Radar ornithology, stopover hotspots, and the conservation of migratory landbirds

Thanks to emerging technologies, citizen science, and powerful new computational tools, we have entered a golden age for studies of animal migration (1). We can now track individual birds on seemingly impossible migratory journeys, link the timing of migration with resource phenology (2), accurately predict how weather affects migratory movements (3), and determine connectivity between breeding and wintering populations (4). Some of these tools can also be used to identify stopover hotspots where migrants concentrate in large numbers between migratory flights to refuel and recover physiologically (5, 6). Guo et al. (7) provide a compelling example of how the system of weather radar stations spread across eastern North America can be used to measure the population density of migrants at potential stopover sites at many spatial scales and how these data can inform both local and regional conservation strategies.

Identifying migratory hotspots has become an urgent need because migratory birds as a group are undergoing severe and rapid declines (8). Conserving migrants is challenging because they face threats on often-distant breeding and wintering grounds and along the routes used during migration (9). Migrant population dynamics therefore can be understood only with studies that cover the entire annual cycle (10). The declines of migrants, for example, have been linked to loss of winter habitat, fragmentation and loss of breeding habitat, and climate change–related decreases in survival and breeding productivity (10). Yet, many, if not most, studies of long-distance migrants have concluded that mortality rates are highest during migration, which suggests that loss of migratory stopover sites contributes to their declines (7, 10).

Guo et al. (7) used weather radar station imagery from a 5-y period during the Fall migration period to measure the population density of migrants departing from stopover sites. They worked at multiple spatial scales ranging from comparisons of migrant density between radar stations spread across eastern North America to specific sites and habitats within a fixed radius of each radar station. It is only recently that we have had the computational tools and power needed to analyze the enormous amounts of data generated by these radar stations (7, 11).

At the largest scale (eastern North America), Guo et al. (7) showed that migrants move across a broad front rather than move along specific migratory flyways but that they concentrate more along the base of the Appalachians and in the Mississippi River Valley. Guo et al. (7) also confirmed the value of deciduous forests, which might contain richer food resources for migrants than coniferous forests, especially in the Fall when both fruit and insects are abundant. Migrants also occurred more consistently and at the highest population densities in more forested landscapes, which suggests that a conservation strategy aimed at increasing forest cover to decrease fragmentation-related problems with nesting success (12), will also benefit birds during migration.

The midwestern US landscape, which has long been iconic in studies of landscape ecology and migration (13, 14), provides a concrete example of the insights that can be obtained from analyzing weather radar data. The Midwest contains a large “prairie peninsula” [see figures 2 to 3 in Guo et al. (7)] that was formerly dominated by grasslands and large riparian forest tracts but has mostly been converted to row crops. This peninsula is bordered to the north and south by extensive forest tracts such as those in southern Illinois (see Insets in figures 2 and 3). A bird flying from southern Illinois to the nearest extensive forest tracts to the north would have to cross nearly 1,000 km of landscape dominated by corn and soybean fields. Nevertheless, patches of deciduous forest remain within the prairie peninsula. Migrants crossing this gap therefore have a choice between making a single long flight across the prairie peninsula to the nearest large forest tract or making shorter flights using the remaining forest fragments as steppingstones where they refuel quickly (15, 16).

Guo et al. provide a compelling example of how the system of weather radar stations spread across eastern North America can be used to measure the population density of migrants at potential stopover sites at many spatial scales and how these data can inform both local and regional conservation strategies.

The decision each bird makes between migrating in a few long flights or many short flights has been the focus of intensive research over the last few decades and is the basis of optimal migration theory (5). Many migrants facing the need to cross large gaps such as the Gulf of Mexico and the Caribbean Sea spend extended periods in large forest tracts in southern Central America or along the northern edge of South America, during which they recover from previous flights and store the large amounts of energy necessary for extended (up to at least 3,500-km) flights (17). Other birds, however, migrate in more gradual increments as they move overland through Central America or island-hop across the Caribbean. The same individuals can switch between these two migratory strategies in different years (18), which suggests that migratory decisions are flexible and may depend upon local conditions encountered during migration.

Guo et al.’s (7) data suggest that both strategies are being used by migrants crossing the prairie peninsula. Migrants consistently concentrate in the large forest tracts that border the southern edge of the prairie peninsula but also occur at high densities in the remnant forest patches within the prairie peninsula. Intensive local field studies in this landscape could test the hypothesis that migrants concentrating in the large forest tracts bordering the prairie peninsula are mostly composed of individuals that cross the prairie peninsula in a single flight, and those using the small woodlots within the prairie peninsula are making a series of short flights punctuated by brief stopovers. Some of the earliest, and often heroic, studies of migrants using radiotelemetry from fixed-wing aircraft in this landscape show that migrants can make long (>500-km) flights after departing small midwestern woodlots (13). Newer technologies such as automated telemetry systems using lightweight transmitters (19) make relocating radio-tagged individuals much easier. Linking weather patterns with migrant use of these small woodlots would further allow us to test the hypothesis that the primary value of small, isolated woodlots is as emergency stopover habitat during inclement weather (15, 16).

Based on their results, Guo et al. (7) argue that any conservation strategy aimed at improving stopover habitat must include both preservation of the remaining large forest tracts and conserving and restoring forest patches within the prairie peninsula. Even if these small woodlots are “sink” habitats for breeding birds where their nesting success is too low to compensate for adult mortality (14), their value during migration may far outweigh any population losses during the breeding season. Given the high concentrations of migrants in small woodlots within the prairie peninsula (16), it would also be valuable to measure how effectively migrants are able to store energy in the face of potentially severe competition for food resources. Local studies could also quantify what plant species and communities are used most heavily by migrants, which could translate directly to local management practices (e.g., restoring floodplain forests and planting favored food trees); this could have enormous benefits for migrating birds. Similar studies in the urban areas identified by Guo et al. (7) as migratory hotspots might be especially important if migrants are being lured to these habitats by urban light pollution (20).

Guo et al. (7) concluded that “a well-distributed network of protected areas at multiple scales across the eastern US is essential to maintaining healthy populations of migratory birds” (lines 311 to 313). Clearly, maintaining the large forest tracts bordering the prairie peninsula is a top priority as they provide the conditions necessary for both successful nesting (14) and the storage of fuel for long migratory flights (17). They further argued that local conservation actions are critical, especially in human-dominated landscapes where deciduous forest cover is concentrated in residential areas and small fragmented woodlots. Guo et al.’s (7) results also suggest directions for future research such as measuring the time spent by individual birds in stopover sites, detailed studies of the movements of individual birds, measuring energy storage, and coupling the results from radar studies with the species-specific data provided by citizen science databases (21).