Seaside Sparrow Research Articles

Mercury concentrations in Seaside Sparrows and Marsh Rice Rats differ across the Mississippi River Estuary

Mercury (Hg) concentrations and their associated toxicological effects in terrestrial ecosystems of the Gulf of Mexico are largely unknown. Compounding this uncertainty, a large input of organic matter from the 2010 Deepwater Horizon oil spill may have altered Hg cycling and bioaccumulation dynamics. To test this idea, we quantified blood concentrations of total mercury (THg) in Seaside Sparrows (Ammospiza maritima) and Marsh Rice Rats (Oryzomys palustris) in marshes west and east of the Mississippi River in 2015 and 2016. We also tested for a difference in THg concentrations between oiled and non-oiled sites. To address the potential confounding effect of diet variation on Hg transfer, we used stable nitrogen (δ15N) and carbon (δ13C) isotope values as proxies of trophic position and the source of primary production, respectively. Our results revealed that five to six years after the spill, THg concentrations were not higher in sites oiled by the spill compared to non-oiled sites. In both species, THg was higher at sites east of the Mississippi River compared to control and oiled sites, located west. In Seaside Sparrows but not in Marsh Rice Rats, THg increased with δ15N values, suggesting Hg trophic biomagnification. Overall, even in sites with the most elevated THg, concentrations were generally low. In Seaside Sparrows, THg concentrations were also lower than previously reported in this and other closely related passerines, with only 7% of tested birds exceeding the lowest observed effect concentration associated with toxic effects across bird species (0.2 µg/g ww). The factors associated with geographic heterogeneity in Hg exposure remain uncertain. Clarification could inform risk assessment and future restoration and management actions in a region facing vast anthropogenic changes.

Ecological characteristics explain neutral genetic variation of three coastal sparrow species.

Eco-phylogeographic approaches to comparative population genetic analyses allow for the inclusion of intrinsic influences as drivers of intraspecific genetic structure. This insight into microevolutionary processes, including changes within a species or lineage, provides better mechanistic understanding of species-specific interactions and enables predictions of evolutionary responses to environmental change. In this study, we used single nucleotide polymorphisms (SNPs) identified from reduced representation sequencing to compare neutral population structure, isolation by distance (IBD), genetic diversity and effective population size (Ne) across three closely related and co-distributed saltmarsh sparrow species differing along a specialization gradient-Nelson's (Ammospiza nelsoni subvirgata), saltmarsh (A. caudacuta) and seaside sparrows (A. maritima maritima). Using an eco-phylogeographic lens within a conservation management context, we tested predictions about species' degree of evolutionary history and ecological specialization to tidal marshes, habitat, current distribution and population status on population genetic metrics. Population structure differed among the species consistent with their current distribution and habitat factors, rather than degree of ecological specialization: seaside sparrows were panmictic, saltmarsh sparrows showed hierarchical structure and Nelson's sparrows were differentiated into multiple, genetically distinct populations. Neutral population genetic theory and demographic/evolutionary history predicted patterns of genetic diversity and Ne rather than degree of ecological specialization. Patterns of population variation and evolutionary distinctiveness (Shapely metric) suggest different conservation measures for long-term persistence and evolutionary potential in each species. Our findings contribute to a broader understanding of the complex factors influencing genetic variation, beyond specialist-generalist status and support the role of an eco-phylogeographic approach in population and conservation genetics.

Observations of mixed parentage and male-biased primary sex ratio in Seaside Sparrow (Ammospiza maritima) nests along the Gulf of Mexico

Observations of mixed parentage and male-biased primary sex ratio in Seaside Sparrow (Ammospiza maritima) nests along the Gulf of Mexico

Genetic analysis of federally endangered Cape Sable seaside sparrow subpopulations in the Greater Everglades, USA

The federally endangered Cape Sable seaside sparrow (Ammospiza maritima mirabilis) is endemic to the Greater Everglades ecosystem in southern Florida, inhabiting fragmented marl prairies in six individual subpopulations. The subspecies is threatened by loss of breeding habitat from fire and water management. Genetic information is severely limited for the subspecies but could help inform decisions regarding subpopulation protections and potential translocations for genetic rescue. To provide genetic data and inform management efforts, feather samples were collected across five subpopulations (designated A–E) and protocols were tested to optimize DNA extraction yields. We assessed four mitochondrial DNA markers (N = 36–69) and 12 nuclear microsatellite loci (N = 55) in 108 sparrows. Mitochondrial DNA sequences revealed low haplotype diversity, with NADH dehydrogenase-2 haplotypes matching to most other extant subspecies and to the Atlantic coast subspecies. Nuclear diversity was low compared to other subspecies, but similar across subpopulations. Samples grouped as one population when analyzed by Principal Component Analysis, Bayesian modelling and genetic distance metrics. Limited genetic emigration was detected from one putative migrant. Relatedness was significantly different for sparrows in the most geographically distant subpopulation (A), likely reflecting high self-recruitment and natal site fidelity (P = 0.003). The low to moderate effective population size (NE = 202.4; NE:NC = 0.06) and generation time estimates indicated that unique genetic variation could be lost quickly during stochastic events. The sample sizes were limited, which reduced the power to comprehensively address recent population size reductions and any subsequent loss of genetic diversity.

The Importance of Paleoecology in Everglades Restoration Science and Management

Paleoecology studies provide pre-instrument data from the Everglades painting an informative picture of physical, biological, and ecological conditions prior to human intervention. Because Florida’s development history is relatively recent, and because observational data span only the last half-century, managers rely on paleoecology data as the basis for important decisions regarding multi-decadal and expensive restoration. Effective restoration depends strongly on the establishment of restoration targets, especially pre-development vegetation and hydropatterns (flow, depth, timing, distribution). Coring data were instrumental in the reconstruction of paleo hydrologic and vegetation trends in the ARM Loxahatchee National Wildlife Refuge, helping managers understand the magnitude and causes of 20th century problems. Similarly, sediment cores from the Big Cypress National Preserve led managers to understand that the development of marl prairies in Everglades National Park – critical habitat for the endangered Cape Sable Seaside Sparrow – was a 20th century phenomenon resulting from past water management practices. Finally, paleoecological and modeling studies in Florida Bay and the Everglades for the $20B+ Comprehensive Everglades Restoration plan established freshwater flow targets now used by managers. Although restoration of historic freshwater flow is not feasible, knowledge about Everglades hydrology and ecology prior to human intervention plays a pivotal role in the design, selection, and construction of restoration projects. Paleoecological data, coupled with decadal-scale monitoring and other long-term studies, provide the long-term perspective necessary to understand decadal, to centennial, and to millennial time-scale processes. While these data are cost-effective to procure, a long-term commitment to funding these types of studies is essential to provide the scientific foundation for restoration.

Estimating detection for salt-marsh songbirds during winter using the double-pass rope-drag technique.

Bird species that are restricted to tidal marshes during one or all of their life stages are under increasing pressure from sea-level rise. To date, most of the research focused on this group has been conducted during the breeding season despite the fact that more than half of the annual cycle is spent on wintering grounds and the high likelihood that the winter period is the most critical time for adult survival. We used a double-pass rope-drag technique to estimate the winter abundance of sharp-tailed sparrows (Ammospiza nelson and A. caudacutus collectively), seaside sparrows (A. maritimus) and marsh wrens (Cistothorus palustris) within tidal marshes of Virginia along 102 60X250 m transects between January and March, 2014. We used the first pass to remove birds from the transect and the second pass was used to estimate detection probabilities. The technique was highly effective producing detection rates of 98% for sharp-tailed sparrows, 95% for seaside sparrows, and 91% for marsh wrens. We conducted three rounds of surveys and found that species-specific detection rates were comparable when we restricted our analyses to two survey rounds. Availability and abundance estimates deviated to a greater degree than detection rates when restricting data to that collected during only two rounds but confidence intervals overlapped for all three taxa, regardless of which two survey periods were used for the comparison. However, results were less precise when we restricted our analyses to two of three rounds with confidence intervals averaging 13%, 45%, and 14% larger for detection, availability, and abundance respectively. The double-pass rope-drag technique provides an effective, unbiased sampling technique to estimate winter songbird abundance in saltmarsh habitat provided that at least two rounds are used and increasing the number of survey rounds will result in more precise estimates.

Sea level rise may pose conservation challenges for the endangered Cape Sable seaside sparrow

Biodiversity conservation under a changing climate is a challenging endeavor. Landscapes are shifting as a result of climate change and sea level rise but plant communities in particular may not keep up with the pace of change. Predictive ecological models can help decision makers understand how species are likely to respond to change and then adjust management actions to align with desired future conditions. Florida’s Everglades is a wetland ecosystem that is host to many species, including a large number of endangered and endemic species. Everglades ecosystem restoration has been ongoing for decades, but consideration of sea level rise impacts in restoration planning is more recent. Incorporating potential impacts from sea level rise into restoration planning should benefit species and their coastal habitats, most notably at the southern Florida peninsula. The endangered Cape Sable seaside sparrow (Ammospiza maritima mirabilis) occurs in marl prairie habitat at the southern end of the Everglades. The locations of three of its six subpopulations are proximate to the coast. We used a spatially explicit predictive model, EverSparrow, to estimate probability of sparrow presence considering both hydrologic change from restoration and sea level rise. We found that the probability of sparrow presence decreased with increasing sea level rise. Within approximately 50 years, probability of presence significantly decreased for all three coastal subpopulation areas, with areas above 40% probability increasingly limited. Given the exceptionally low dispersal ability of this species and the geographic restrictions for habitat expansion, our results highlight the importance of freshwater flow into the southern Everglades marl prairie for habitat conservation.

Variation in song between coastal lineages of Seaside Sparrow (Ammospiza maritima)

El canto del gorrión Ammospiza maritima es un canto de 2 frases, con una introducción compleja y un zumbido final, con cierta variación interindividual y entre poblaciones mostrada anteriormente. En comparación a lo anterior, se conoce poco de la variación geográfica de su canto o si la estructura del canto corresponde a subespecies o a linajes costeros (del Atlántico vs el Golfo de México). Recopilamos grabaciones de todas las subespecies reconocidas a través del rango de la especie para investigar si la variación del canto de este gorrión corresponde con subespecies o con denominaciones de linajes costeros. Los linajes de grupos costeros de gorriones variaron significativamente con respecto al número de frecuencia y caracteres temporales. Notablemente, las frecuencias de la sílaba introductoria en la primera oración eran más altas para el linaje costero del Golfo mientras que las medidas de frecuencia de la sílaba terminal dentro de la primera oración eran más bajas. Además, el número de sílabas de la primera frase era mayor en el grupo costero del Golfo. Un análisis de regresión múltiple usando 6 medidas de canto clasificados en grupos de linajes costeros con 96% de precisión pero falló en distinguir 7 subespecies que tenían datos insuficientes. Palabras clave: acústica, aves canoras, frase, frecuencia, sílaba, subespecie.

Using autonomous recording units to detect individual marsh birds in the Northern Gulf of Mexico.

Marsh birds are highly elusive and select wetland habitats that are difficult to navigate, as well as easily damaged by human observers. Autonomous recording units (ARUs) have been used to determine presence or absence of marsh bird species, but little is known about distance effects on detection probability. Therefore, our objectives were to 1.) evaluate if ARUs can be used to accurately count three marsh bird species (e.g., Clapper Rail, Least Bittern, Seaside Sparrow), 2.) estimate detection probability curves based on ARU recordings, and 3.) determine the area surveyed by the ARU for the three species. We arranged ARUs to record calls from our marsh bird species broadcasted from Bluetooth speakers at fixed distances. We replicated natural calling scenarios by playing calls in different number combinations of individuals, ranging from 1 to 10 birds. To reduce interference from real bird vocalizations, we conducted our experiment in a recently burned pine savanna habitat that had similar herbaceous vegetation structure to the coastal emergent wetland habitats preferred by these species in southern Mississippi. We used Raven Pro bioacoustics software to produce sonogram images of the broadcasted calls to count individuals in each recording. Detection probability of each species by distance was calculated in program R. Results showed that ARUs may be useful for counting individuals at close distances for some species (< 100m), but most counts biased low. In Clapper Rails and Least Bitterns, count accuracy decreased between 100 and 125 meters from the ARU but closer (50-100 meters) for Seaside Sparrows. There was also a significant decline in count accuracy with increasing chorus size. With further study and advancing technology, ARUs may be able to supplement marsh bird surveys and limit logistical issues.

Diet and disturbance: Seaside Sparrow resource use driven by oiling and Hurricane Isaac

The 2010 Deepwater Horizon (DWH) disaster led to extensive oil deposition in Louisiana’s coastal wetlands, followed two years later by Hurricane Isaac. These disturbances led to changes in saltmarsh invertebrate communities, potentially affecting higher-level predators and the saltmarsh food web. Seaside Sparrows (Ammospiza maritima) are ubiquitous, year-round residents of the coastal saltmarsh affected by the DWH spill, where they consume diverse invertebrates and may be considered an indicator species for the ecosystem’s integrity. We used DNA metabarcoding to evaluate prey consumed by Seaside Sparrows to understand how sparrows responded to residual contamination from the DWH oil spill and ecosystem disturbance caused by Hurricane Isaac. To do so, we evaluated metrics of diet (prey richness, diversity, overall diet composition) and resource use (total niche width, individual specialization) from 2011 to 2017 on oiled, unoiled, and reference sites. We found that while diet composition varied across years and site type, Hurricane Isaac had an even greater effect on the richness and diversity of prey consumed. Resource use—as measured by the total niche width of the populations and degree of individual specialization—was most stable on unoiled sites compared to unoiled and reference sites. Finally, we analyzed resource use for each combination of site type and year (i.e.: “2014 oiled sites”), which indicated a strong correlation between individual specialization and total niche width: as total niche width increased, individuals became more specialized, following the predictions of the Niche Variation Hypothesis.

Habitat selection of nesting and fledgling salt marsh songbirds in northeast Florida

AbstractUnderstanding habitat selection by breeding birds and their newly fledged young can be an essential aspect of the conservation of vulnerable species. During 2015–2017, we examined nest site selection of Worthington's marsh wren (Cistothorus palustris griseus) and MacGillivray's seaside sparrow (Ammospiza maritima macgillivraii), and fledgling habitat use by Worthington's marsh wren, 2 imperiled species in northeast Florida, USA. We compared vegetation at unused points to vegetation at nests of both subspecies and at locations used by radio‐tagged marsh wren fledglings. Vegetation was taller and stem counts were greater at nest sites compared to unused points. Worthington's marsh wrens also used nest sites with a greater proportion of tall‐form smooth cordgrass (Spartina alterniflora) than was observed at unused points. Worthington's marsh wren fledglings also used locations with taller, denser vegetation, but vegetation use changed with fledgling age and tidal stage; older fledglings more frequently used areas with short‐form smooth cordgrass and bare ground (and more so during low tides). In contrast, so few nests and nestlings were in black needlerush (Juncus roemerianus) that we could not consider it in our analysis despite its prevalence within our study sites. Our results indicate that tall, dense cordgrass is an important habitat component for these subspecies during the nesting and fledgling life stages in southeastern Atlantic salt marshes.

Habitat edges influence the distribution of nest predators for Seaside Sparrows, but not nest placement or success

Abstract Nest failure for coastal marsh bird species is primarily caused by predation and nest flooding. As sea level rise makes nest flooding more likely, the threat of nest predation will constrain the potential adaptive responses of marsh nesting species. Thus, understanding the predictors of nest predation is important for the conservation of salt marsh-dwelling bird species, such as the Seaside Sparrow (Ammospiza maritima). Predator activity may be influenced by landscape features (particularly habitat edges), potentially making nest predation predictable. We aimed to understand the predictability of Seaside Sparrow nest predation relative to two major landscape features: roads and tidal rivers, as both of these edges may be entryways or attractants for predators in marshes. In coastal Georgia, USA, we assessed mammalian predator activity relative to the two features of interest, and hypothesized that mammalian predator activity would be greater close to roads and tidal rivers. We also recorded Seaside Sparrow nest locations and nest predation events and hypothesized that nest predation events would increase with increasing predator activity. Consistent with our first hypothesis, mammalian predator activity increased close to roads and tidal rivers, but mammalian predator distribution did not explain the spatial variation in Seaside Sparrow nest predation thus not supporting our second hypothesis. Seaside Sparrows also placed their nests in locations with high mammalian predator activity, indicating that the ability to avoid nesting in high-risk areas may be constrained by habitat or resource limitations. Additionally, mammals may not be the primary nest predators, as we found that one bird species—Marsh Wren (Cistothorus palustris)—contributed substantially to nest predation rates. Understanding the predictability of mammalian predator distribution can allow for focused predator management efforts, such as exclusion, to habitat edges where we found the highest mammalian predator activity, which could relax the constraint of nest predation on Seaside Sparrow’s ability to respond to the intensifying threat of sea level rise.

Imperiled sparrows can exhibit high nest survival despite atypical nest site selection in urban saltmarshes

Understanding habitat selection and its fitness consequences in remnant populations of birds in fragmented and urbanized habitat may provide guidance to land managers for imperiled species conservation. We studied Saltmarsh Sparrow, <em>Ammospiza caudacuta</em>, and Seaside Sparrow, <em>A. maritima</em>, nest site selection and nest survival at five sites in New York City (NYC) and one site on western Long Island, New York in 2012 and 2013. We compared marsh vegetation and nest structure characteristics between our study sites and other studied sites for these species in the Northeastern United States. Saltmarsh Sparrows in NYC selected nest sites with the tall form of low-elevation marsh grass (at two sites this was above the proportion available), which is atypical for this species, and Seaside Sparrows selected an upland shrub species at one site, possibly to compensate for a lack of tall low-elevation marsh grasses. Daily nest survival at New York sites increased with nest height above the ground for both species, contrary to previous studies for Saltmarsh Sparrows in intact habitat. Mean nest height for both species was found to be taller in NYC than at other studied locations, and NYC sites may have had taller than average vegetation available, indicating that these sparrows may be able to adapt to some changing marsh characteristics. The difference in nest site selection observed in NYC populations from published studies did not appear to have a cost to nest survival in at least one year. Although high-elevation marsh is the optimal habitat restoration target for Saltmarsh Sparrows, it may be difficult to create in small, urbanized marshes, and our results suggest that other approaches may be successful. A mix of substrates and vegetation heights may allow sparrows to exhibit variation in nest site selection and promote nest survival in the face of changing limiting factors.

Endangered Cape Sable seaside sparrow ecology: actions towards recovery through landscape-scale ecosystem restoration

Understanding the ecology of endangered taxa and the factors affecting their population growth and decline is imperative for their recovery. In the southeastern USA, the Everglades wetland ecosystem supports a high diversity of species and communities, including many endemic and imperiled taxa, such as the federally endangered Cape Sable seaside sparrow Ammospiza maritima mirabilis (CSSS). The Everglades, once a completely connected wetland with a slow-moving sheet flow of water, is now compartmentalized into separated wetland units where water distribution is managed year-round. The CSSS is affected by, and at the crux of, many Everglades ecosystem restoration decisions. The CSSS faces conservation challenges, including limited habitat availability, low population numbers, dispersal limitations, and constraints on suitable breeding conditions owing to wetland water levels. Despite these challenges, ecological knowledge of the factors affecting CSSS population numbers in the context of ongoing ecosystem-level restoration can help inform protection of this bird while restoring the Everglades. Existing research shows target hydroperiods between 90 and 210 days, a minimum of 90 consecutive dry days during the breeding season, and non-breeding season fires approximately every 5-10 years may aid in CSSS recovery. There are numerous tools and models to support habitat and water management for the CSSS, and the most recent ecosystem-level water operations plan for the Everglades indicates potential for increased CSSS habitat. Here, we provide a review on the ecology of the CSSS, factors affecting population decline, and ecosystem-level restoration actions that may aid in CSSS recovery.

Metabarcoding of stomach contents and fecal samples provide similar insights about Seaside Sparrow diet

Abstract DNA metabarcoding is a popular tool for animal diet studies to address a variety of research topics across disciplines and taxa. Despite its widespread use, there has been relatively little focus on how digestion influences prey DNA detection and the description of a predator’s diet. Fecal samples are a compelling source of dietary DNA because they are collected non-invasively. However, these samples may provide incomplete or inaccurate descriptions of diet because of differential digestion and DNA degradation across prey taxa during gut passage. This is especially pertinent for avian diet studies, which have widely adopted the use of fecal samples as a proxy for overall diet. To explore how digestion affects the recovery and detection of prey DNA in passerines, we used DNA metabarcoding to compare the recovery of prey DNA from paired stomach contents and fecal samples in Seaside Sparrows (Ammospiza maritima). Stomach contents produced ~2.5 times greater DNA concentrations than fecal samples and—while this difference in DNA concentration was not statistically significant—stomach contents produced significantly more read identifications than fecal samples. However, these differences did not influence the description of diet, as similar measures of richness and diversity were found in both sample types. The relative read abundance of common prey families remained consistent between sample types, suggesting that while less DNA may survive digestion, the proportions of prey remain largely unaffected. We found no difference in the description of diet based on sample type at the population level, but our results show that comparing stomach and fecal samples from the same individual can reveal distinct foraging bouts. With no clear benefit to using stomach contents, we conclude that fecal samples are the preferred sample type for avian metabarcoding diet studies, unless research goals necessitate otherwise.

Genome-wide assessment of population structure in Florida’s coastal seaside sparrows

Genome-wide assessment of population structure in Florida’s coastal seaside sparrows

Nest survival of Seaside Sparrows (Ammospiza maritima) in the wake of the Deepwater Horizon oil spill.

In 2010, the Deepwater Horizon oil spill released an estimated 4.9 million barrels of oil into the Gulf of Mexico, damaging coastal ecosystems. Seaside Sparrows (Ammospiza maritima)—a year-round resident of Gulf Coast salt marshes—were exposed to oil, as shown by published isotopic and molecular analyses, but fitness consequences have not been clarified. We monitored nests around two bays in Plaquemines Parish, Louisiana, USA from 2012–2017 to assess possible impacts on the nesting biology of Seaside Sparrows. A majority of nests failed (76% of known-fate nests, N = 252 nests, 3521 exposure-days) during our study, and predation was the main cause of nest failure (~91% of failed nests). Logistic exposure analysis revealed that daily nest survival rate: (1) was greater at nests with denser vegetation at nest height, (2) was higher in the more sheltered bay we studied, (3) decreased over the course of the breeding season in each year, and (4) was not correlated with either sediment polycyclic aromatic hydrocarbon concentrations or estimated predator abundance during the years for which we had those data. Although the Deepwater Horizon spill impacted other aspects of Seaside Sparrow ecology, we found no definitive effect of initial oiling or oiled sediment on nest survival during 2012–2017. Because predation was the overwhelming cause of nest failure in our study, additional work on these communities is needed to fully understand demographic and ecological impacts of storms, oil spills, other pollutants, and sea-level rise on Seaside Sparrows and their predators.

Landscape-scale drivers of endangered Cape Sable Seaside Sparrow (Ammospiza maritima mirabilis) presence using an ensemble modeling approach

The Florida Everglades is a vast and iconic wetland ecosystem in the southern United States that has undergone dramatic changes from habitat degradation, development encroachment, and water impoundment. Starting in the past few decades, large restoration projects have been undertaken to restore the landscape, including improving conditions for threatened and imperiled taxa. One focus of restoration has been the marl prairie ecosystem, where the federally endangered Cape Sable Seaside Sparrow (Ammospiza maritima mirabilis; CSSS) resides. The CSSS is endemic to the Everglades where populations have been steadily declining, signaling the importance of decision support tools for natural resource managers for evaluating water management and restoration scenarios. Here we developed an ensemble logistic regression, combining a frequentist and Bayesian approach, to model CSSS presence and measure how environmental factors such as hydrometrics, fire occurrence, and vegetation structure impact CSSS habitat suitability. This is the first analysis to quantitatively assess the interdependent relationships between a broad range of environmental factors and CSSS presence across the landscape. Our results show that the probability of CSSS presence was highest in areas with dry conditions, hydroperiods between 80 and 120 days, percentages of canopy cover and woody vegetation less than 10%, and more than six years post-fire where 75% or more of the area was burned. Because the frequentist and Bayesian models had nearly identical spatial outputs with the Bayesian model having slightly higher validation metrics, we used the Bayesian approach as our final model (EverSparrow). The results from our analysis can provide a valuable decision support tool as natural resource managers work to restore the Everglades landscape.

Corrigendum to: Range-wide population genetic analysis of Seaside Sparrows (Ammospiza maritima) supports at least five distinct population segments that do not align with current subspecies descriptions

Corrigendum to: Range-wide population genetic analysis of Seaside Sparrows (<i>Ammospiza maritima</i>) supports at least five distinct population segments that do not align with current subspecies descriptions

Geographic variation of mercury in breeding tidal marsh sparrows of the northeastern United States.

Saltmarsh sparrows (Ammospiza caudacuta) and seaside sparrows (A. maritima) are species of conservation concern primarily due to global sea-level rise and habitat degradation. Environmental mercury (Hg) contamination may present additional threats to their reproductive success and survival. To assess site-specific total mercury (THg) exposure and identify environmental correlates of THg detection, we sampled blood from adult male saltmarsh and seaside sparrows at 27 sites between Maine and Virginia, USA. The mean THg concentration (±1 SD) throughout the entire sampling range was 0.531 ± 0.287 µg/g wet weight (ww) for saltmarsh sparrows and 0.442 ± 0.316 µg/g ww for seaside sparrows. Individual THg concentrations ranged from 0.135-1.420 µg/g ww for saltmarsh sparrows and 0.153-1.530 µg/g ww for seaside sparrows. Model averaging from a suite of linear mixed models showed that saltmarsh sparrows averaged 20.1% higher blood THg concentrations than seaside sparrows, potentially due to differences in diet or foraging behavior. We found no evidence for a relationship between sparrow THg concentrations and land cover surrounding sampled marshes or average precipitation-based Hg deposition. Overall, our results suggest considerable, unexplained variation in tidal marsh sparrow blood THg concentrations over their co-occurring breeding ranges.