Two Bird Monitoring Programs Tell Different Stories About the Same Great Plains Songbirds

Across the arid grasslands and sagebrush plains of central Montana, two very different ways of counting birds are telling two very different stories about the same species. A new study published in Frontiers in Bird Science by Kelsey Miller, Kaitlyn M. Reintsma, and Victoria J. Dreitz of the University of Montana’s Avian Science Center compared population trend estimates for seven migratory songbirds using two monitoring programs: the volunteer-driven North American Breeding Bird Survey, which has tracked birds across the United States and Canada since 1966, and a targeted local study conducted near the town of Roundup from 2013 to 2019. The comparison revealed partial agreement between the two approaches, but also striking discrepancies that depended on how statistical uncertainty and trend direction were interpreted. The most dramatic divergence involved the thick-billed longspur, a sagebrush specialist that appeared to be increasing statewide according to the Breeding Bird Survey while simultaneously declining sharply in the targeted study area.

The stakes of this methodological question are considerable. Songbirds in the Great Plains have undergone some of the most dramatic and consistent population declines in North America, driven by a complex suite of threats including habitat fragmentation and conversion to agriculture, woody vegetation encroachment, invasive species, and climate change. As a result, Great Plains ecosystems are now considered among the most imperiled in the world. Because songbirds serve as ecological indicators, reflecting changes in habitat quality and broader ecosystem trends, robust population trend estimates across spatial scales are essential for effective conservation. The central question of the new research was deceptively simple: do the large-scale trend estimates that guide regional and continental conservation assessments actually reflect what is happening to birds on the ground, at the local scales where management decisions are made?

To answer it, the researchers focused on seven migratory songbird species representative of arid ecosystems in Montana, spanning a gradient of habitat preferences from grassland to sagebrush. Five were grassland specialists: the chestnut-collared longspur, horned lark, lark bunting, thick-billed longspur, and western meadowlark. The Brewer’s sparrow represented sagebrush-dependent species, while the vesper sparrow served as a habitat generalist. By including species with varied habitat use and life history strategies, the team aimed to capture a broad perspective on population trends in the northern Great Plains.

The two monitoring programs differ fundamentally in design, scope, and objectives. The Breeding Bird Survey operates through a standardized framework in which a single volunteer observer conducts 3-minute point counts at 50 stops along a 39.4-kilometer roadside route, beginning 30 minutes before sunrise and recording all birds seen or heard within a 400-meter radius between late May and early July. Montana has 103 routes potentially surveyed each year, distributed statewide across multiple habitat types including forests, grasslands, and sagebrush steppe. Because these routes are largely road-based and more routes are completed near human population centers, the design can under-sample remote or specialized habitats, and spatial overlap with the targeted study area was limited. The targeted study, in contrast, encompassed roughly 2,160 square kilometers of mixed-grass prairie and sagebrush habitat near Roundup, where the landscape is characterized by Wyoming big sagebrush interspersed with western wheatgrass, needle-and-thread grass, blue grama, and prairie Junegrass. The team randomly selected 66 sampling plots, each measuring 500 by 500 meters, from public and private lands with pre-established access, ensuring a minimum spacing of 500 meters between plots.

The targeted surveys employed a technique known as the dependent double-observer transect method, which has been shown to provide higher and more precisely estimated detection probabilities in arid systems relative to single-observer methods. Two observers walk a U-shaped transect with the secondary observer walking 3 to 5 meters behind the primary observer, allowing the pair to survey up to 125 meters on each side of the transect and effectively cover the entire plot. The primary observer identifies each bird’s species and location and relays this information to the secondary observer, who records detections and notes any additional observations missed by the primary observer. Auditory detections required visual confirmation before being recorded, and observer roles were alternated between surveys to estimate and account for individual variation in detection. Each plot was surveyed three times over the breeding season from May through early July, and each year eight observers conducted the surveys in four two-person teams, resulting in 56 observer-years across the study period. The raw observation totals illustrate the different scales of the two programs: the Breeding Bird Survey recorded 93,330 observations for all focal species in Montana over the seven years, with annual counts ranging from 8,469 in 2013 to 16,784 in 2017, while the targeted study recorded roughly half as many, 46,869 observations, with the number and location of sampling plots remaining constant across years.

Both sets of trend estimates were generated using hierarchical models that separate biological and observational processes, allowing for estimation of abundance while accounting for detection probabilities, and each model was implemented within a Bayesian framework using Markov chain Monte Carlo methods. The Breeding Bird Survey estimates used an overdispersed Poisson regression framework that accounts for multiple sources of variation in bird count data, including year effects describing temporal patterns in relative abundance, observer effects capturing differences among observers and changes within observers over time, route-level variation incorporated through random effects, and overdispersion effects capturing extra variation beyond what a standard Poisson model predicts. The targeted study analysis used a multispecies dependent double-observer abundance model, a type of N-mixture model implemented in a Bayesian hierarchical framework, to estimate detection probabilities and species-specific absolute abundances, with species-specific fixed effects for year and a random effect for site. Crucially, however, the two approaches summarize temporal change differently: Breeding Bird Survey trends are derived from endpoints, calculated as the ratio of annual indices corresponding to the first and last years of the interval, 2013 and 2019, whereas the targeted study trends were estimated from a linear regression across all years, regressing the log of posterior-predicted abundance on year and exponentiating the slope to express the trend as an annual percent change. Endpoint-based estimates may be more sensitive to interannual variability, particularly for songbirds whose local abundances fluctuate substantially among years in response to changing environmental conditions, while regression-based estimates incorporate information across the full time series.

The results showed that agreement between the two programs was conditional and depended strongly on how trend magnitude, uncertainty, and direction were considered. Breeding Bird Survey estimates were generally less precise, with 95 percent credible intervals that frequently overlapped zero, resulting in most statewide trends being classified as uncertain, whereas the targeted study estimates were typically more precise and more often indicated directional change. For example, Brewer’s sparrow and horned lark both showed declining trends in the targeted study, with Breeding Bird Survey estimates suggesting similar but more uncertain declines. In contrast, the chestnut-collared longspur exhibited a positive trend in the targeted study, at 3.75 percent per year, while the Breeding Bird Survey estimate was near zero with a wide credible interval. For four species, the horned lark, thick-billed longspur, vesper sparrow, and western meadowlark, the credible intervals from the two programs did not overlap at all, suggesting genuine differences in estimated trends. The clearest divergence in magnitude occurred with the thick-billed longspur, for which the Breeding Bird Survey estimated a positive but highly uncertain trend of 11.34 percent per year, while the targeted study estimated a strong decline of 13.95 percent per year, a difference of more than 25 percentage points. An exploratory Wilcoxon signed-rank test did not detect a systematic difference in trend estimates across the seven species, but the authors cautioned that the small sample size and limited power mean this result should not be interpreted as evidence of equivalence between the approaches.

Disentangling the causes of these discrepancies is challenging, because some mismatches are consistent with biological processes while others more plausibly reflect differences in sampling design or analytical choices, and in most cases both factors are likely to contribute. The spatial heterogeneity documented by the study aligns with other monitoring efforts: national-level Breeding Bird Survey estimates from 2013 to 2019 suggest Brewer’s sparrows are broadly stable in the United States, at 0.52 percent per year, while Montana-specific routes show more declining patterns. Other targeted studies in Montana have documented variable trends depending on site-specific conditions, with some habitats functioning as population strongholds, such as high nest densities and success in native grasslands for western meadowlark in eastern Montana, and others reflecting sink dynamics, such as reduced productivity in cropland sites for thick-billed longspur in northern Montana. The thick-billed longspur’s highly specific habitat preferences make it particularly susceptible to these design differences, and the authors noted that targeted studies, while offering finer ecological resolution, may overrepresent population status relative to degraded areas if they focus on intact or managed sites.

The authors argue that the two monitoring programs are best understood as complementary rather than competing. Targeted monitoring can reveal local population declines and refugia that large-scale surveys miss, while programs like the Breeding Bird Survey provide essential regional context and the statistical power to detect widespread shifts. Looking forward, the researchers suggest that future work could formally integrate broad-scale and targeted surveys within a unified analytical framework such as an integrated population model, which combines multiple data sources within a hierarchical structure, allowing separation of biological process variation from observation error while leveraging the complementary strengths of different survey designs. Such integration could improve precision, reduce survey-specific bias, and provide insight into the demographic mechanisms underlying observed trends. In the nearer term, the findings support a practical conservation message: by combining insights from both large-scale and targeted monitoring, researchers and managers can more accurately assess population status, identify priority habitat areas, and clarify whether observed declines reflect widespread drivers or site-specific constraints. Maintaining habitat heterogeneity and landscape connectivity will be critical for sustaining viable populations under continued environmental change, and interpreting population trends cautiously and concurrently across scales, rather than relying on any single monitoring framework alone, offers the best opportunity to detect change, understand its drivers, and evaluate the effectiveness of management strategies in one of North America’s most imperiled ecosystems.

Subject of Research: Large-scale context and local insight: songbird population trends in the Great Plains

Article Title: Large-scale context and local insight: songbird population trends in the Great Plains

Article References: Miller, K., Reintsma, K. M., & Dreitz, V. J. (2026). Large-scale context and local insight: songbird population trends in the Great Plains. Frontiers in Bird Science, 5, Article 1834611. https://doi.org/10.3389/fbirs.2026.1834611

Image Credits: AI Generated

DOI: 10.3389/fbirs.2026.1834611

Keywords: Large-scale, context, local, insight, songbird, population, trends, Great, Plains, scientific research