres, suggesting that the relatively fast growth barn swallows experience early in development is more costly than the relatively slower growth later in development. These effects were only found in the first year of study. Telomere lengthening may be due to the initiation of new hematopoietic cell lines during development or the expression of telomerase early in life. Favorable early life conditions and high parental investment could allow for more growth with little to no cost to telomere length or dynamics.

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.

Long-term trends in grassland bird relative abundance on focal grassland landscapes in Missouri.

North American grassland birds have widely declined over the past 50 years, largely due to anthropogenic-driven loss of native prairie habitat. In response to these declines, many conservation programs have been implemented to help secure wildlife habitat on private and public lands. The Grasslands Coalition is one such initiative established to advance the conservation of grassland birds in Missouri. The Missouri Department of Conservation conducted annual point count surveys for comparison of grassland bird relative abundance between focal grassland areas and nearby paired (i.e., containing no targeted management) sites. We analyzed 17 years of point count data with a generalized linear mixed model in a Bayesian framework to estimate relative abundance and trends across focal or paired sites for nine bird species of management interest that rely on grasslands: barn swallow (Hirundo rustica), brown-headed cowbird (Molothrus ater), dickcissel (Spiza americana), eastern meadowlark (Sturnella magna), grasshopper sparrow (Ammodramus savannarum), Henslow's sparrow (A. henslowii), horned lark (Eremophila alpestris), northern bobwhite (Colinus virginianus), and red-winged blackbird (Agelaius phoeniceus). Relative abundance of all species except eastern meadowlarks declined regionally. Relative abundance of barn swallows, brown-headed cowbirds, dickcissels, eastern meadowlarks, Henslow's sparrows, and northern bobwhites was higher in focal than paired sites, though relative abundance trends were only improved in focal vs. paired areas for dickcissels and Henslow's sparrows. Relative abundance increased with increasing grassland cover at the local (250-m radius) scale for all species except horned larks and red-winged blackbirds and at the landscape (2,500-m radius) scale for all species except dickcissels, eastern meadowlarks, and northern bobwhites. Our results suggest focal areas contained greater relative abundances of several grassland species of concern, likely due to increased availability of grassland habitat at local and landscape scales. Further efforts to decrease landscape-scale fragmentation and improve habitat quality may be needed to achieve conservation goals.

The detection of Mycoplasma sturni and Mycoplasma moatsii from the choana of a barn swallow (Hirundo rustica): a case report.

BACKGROUND: Mycoplasmas are found in many different species. Until now 26 avian mycoplasma species have been described, but in the most free ranging bird species the prevalence and significance of Mycoplasma spp. is still unclear. CASE PRESENTATION: In May 2021 a barn swallow (Hirundo rustica) was brought to a veterinary clinic after it hit a window. As part of the routine exam a choanal swab was taken for mycoplasma culture and for the detection of mycoplasmas using a Mycoplasma-genus-specific Polymerase chain reaction. Six single colony subcultures were obtained by the cultivation. Obtained subcultures were investigated by sequencing the 16S rRNA and the 16S-23S rRNA intergenic transcribed spacer region sequence. The 16S rRNA gene sequence from one subculture had a homology of 99.03% and the 16S-23S rRNA intergenic transcribed spacer region sequence of 100% with the sequence of Mycoplasma sturni. The 16S rRNA gene sequence from the other five subcultures shared a homology of 99.89% and the 16S-23S rRNA intergenic transcribed spacer region sequence of 99.81% with the sequence of Mycoplasma moatsii. CONCLUSIONS: According to the available literature this is the first report about the detection of M. moatsii, in the respiratory tract of a barn swallow. M. moatsii was previously only found in grivit monkeys (Cercopithecus aethiops), Norway rats (Rattus norvegicus) and a mute swan (Cygnus olor). The role of mycoplasmas in barn swallows is still unknown, especially as in the present case both mycoplasma species do not seem to cause clinical symptoms.

Genomic architecture of migration timing in a long-distance migratory songbird.

The impact of climate change on spring phenology poses risks to migratory birds, as migration timing is controlled predominantly by endogenous mechanisms. Despite recent advances in our understanding of the underlying genetic basis of migration timing, the ways that migration timing phenotypes in wild individuals may map to specific genomic regions requires further investigation. We examined the genetic architecture of migration timing in a long-distance migratory songbird (purple martin, Progne subis subis) by integrating genomic data with an extensive dataset of direct migratory tracks. A moderate to large amount of variance in spring migration arrival timing was explained by genomics (proportion of phenotypic variation explained by genomics = 0.74; polygenic score R2 = 0.24). On chromosome 1, a region that was differentiated between migration timing phenotypes contained genes that could facilitate nocturnal flights and act as epigenetic modifiers. Overall, these results advance our understanding of the genomic underpinnings of migration timing.

The influence of environmental and ecological factors on the accumulation and distribution of short- and long-chain perfluoroalkyl acids in a mid-trophic avian insectivore.

Perfluoroalkyl acids (PFAAs) include perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), perfluorohexane sulfonic acid (PFHxS), and perfluorodecane sulfonic acid (PFDS), as well as increasingly used alternative short-chain perfluorosulfonic acids (PFSAs) and short- and long-chain (≥C9) perfluorocarboxylic acids (PFCAs). In the present study, tissues of tree swallows (Tachycineta bicolor) from two sites in southern Ontario, Canada, were analyzed for 17 individual PFAAs and showed egg and nestling tissue (liver, carcass) profiles dominated by PFOS (57-66%). The remaining PFAAs contributed ≤7% each, although collectively the long-chain PFCAs comprised 21-29% of the PFAAs. The short-chain PFSAs and PFCAs were among the lowest concentrations, suggesting that despite increased production and use of these alternative PFAAs, they are not accumulated to the same extent as the long-chain PFSAs and PFCAs. PFOS, PFDS, and some long-chain PFCAs were significantly higher in eggs than in livers and carcasses, whereas PFOA and the two short-chain PFCAs were significantly higher in nestling tissues than in eggs. For the two short-chain PFSAs, concentrations were similar among tissues. Tree swallow tissues at the site near a wastewater treatment plant (WWTP) outfall showed higher concentrations of PFOS, PFDS, PFHxS, and some long-chain PFCAs than tree swallows sampled at the nearby reference site; however, the influence of the WWTP was more equivocal for PFOA, other long-chain PFCAs, and short-chain PFSAs and PFCAs. Carbon stable isotopes (δ13C) and fatty acid signatures indicated that the diets of the WWTP swallows were more terrestrial than the reference swallows. Nonetheless, models considering environmental and ecological variables indicated that site was often the primary driver of PFAA variation among the swallows, with less or no influence of dietary patterns, or sex or body condition, revealing that of WWTP effluent can be an important environmental source of the major PFAAs in tree swallows.

A chromosome-level reference genome and pangenome for barn swallow population genomics.

Insights into the evolution of non-model organisms are limited by the lack of reference genomes of high accuracy, completeness, and contiguity. Here, we present a chromosome-level, karyotype-validated reference genome and pangenome for the barn swallow (Hirundo rustica). We complement these resources with a reference-free multialignment of the reference genome with other bird genomes and with the most comprehensive catalog of genetic markers for the barn swallow. We identify potentially conserved and accelerated genes using the multialignment and estimate genome-wide linkage disequilibrium using the catalog. We use the pangenome to infer core and accessory genes and to detect variants using it as a reference. Overall, these resources will foster population genomics studies in the barn swallow, enable detection of candidate genes in comparative genomics studies, and help reduce bias toward a single reference genome.

A new isosporan (Apicomplexa: Eimeriidae) from barn swallow, Hirundo rustica (Aves: Passeriformes: Hirundinidae: Hirundininae), in Oklahoma, USA.

Compared to other commonly examined passerine birds for coccidian (Apicomplexa) parasites, little is available on the coccidia of birds of the family Hirundinidae, including barn swallows (Hirundo rustica). Feces was collected in May 2022 from two nesting H. rustica in McCurtain County, Oklahoma, USA; the samples contained oöcysts representing a new species of Isospora. Oöcysts of Isospora zimmermani n. sp. are subspheroidal to ovoidal with a smooth bi-layered wall, measure (L × W) 22.8 × 20.8 µm, and have a length/width (L/W) ratio of 1.1; a micropyle and oöcyst residuum were absent but polar granule(s) are present. Sporocysts are ellipsoidal to ovoidal and measure 15.2 × 9.6 µm, L/W 1.6; a knob-like Stieda body and a rounded sub-Stieda body are present. The sporocyst residuum is composed of compact spheroid with a dense, irregular mass of finer granules lying between and dispersed among the sporozoites. The new species represents only the second isosporan reported from H. rustica but the first from the USA, and fourth species documented from the avian family Hirundinidae. This article was registered in the Official Register of Zoological Nomenclature (ZooBank) as urn:lsid:zoobank.org:pub:1F1E71C3-56E8-4EA7-A9B0-1E585FAA9DE0.