Radar love: Weather data detects hotspots for migratory birds

Have you ever taken a long road trip covering hundreds or even thousands of miles, during which you were pretty much flying by the seat of your pants navigationally? Let me guess: You were broke, moving, between jobs, between semesters, on a journey of discovery, or all of the above. And if your trip took place in the not-so-distant pre-smartphone era, you were also constantly trying to figure out your next move based on maps, vacancy signs, and billboards promising all-you-can eat anything, though usually not salad.


Migratory birds like yellow warbler need to make many stops to rest and refuel during their annual journeys to wintering grounds as far away as South America. Credit: FWS

Migratory birds that breed in North America embark on a similar journey every fall to reach wintering grounds as far away as South America. Similar in that they are also flying by the seats of their pants, but different in that they are actually flying, are not wearing pants, and face much graver dangers than overindulging at the waffle bar.

Tall buildings, food scarcity, high winds, cats — rife with threats, migration is a stressful time in the life of birds. They suffer higher mortality in this period than in any other phase in their annual life cycles, and that can put a strain on entire populations. It’s critical that birds have safe places to rest and refuel along this perilous journey, and the more we can do to help manage and protect important stopover sites for them, the more likely they will be to reach their destinations.

The first step to protecting the most important stopover sites is to figure out where they are located. I know what you’re thinking: Bird migration road trip


Radars detect birds initiating migratory flights from stopover sites, and provide an estimate of the relative density of birds leaving a given location

Dr. Jeff Buler at the University of Delaware had a better idea. “In the Northeast, nothing provides more comprehensive coverage of the land surface than radar,” he points out. “It detects birds over more than a third of the land area in the Northeast.” 

With support from the U.S. Fish and Wildlife Service and other partners, Buler and his colleagues analyzed seven years of weather surveillance radar data to predict potentially important stopover sites for migratory landbirds in the region. They also conducted surveys for two fall seasons at 48 sites in the Delmarva peninsula and mainland Virginia to corroborate the radar detections with that they observed on the ground.

“We wanted to know: What are birds doing during stopovers, and why are they choosing certain sites over others?” says Buler. Here is what they found out:

Bright lights, bird city

When was the last time you gazed up at the Milky Way galaxy shimmering in the night sky from your front steps? For 70 percent of the population of the United States, it’s been awhile. Light pollution has increased dramatically in the past century, and the Northeast is one of the brightest areas on the entire planet. It’s affecting more than star gazers.

“Birds flying at about 500 meters above the ground can always detect the sky glow of some large city on the horizon,” says Buler. For migratory birds, artificial light is never out of sight, and it appears to be attractive. The study showed that migrant bird density increased with proximity to the brightest areas. Also known as cities.


A composite satellite image of the Northeast and Midwest shows the extent of artificial night light in the region. Credit: NASA

Two other key factors that determine where birds are likely to stop are the distance to the Atlantic coast, and the amount of hardwood forest cover on the landscape. “Migrant density is higher in places where there is more food, but migrants also become concentrated in coastal areas because winds push them out over the ocean, and they need need to retreat back to land,” explains Buler.

Across the landscape, results showed that birds responded positively to the coast, bright lights, hardwood forest, and any combination of those variables, indicating that migrant stopover was extra concentrated in woods of urban and suburban parks near the Atlantic coast.

“We need to recognize the importance of urban parks for migratory birds,” says Buler. For the thousands of individual birds representing hundreds of species that are passing through in the fall, these areas are like oases in the concrete jungle. Enhancing the habitat quality of urban forests for migrants by planting native vegetation that will host more insects and fruit during migration may be the best way to maximize the conservation value of these parks.

He also points out an important demographic detail: The relative intensity of use of urban areas is higher in the fall than in the spring. “Fall is when you see naive birds that are migrating for the first time, and studies show that juveniles have strong orientation to bright lights,” says Buler. “Probably in fall, parks are hosting a lot of young birds.” Which represent future generations of their species.

Between meals

If in the midst of a long road trip, you crashed on the floor of a friend’s studio apartment overlooking a busy intersection, you might be inclined to hit the road first thing the next morning. Especially if it’s that friend with the enormous dog who could use a bath and always seems to want to lick your face. If during that same trip, however, you crashed in the guest room of your grandparents’ quiet country home, and they were eager to cook for you, you’d probably stay until they kicked you out.

The ground surveys that Buler and his colleagues conducted at stopover sites in Virginia, Delaware, and Maryland not only helped them corroborate what they were seeing on the radar, they helped them figure out how long birds stayed at certain stopover sites, and why. As with people, it has a lot to do with the quality and quantity of resources available to help them rest and refuel.

“We could see the whole spectrum of uses across the framework of site types,” said Buler, explaining that the framework ranges from so-called “fire escape” sites that only offer a safe place to land in an emergency, to sites with lots of food and space where birds can comfortably rest and refuel.

“The fire-escape sites – which are typically along the coast and in urban areas — get heavy day-to-day use, but there is a lot of turnover,” he says. “Whereas birds tend to stay longer at sites away from the coast with lots of food.”


Shrubs that fruit in the fall, like American cranberry bush, are an important food source for birds during their seasonal migration. Credit: USDA

But the surveys showed that birds tend to stay the longest at sites in the middle of the resource spectrum. While that may seem counterintuitive to those of us who can’t get enough of grandma’s cooking, it’s consistent with what ornithologists call optimal migration theory. “If food is really good or really poor at a given site, you will only stay for a short time: Either there is nothing to eat, or you get a giant satisfying meal right away,” explains Buler. “At places where there is only a moderate amount of food available, it takes longer to refuel, so birds tend to stay longer.”

The surveys also provided detailed intelligence on what exactly migratory species are eating during their stopovers, and the researchers used that information to model habitat relationships for 14 of the most common species.

“For example, both black-throated blue warblers and American redstarts seem to be more closely associated with Lepidoptera larvae – caterpillars of moths and butterflies — than just insects in general,” says Buler.

Sky’s the limit for use on the ground

The combination of the regional radar data and the survey data equips people involved in conservation at any scale to identify important stopover sites and make management decisions that reflect the needs of specific species, such as ground foragers that feed on insects in the leaf litter.

The radar data capture nearly half of the National Wildlife Refuges in the Northeast, emphasizing their importance as stopover habitat, particularly in properties near the Atlantic Coast.

While the maps are useful for informing management strategies on protected lands — Buler says the data can help identify new priorities as well. “We can see many places with heavy use by migratory birds that are not yet protected.”

When Gwen Brewer of the Maryland Department of Natural Resources looked at the study results, she said, “The Pocomoke River corridor on the Eastern shore just lit up like crazy as a migratory hotspot.”

Pocomoke State Forest - Pocomoke River

The Pocomoke River corridor shows up as a hotspot for migratory birds on the radar. Credit: Maryland DNR

The DNR provided funding to help ground truth the radar data in coastal Maryland and the Delmarva Peninsula through the Resource Assessment Service Power Plant Research Program. Brewer, who is the Science Program Manager for the Wildlife and Heritage Service, said the study can direct her agency to other priority areas where they can use fine-scale data to narrow in on the forest patches that offer the greatest value to migratory birds.

“By showing us what stands out as important in Maryland, the study also helps us understand what our role should be in the big conservation picture,” she said. “It helps us think about the responsibility we have as part of the larger landscape, and that can inform our in-state process for acquisition, easements, and grant proposals.”

The full report, maps, and data depicting predicted bird density during fall migration are now available in the Northeast Stopover Sites for Migratory Landbirds gallery on DataBasin, and you can find a short video that the University of Delaware produced about the artificial light finding here.

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