It’s enough of a challenge to develop effective conservation plans for resident birds. But finding ways to maximize our efforts to conserve migratory birds can seem daunting. For a species like a blackpoll warbler or a bobolink, mortality is possible on the wintering grounds, on the breeding grounds and twice a year during migration.

We have plenty of evidence that wintering habitats are diminishing because of deforestation and pollution. Similarly, habitat loss affects the amount of suitable breeding habitat. Mortality during migration is thought to be the most significant threat to migratory birds.

It therefore behooves us to identify the most important stopover areas and conserve them as best we can. Some species like shorebirds and waterfowl engage in connect-the-dot migrations, massing in large numbers at traditional stopovers as migration proceeds. Familiar examples are the Bay of Fundy for semipalmated sandpipers, Grays Harbor, Washington and the Copper River Delta, Alaska for western sandpipers, Delaware Bay for red knots and Middle Creek Wildlife Management Area, Pennsylvania for snow geese.

Passerines migrate along a broad front with no traditional stopover areas. That’s why you might see a Maine mourning warbler or a Lincoln’s sparrow anywhere in the southeastern United States during migration. The migrating birds land either because of exhaustion or weather impacts.

We now have a more nuanced view of migratory stops for passerines. Some areas seem to attract higher densities of migrants. Shouldn’t those areas have high priority for conservation?

Of course, the answer is yes but we still need definitive evidence to document the number of migrants that stop at a particular place. To do so demands that we have a reliable way to quantify the birds.

Bird-banding stations provide quantitative information on the number of migrants passing through a local area. Running a banding station is a huge undertaking, requiring funding to buy nets and poles, and to recruit and train volunteers. We simply don’t have the capacity to set up banding sites broadly across the country.

We do have more modern technology that can be brought to bear in our efforts to identify important stopover areas. Radar ornithology has been an important subfield since World War II. During the war, radar observers noted enigmatic shapes on their radar screens, calling them angels. We know now that the angels were flocks of migrating birds.

Now with the development of Doppler weather technology, radar ornithologists can visualize individual birds, bats and even migrating insects like butterflies and dragonflies. Some Doppler radar is so sensitive that the rate of wing flapping can be discerned.

Fengyi Guo, from Princeton, and three colleagues recently published a paper that makes a huge leap in our understanding of migration hotspots. They used 60 weather radar stations in the eastern United States to examine the density of migrants at several geographic scales. Two were in Maine, one south of Portland and one in the Houlton area. The study was done for five autumn migrations (2015-19).

At the broadest scale, the data supported the broad-front hypothesis for passerine migration. Migrating nocturnal passerines could be detected anywhere as they took flight and embarked on the next migratory leg. Even at this scale, the Mississippi River and the Appalachian Mountains had more migrants.

Looking at smaller scales, the authors showed there were distinct habitats that consistently had high densities of nocturnal migrants. Over all five years, higher migrant densities increased with the percentage of forested landscape of all types and decreased as the percentage of croplands and pastures increased. In areas like Illinois where large tracts of forest are scarce, migrant densities were strongly concentrated in patches of forest. Broad swaths of open habitat act as barriers.

The data tell us our efforts in protecting migrants should be to preserve forestland, particularly deciduous forests, in highly altered habitats. Maine is in good shape in this regard.

Herb Wilson taught ornithology and other biology courses at Colby College. He welcomes reader comments and questions at whwilson@colby.edu

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