Autumn stopover hotspots and multiscale habitat associations of migratory landbirds in the eastern United States

Abstract

Halting the global decline of migratory birds requires a better understanding of migration ecology. Stopover sites are a crucial yet understudied aspect of bird conservation, mostly due to challenges associated with understanding broad-scale patterns of transient habitat use. Here, we use a national network of weather radar stations to identify stopover hotspots and assess multiscale habitat associations of migratory landbirds across the eastern United States during autumn migration. We mapped seasonal bird densities over 5 y (2015 to 2019) from 60 radar stations covering 63.2 million hectares. At a coarse scale, we found that landbirds migrate across a broad front with small differences in migrant density between radar domains. However, relatively more birds concentrate along the Mississippi River and Appalachian Mountains. At a finer scale, we identified radar pixels that consistently harbored high densities of migrants for all 5 y, which we classify as stopover hotspots. Hotspot probability increased with percent cover of all forest types and decreased with percent cover of pasture and cultivated crops. Moreover, we found strong concentrating effects of deciduous forest patches within deforested regions. We also found that the prairie biome in the Midwest (now mostly cropland) is likely a migration barrier, with large concentrations of migrants at the prairie–forest boundary after crossing the agricultural Midwest. Overall, the broad-front migration pattern highlights the importance of locally based conservation efforts to protect stopover habitats. Such efforts should target forests, especially deciduous forests in highly altered landscapes. These findings demonstrate the value of multiscale habitat assessments for the conservation of migratory landbirds.

Billions of birds migrate every year, and in the process of doing so, they provide important ecosystem functions with respect to energy flow, nutrient cycling, and plant propagule dispersal (1). However, populations of many migratory species are in steep decline due to habitat loss, overexploitation, and climate change (2, 3). Long-term surveys reveal a staggering 28.3% decline in migratory bird populations in North America since 1970 (4). Some of the steepest population declines among North American birds involve landbirds, primarily passerines, moving through the eastern half of the continent (4). Such declines underscore the need for a better understanding of the habitat requirements of migratory birds.

However, the conservation of migratory landbirds is especially challenging due to the vast spatial extent of their migrations coupled with a lack of understanding of the factors limiting their populations across the entire annual cycle (5, 6). Historically, research has been biased toward studying birds where they are resident for long stretches of time, such as their summer breeding grounds and, increasingly, their winter nonbreeding grounds. The migration period itself remains poorly studied (7), despite the fact that the greatest annual mortality for migratory birds can occur during migration (8, 9). Given that habitat loss and degradation are likely the major threats to migratory birds (10), identifying and protecting key habitats along migratory pathways are crucial to the conservation of these birds (6, 11, 12).

While many migratory waders and waterfowl use well-defined networks of discrete stopover sites and show a high degree of philopatry (13), most migratory landbirds, especially passerines, are thought to migrate across a broad front, taking advantage of multiple stopover opportunities and exhibiting lower interannual fidelity to particular sites (14, 15, but see refs. 16 and 17 for counterexamples involving particular species). Beyond these local case studies, there is little empirical information on the large-scale migration patterns of migratory landbirds due to a lack of broad-scale surveys that match the highly dynamic spatial–temporal patterns of bird migrations. The resulting lack of information regarding migration patterns and habitat requirements en route has made migratory landbirds difficult to protect.

Recent advances in radar ornithology provide novel opportunities to study bird migration and stopover ecology at unprecedented spatiotemporal scales (18, 19). Unlike traditional field surveys conducted by people, radar “observes” birds through reflected electromagnetic radiation, thus comprehensively measuring the aggregate bird biomass in the atmosphere at relatively fine scales across broad extents and with less observer bias. Although individual species cannot be identified from radar signals, the comprehensive spatial and temporal coverage of radar provides density estimates of all migratory birds collectively, thereby generating results that contribute to the conservation of entire migratory bird populations (20, 21).

Radar images have been used to map the stopover density of migratory landbirds near major geographic barriers at coastal regions, including the northeastern United States (18, 20), the Great Lakes (22, 23), and the Gulf of Mexico (24, 25). These regional studies highlight local stopover hotspots with high densities of migrants and reveal positive associations between migrant density and deciduous forest cover. However, broad-scale stopover habitat mapping across the vast interior of the continent away from large open-water barriers has not been done. Although landbirds may have more stopover opportunities within the large tracks of vegetation of the interior, they are not expected to be equally abundant in all locations (26). For instance, forest habitat is limited in the agricultural Midwest due to large-scale deforestation accompanying Euro-American settlement, which could constrain bird migrants throughout this region (27, 28).

Here, we used data from 60 weather surveillance radar stations (NEXRAD) in the eastern United States covering 8.9 million radar pixels (pixel area: 0.4 to 21.8 ha) to determine whether and where there are continental stopover hotspots for migratory landbirds. We defined hotspots as radar pixels showing consistently high seasonal densities of migrants during autumn migration. In addition, we evaluated the habitat and land cover characteristics important to migratory landbirds across multiple spatial scales. In order to assess multiscale habitat associations, we calculated proportional land cover at local (within radar pixel), landscape (5-km buffer), and regional (within radar domain) scales. We also divided our study area into three avifaunal biomes (eastern forest, northern forest, and prairie) to explore biome-specific habitat requirements. We tested whether, as hypothesized in the literature (e.g., ref. 29), autumn landbird migration moves along a broad front when analyzed at the scale of the entire eastern United States. We predicted that, even if landbirds migrate in a broad front across the continent, there would be areas measured at a finer scale that consistently support high densities of migratory landbirds (hotspots), as has been found in previous regional studies (e.g., ref. 20). We also predicted that such hotspots would be tied to intrinsic habitat features at multiple spatial scales (30) and might be clustered in forest patches in largely deforested landscapes (18). Last, we explored whether the habitat associations of migrants differ among the three different biomes included in our study area.

Results

Coarse-Scale Stopover Patterns across Radar Domains.

We mapped seasonal stopover densities across 60 radar domains (sampling areas of ~80 km radius around each radar station) from 2015 to 2019, covering an area of 63.2 million hectares, effectively sampling 24% of the land area of the entire eastern United States. Across radar domains (coarse scale), we found an overall homogenized pattern of average autumn stopover density (Fig. 1; yearly maps from 2015 to 2019 are provided in SI Appendix, Fig. S1), with the seasonal cumulative density of migrants falling below 200 cm²/ha/season for the majority of radar domains (SI Appendix, Fig. S2); this represents fewer than 180 birds/ha stopping over throughout the autumn migration (the conversion is provided in SI Appendix, Methods). The relatively few radar domains supporting higher seasonal migrant densities are along the Appalachian Mountains and the Mississippi River (Fig. 1). We found the highest migrant density (665 cm²/ha/season) at the KHTX radar site in Alabama, which also has restricted spatial coverage due to topography (Figs. 2 and 3).

Conservation Implications.

Our results help to fill some important knowledge gaps relating to the understudied migration period, which, in turn, could benefit conservation efforts directed toward migratory landbirds (11, 26, 46). Our yearly maps of stopover density confirm a broadly diffuse migration pattern of landbirds across the eastern United States. A broad front of migrating birds requires a broad front of stopover habitat. Thus, creating a small set of discrete reserves at key staging areas, as might be feasible for some shorebirds (13), would fail to protect most migrating landbirds in eastern North America. A well-distributed network of protected areas at multiple scales across the eastern United States is essential to maintaining healthy populations of these species, a point also highlighted in the strategic goals of the international Partners in Flight (PIF) conservation initiative (6). Incentive programs to encourage the protection of forests (see next paragraph), analogous to the existing federal Conservation Reserve Program (47), or local ordinances that prevent forest conversion could be developed to achieve this goal. Such efforts are especially important for protecting the sites that we identified as consistently supporting high densities of migrants at the pixel scale, especially in areas where those pixels are clustered into continuous stretches of hotspot habitat. Some existing public/private partnership programs, such as the PIF and the Migratory Bird Joint Ventures, are already engaged in efforts to protect key habitats for migrants (6).

Deciduous forest appears to be the habitat type that harbors the greatest biomass of migratory landbirds in the eastern United States. Forest fragments in largely deforested regions are especially important to migrants. Such sites may represent the “convenience stores” identified in the framework for stopover sites proposed by Mehlman et al. (11); these are places where birds can briefly rest and replenish their energy stores while moving through a generally inhospitable landscape. Mehlman et al. (11) called for more attention to these remnant forest patches because they are often neglected in conservation planning due to their relatively small size and the low nesting success of breeding populations of landbirds within them (48). This is especially relevant to the agricultural Midwest, which we identified as a human-created migration barrier with a long history of land use change and habitat loss (28, 49). Perhaps for this reason, the Midwest has experienced the greatest cumulative loss in migrant biomass over the past decade (4). We therefore urge the protection of remaining forest habitats throughout the Midwest, as well as attention to anthropogenic woodland patches that also provide suitable stopover habitats (35, 50, 51). Protecting and creating more such stepping stones might enable more migratory birds to successfully traverse this human-created migration barrier and thus maintain healthy populations.

Our study also underscores the importance of measuring both cumulative density and consistency of use in order to identify stopover hotspots. Doing so also makes the most effective use of limited conservation resources because the loss of areas that receive heavy and consistent bird use across years is more likely to directly affect the abundance of migratory bird populations (11). Conservation planning based on both metrics is more likely to achieve the mission of “keeping the common birds common” and “helping species at risk” (6). Note, however, that the broad-front migration pattern we document means that conservation measures cannot be limited to only a few places if migrant populations across the eastern half of the continent are to be sustained.

Conclusion

We mapped the distribution of stopover densities of migratory landbirds across the eastern United States using radar data. We found that eastern landbirds migrate across a broad front far different than the pattern exhibited by many shorebirds that congregate massively in a few key sites. At finer scales, we identified stopover hotspot pixels that harbor unusually large numbers of migrants and therefore deserve particular attention from conservationists. However, our general finding of a broad-front migration means that conservation efforts, too, must proceed on a broad front, targeting key habitats throughout the eastern half of the continent. We found that deciduous forest patches and riparian forests, especially those in largely unforested regions, are of particular importance to migrating landbirds. We also flag the agricultural Midwest as likely being a human-created barrier for autumn migrants. Forest remnants within this altered landscape may be of particular importance to migrants. We conclude that the challenge of conserving migratory landbirds must be understood and undertaken at multiple spatial scales if ongoing declines of these birds are to be halted or even reversed.