Patterns of species’ distribution and abundance are dynamic in response to changes in land cover, climate, interspecific relationships, and other factors. There are many examples of latitudinal range expansions, contractions, and shifts associated with warming climate. Global change effects are also manifested along gradients of longitude, and woody plant encroachment westward into the Great Plains might also be creating opportunities for species to expand into new areas. We used data from the North American Breeding Bird Survey (BBS) to examine the abundance of Red-bellied Woodpecker (Melanerpes carolinus) at a western edge of its distribution in Texas, Oklahoma, and Kansas. We compared BBS data from 1968–1992 to data from 1993–2018, fitting a series of generalized linear models to best explain variability in abundance. The best performing model included an interaction between time period and longitude, indicating both a change in abundance and in the distribution of abundance over time. Specifically, we found that Red-bellied Woodpecker increased in abundance near its western distributional limit over the past 5 decades, beginning at approximately −94 degrees longitude.

El carpintero Melanerpes carolinus ha incrementado su abundancia en el margen occidental de su rango

Los patrones de distribución y abundancia de las especies son dinámicos en respuesta a los cambios en uso del suelo, el clima, las relaciones interespecíficas y otros factores. Existen muchos ejemplos de expansiones, contracciones y desplazamientos latitudinales de área de distribución asociados al calentamiento del clima. Los efectos del cambio global también se manifiestan a lo largo de gradientes de longitud y la invasión de plantas leñosas hacia el oeste en las Great Plains también podría estar creando oportunidades para que las especies colonicen nuevas áreas. Utilizamos datos del North American Breeding Bird Survey (BBS) para examinar la abundancia del carpintero Melanerpes carolinus en el límite occidental de su distribución en Texas, Oklahoma y Kansas. Comparamos los datos de BBS de 1968–1992 con los de 1993–2018, ajustando una serie de modelos lineales generalizados para explicar mejor la variabilidad en la abundancia. El modelo con mejores resultados incluyó una interacción entre el periodo de tiempo analizado y la longitud, indicando un cambio en la abundancia y un cambio en la distribución de la abundancia a lo largo del tiempo. Específicamente, encontramos que la abundancia de este carpintero aumentó cerca de su límite de distribución occidental durante las últimas 5 décadas, comenzando aproximadamente a −94 grados de longitud.

Palabras clave: distribution, expansión, longitud

Pressures and opportunities associated with anthropogenic global changes have already influenced bird populations and communities, often indicated by shifts in phenology and broad-scale species distributions (Tingley et al. 2009, Bateman et al. 2015). Range shifts and expansions have also been documented globally for many taxa other than birds, including invertebrates (Hickling et al. 2005), vascular plants (Parolo and Rossi 2008), mammals (Moritz et al. 2008), fishes (Perry et al. 2005), and amphibians (Kusrini et al. 2017). For birds, many studies have found evidence of species distributions shifting toward higher latitudes as a result of climate change (Jackson and Davis 1998, Devictor et al. 2008, Zuckerberg et al. 2009, Kirchman and Schneider 2014), but expansions and range shifts can also be multi-directional (Gillings et al. 2015), and longitudinal shifts have been described (Kirchman and Schneider 2014, Sinnott et al. 2021).

As natural systems continue to experience rapid and directional change in the Anthropocene, honing our understanding of species’ distribution changes will become increasingly important both to predict trajectories in different areas and, for species of conservation priority, better adapt our management focus to changing conditions. There is much to learn about the mechanisms of expansion into new areas (Holt and Keitt 2005). For example, the boundary of an expanding species’ distribution can be extended rapidly by individuals dispersing unusually long distances into new areas, by steady population increase near the boundary of the established distribution, or some combination of factors.

We investigated a potential western margin abundance increase by Red-bellied Woodpecker (Melanerpes carolinus), an annual resident bird of hardwood forests in the eastern United States and Canada (Miller et al. 2020). We selected Red-bellied Woodpecker as a model species for this study due to its relatively high abundance, data richness, widespread distribution, and high detectability in standardized auditory surveys. The Red-bellied Woodpecker is nonmigratory and largely philopatric, which increases confidence that any detected expansion indicates a persistent change in distribution rather than a stochastic change in occupancy. Red-bellied Woodpecker is known to have expanded northward, as warming temperatures at northern limits of the distribution have allowed birds to colonize higher latitudes (Jackson and Davis 1998, Kirchman and Schneider 2014, FitzGerald et al. 2018). Although this northward expansion is well documented and there is some evidence of a westward expansion (Kirchman and Schneider 2014), considerably less is known about dynamics at the western range margins. Examining patterns of abundance at a western distributional margin can help shed light on the mechanisms of expansion. Our central objective was to determine if abundance of Red-bellied Woodpecker has increased along its western limit in the past half-century.

We used Red-bellied Woodpecker count data from the North America Breeding Bird Survey (BBS) during the years 1968–2018 for analysis (Pardieck et al. 2020). The BBS is a continental-scale, citizen-science avian abundance data collection effort. The dataset includes annually collected point count data across thousands of 41 km routes in North America (50 point-counts at each route). We defined the western margin of Red-bellied Woodpecker range to include BBS routes in Kansas, Oklahoma, and Texas, USA (Fig. 1). Before analysis, we systematically cleaned the dataset to better increase confidence in the quality of the data relative to our objective. To avoid including routes that are outside of the Red-bellied Woodpecker’s range, we included only routes on which at least 1 Red-bellied Woodpecker had been detected within the temporal extent of the study. We also removed any survey data that did not meet the BBS quality standard to avoid introducing unnecessary noise (i.e., surveys conducted during unfavorable weather conditions or outside the breeding season window). Finally, we removed any data collected by first-year observers (resulting in approximately a 10% data reduction) to minimize the novice effect and better control data quality (Kendall et al. 1996, Jiguet 2009).

We structured our statistical analysis under an information-theoretic framework to allow for multiple competing hypotheses. Specifically, we ran 3 generalized linear models, all of which included annual, route-level, Red-bellied Woodpecker count data as the response variable (i.e., total route-level abundance in each year, or the sum of all point counts along each route) and a Poisson error distribution (R package lme4; Bates et al. 2015). To address change over time, we binned count data into 2 time periods: “contemporary” (i.e., 1993–2018) or “past” (i.e., 1968–1992). Our intention for splitting the data was to keep year sample sizes as equal as possible (i.e., 25 years for the past time bin and 26 years for the contemporary time bin). Due to concerns over whether this decision may have influenced results, we performed a sensitivity analysis to test the effect of varying time bin decisions (i.e., which years were considered “past” and “contemporary”) by (1) changing the midpoint year, and (2) systematically creating time gaps in the middle of the data (i.e., removing years in the middle of the temporal extent to create 2 separated time bins). We found that results were robust across reasonable time binning decisions (Supplemental Fig. S1).

We tested models including (1) an interaction between time period and longitude (to test the hypothesis that changes in Red-bellied Woodpecker abundance over time vary with longitude, and thus detect whether abundance has increased at the western range margin), (2) time as the only explanatory variable (to test the hypothesis that abundance has changed over time across all analyzed routes, but not in relation to longitude), and (3) longitude as the only explanatory variable (to test the hypothesis that abundance had changed longitudinally but irrespective to time). Longitude was determined by the BBS route’s starting coordinates. We included a null model for reference (i.e., an intercept-only model where bird abundance is modeled without an independent variable) and then used Akaike’s information criterion (AIC) to compare and rank all 4 models (R package bbmle; Bolker and R Development Core Team 2020). AIC is an informational-theoretic method that determines relative support of multiple hypotheses at once. Models with ΔAIC less than 2.0 were considered competitive.

To qualitatively assess the bird’s abundance dynamics at its western range margin, we also mapped relative bird density in both the past and contemporary time bins (mean count at each route during each time period). To allow easier visualization, bird counts were interpolated and mapped using kernel density using a 1 decimal degree search radius and 0.03 decimal degree output resolution (in ArcPro 3.0.0). We only included points that were surveyed in both time periods to allow maps to be visually comparable (n = 131).

Our analysis included data from 217 BBS routes spanning 7 degrees of longitude from −94° to −101°W in the US states of Kansas, Oklahoma, and Texas. Across all routes and the entire temporal extent of the study, mean abundance of Red-bellied Woodpecker at each BBS route was 5.3 (SD = 5.5). During the contemporary time period (1993–2018), overall mean abundance estimate at each route was 6.3 (SD = 5.7) and during the past time period, the mean abundance estimate at each route was 3.9 (SD = 4.8). Across both time periods, birds were always more abundant in the eastern part of their range than the western part of their range (i.e., woodpecker abundance tapers off at the western range margin during both time periods; Fig. 1).

Of the 3 generalized linear models we tested to explain variability in abundance across space and time, the only competitive model included an interaction between time period and longitude (interaction slope = 0.80 with 95% confidence intervals 0.82–0.85). The top-performing model performed overwhelmingly better than the other 2 models in the set and had an AIC weight of approximately 1 (the second-best model was over 1,000 ΔAIC from the first; Table 1). We found that Red-bellied Woodpecker had become more abundant at its western range margin in recent decades, beginning at approximately −94° longitude (Fig. 2, 3, and Supplemental Fig. S2). For example, the Red-bellied Woodpecker abundance at the −101 meridian in the recent time period (1993–2018) was approximately equivalent to its historical (1968–1992) abundance 4 degrees of longitude (approximately 360 km) east at the −97 meridian.

Our objective in this study was to determine if abundance of Red-bellied Woodpecker near a western distributional limit was higher in a contemporary period than in the past. Analysis of data from the North American Breeding Bird Survey indicated that Red-bellied Woodpecker had indeed increased in abundance near its western distributional limit in the US Great Plains states of Kansas, Oklahoma, and Texas. This area spans at least 860,000 km2 of the gradient of Great Plains’ ecoregions in these 3 states, from tallgrass to shortgrass prairie.

It might also be the case that the limit of the distribution is expanding westward as well, and Red-bellied Woodpecker has been increasingly reported on BBS routes west of our study region in the US states of Colorado and New Mexico (Sauer et al. 2019). Specifically, our analysis focused on another aspect of expansion, i.e., increase in abundance in areas approaching a distributional limit. For Red-bellied Woodpecker, the mechanism of expansion appears to include an increase in abundance approaching the western limit of the distribution over approximately the past 5 decades.

We did not directly evaluate factors contributing to this westward expansion. Red-bellied Woodpecker is recorded on BBS routes in at least 35 US states and in Ontario; data from 1966–2019 indicate that the population is increasing in every one of those except Florida (Sauer et al. 2019). Thus, it might be that Red-bellied Woodpecker has increased near its western limit in our study area simply as part of a broader pattern of increase rangewide. That long-term trend might be related to increased productivity and/or survivorship from increasingly milder winters, nutritional supplementation at suburban bird feeders, competitive release, or an expansion of niche space (FitzGerald et al. 2018). In our study area of the central and western Great Plains there is good reason to suspect that Red-bellied Woodpecker has increased in abundance as more suitable habitat has spread across the region. This area contains significant east–west precipitation, woodland-to-grassland, and elevation gradients. When considered in combination with the east–west bird expansion pattern we observed, this may indicate that either these gradients are not as limiting as we might expect, or that they are also moving west, and birds are tracking their niche. Because multiple habitat gradients occur along an east–west direction, this may make teasing out any individual driver of this expansion difficult without further research incorporating avian and environmental data at fine spatial and temporal scales.

Coinciding with the increase in Red-bellied Woodpecker in our study region, woody vegetation has increased dramatically in the Great Plains (Rice and Penfound 1959, Briggs et al. 2005, Twidwell et al. 2013, Archer et al. 2017). As a cavity nester associated with hardwood forests, Red-bellied Woodpecker has likely benefited from this increase in forested land cover. Although woody plant encroachment in central North America is most often discussed from the perspective of Juniperus virginiana expansion, oak (Quercus spp.) canopy cover is also increasing as a result of fire suppression (Engle et al. 2008, Scholtz et al. 2018). In addition, maturing and expanding riparian forests with dominant elm (Ulmus spp.), ash (Fraxinus spp.), hackberry (Celtis spp.), and pecan (Carya illinoensis) support breeding bird assemblages that include multiple species in the Great Plains with population centers hundreds of kilometers east (e.g., Kentucky Warbler [Geothlypis formosa]; Heinen and O’Connell 2009).

Species’ expansions and range shifts are hallmark impacts of anthropogenic climate change. Although latitudinal and elevational range shifts are well studied (e.g., organisms moving to higher elevations and latitudes to track their climate optima), east–west expansions are becoming increasingly prevalent in the literature (e.g., Lenoir and Svenning 2015, Sinnott et al. 2021). Here, we contribute to the growing body of knowledge by documenting a directional and persistent increase in abundance of a species from temperate hardwood forests of eastern North America at a western margin of its distribution in the Great Plains.

A prevailing pattern in the literature on global change ecology is the anticipation of winners and losers in the face of change (O’Brien and Leichenko 2003). That is, changing conditions are likely to benefit some species while burdening others. We know that the Great Plains region of the US is being encroached by woody vegetation (Archer et al. 2017). This environmental change drives out many ground-nesting and grassland-obligate bird species, but there is evidence that woodland-inhabiting and generalist species benefit from increased tree cover (Coppedge et al. 2001, Andersen and Steidl 2019). In this study, we highlight an example of a species likely to benefit from one facet of global change, the large-scale land cover change occurring due to advancement of the eastern forest biome into the previously grassland-dominated Great Plains biome. Specifically, as nest cavity availability and foraging substrates in the West increasingly resemble habitat in the core of the distribution, Red-bellied Woodpecker is likely to continue its expansion.

Our study is immediately relevant to the Red-bellied Woodpecker itself, as it contributes to our knowledge of the species’ life history and recent distributional changes. Additionally, our results may have conservation implications for other species influenced by this expansion. When species undergo range expansions into new areas, recipient communities may experience novel interactions. For example, Red-bellied Woodpecker might compete for nest cavities with Red-headed Woodpecker (Melanerpes erythrocephalus), a species that seems to have historically segregated from Red-bellied Woodpecker in the central US (Emlen et al. 1986) and is in decline (Ingold 1994). Although Koenig et al. (2017) concluded that interactions with Red-bellied Woodpecker were not limiting populations of Red-headed Woodpecker overall, it is still plausible that expansion of Red-bellied Woodpecker in the Great Plains introduces conflicts for resources that result in fine-scale displacement of Red-headed Woodpecker family groups. Further studies might examine the potential causal link between the process of woody plant encroachment and woodpecker abundance, as well as effects of encroachment on relevant demographic processes that can influence distributions and thus interactions with other species, such as fecundity and natal and breeding dispersal. For example, there is much to learn regarding the types of woodlands that are supporting Red-bellied Woodpeckers in the Great Plains or the degree to which they are using anthropogenic food sources.

Our study aligns with existing global change literature describing species range movements as sometimes more complex than a steady northward or upslope expansion (Gillings et al. 2015). As the Anthropocene progresses, biome shifts are occurring globally, and such changes are not strictly north–south (Engle et al. 2008, Seager et al. 2018). Species will need to move or adapt in order to persist as habitat conditions change along multiple directional axes, underlining the importance of developing our understanding of mechanisms driving multidirectional range movements or expansions. Red-bellied Woodpecker is just one of multiple species with population centers in the eastern US likely experiencing an increase in abundance and potential expansion into novel areas in the Great Plains. Given the prominence of area importance (i.e., the proportion of a population supported within a single geographic area) in developing conservation rankings and priorities for North American birds (Rosenberg et al. 2016), the potential for rapid changes in distributions places an additional challenge on land managers and conservationists to be nimble in our approaches moving forward.

Funding for this project was provided by the USDA National Institute of Food and Agriculture, McIntire-Stennis project 1006615 administered through the Oklahoma Agricultural Experiment Station at Oklahoma State University. We are grateful to scientists at the USGS Patuxent Wildlife Research Center for curating and providing data, and to the thousands of North American Breeding Bird Survey participants for their contributions to data collection used in this analysis.

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Supplementary data