Migration, The Black Box of Neotropical Bird Conservation

Migration is a dangerous undertaking. For conservation efforts to be effective, scientists need to better understand just what threats birds face along the way, and how those obstacles are changing in an increasingly human-dominated world.

A new paper from Smithsonian, Nature Conservancy, and other agency scientists assesses the state of the science on bird migration across the Gulf of Mexico, and identifies both research gaps and new technology that can help fill them.

Mysteries on the Wing

Each spring and fall, neotropical songbirds amass by the millions to migrate between North and South America. Warblers, vireos, and other passerines typically fly across the Gulf of Mexico in a single, brutal 18-to-24 hour flight, while raptors circumnavigate the Gulf’s western shores.

Many birds don’t survive. By measuring populations at the breeding and wintering grounds, scientists know that the majority of mortality for these species occurs during migration. What obstacles do these birds face along their journey, and just how, when and where are these birds dying?

“Migration is still a bit of a black box,” says Emily Cohen, a researcher at the Smithsonian Migratory Bird Center (SMBC) and lead author on the research. “For nearctic neotropical long distance migrants, we just don’t have a holistic picture of sources of mortality during the migratory periods.”

Scientists from the SMBC, The Nature Conservancy, and their colleagues set out to assess the state of the science surrounding bird migration in the Gulf. Their paper, published recently in The Condor, outlines what is known about major events and obstacles facing neotropical migrants, identifies research gaps, and reviews emerging technology that could help fill those gaps.

Migration is still a bit of a black box.

Emily Cohen

A Region-Wide View of Migration

Much of what scientists do know to date has focused on either specific species or specific locations. The exact movements of American redstarts during migration, or the use of a particular stopover site are both key pieces of the puzzle, but they don’t reveal the bigger picture.

“There haven’t been a lot of region wide studies, either on the whole migratory route of a species or the movement of all species,” says Cohen. “Now we have the tools to do this quantitatively for the first time on a large scale.”

Two complementary new datasets could help unlock some of those questions — radar imagery and citizen science. Amazingly, bird migration is so massive that it can often be picked up on radar networks. “You can use radar data to look at birds moving through the airspace or even lifting up from stopover habitat,” says Cohen. “And the data source is phenomenal, because it’s consistently collected, archived, and freely available.”

The researchers propose that radar data will be a key tool in both identifying and characterizing stopover habitat — particularly in the less-studied areas of Mexico and the Caribbean — and understanding how consistently those locations are used over time.

It can also help scientists map airspace. For migratory birds the air itself is habitat, but one that, until recently, has been largely overlooked and under-studied. And like habitat on land, airspace is increasingly threatened by flight paths, buildings, light pollution, wind development, and even drones. Radar data can help map this habitat and allow scientists to better understand how birds are using it.

The second data source is just about the exact opposite of region-wide radar: the birder’s checklist. The citizen science platform eBird allows birders across the country to log checklists of where and when they see an individual species. Pooled together, these data are an incredible resource for scientists to better understand species distributions, their associations with particular landscapes, and the timing of their migrations.

The Population Perspective

Cohen explains that another key research gap is knowledge about populations of a specific species. Wood thrush populations are declining rapidly, but that decline is not even across their breeding range. Data from the North American Breeding Bird Survey show that populations in the western part of the range (Missouri, Wisconsin, and parts of Illinois) appear stable or increasing, while in much of the northeast and southeast their numbers are plummeting.

It’s possible that these populations also have different wintering grounds or migratory routes, where threats could be contributing to an en-route decline to the breeding grounds. For this species, scientists recently discovered that they follow different migratory routes each season, flying south along the Florida peninsula in fall and then making landfall in coastal Louisiana on their way north. “So we need to have migratory connectivity information to put it into perspective in a population context,” says Cohen.

Cohen and her colleagues at SMBC are using the new method of stable isotope analysis to start asking those questions. At banding stations in coastal Texas, SMBC field teams collect a single tail feather from each bird they catch and band during spring migration. “That tail feather holds the isotopic signature of where the feather was grown,” says Cohen, “and they’ve carried that feather since the previous breeding season.”

Cohen and her colleagues analyze the isotopic ratio of hydrogen in the feathers. These ratios change with latitude, which provides coarse-scale information about where that individual bird bred the previous summer. “It won’t tell you Washington DC versus Baltimore,” she says, “but it can tell you Canada’s boreal forest versus Michigan.”

The Holy Grail of Migration Science

Another puzzle is working out exactly how many birds die en route to the breeding or wintering grounds, and where that mortality occurs.

“Some new technology has come along that is helping unravel that question,” says David Mehlman, director of The Nature Conservancy’s migratory bird program and co-author on the research. “Nifty things like light-level geolocators and smaller and lighter radio transmitter technology are letting us track these small birds over long distances.”

Tracking technology is always limited by the size of both battery and bird. Batteries need to be large enough to gather data for long periods of time, but small enough not to hinder — or exhaust — already-taxed and often tiny migrant species.

Increasingly smaller batteries are allowing scientists to track smaller species, a trend which the researchers expect to continue. But they come with a high price tag, and the need to re-capture the bird to retrieve data. “They’re great at helping us understand habitat quality and population-level distributions,” says Cohen, “But they don’t tell you where birds die, which is sort of the holy grail.”

Other technological advances might improve the situation by taking advantage of existing infrastructure to retrieve data from passing birds. “It’s within realm of possibility that we can have networks of radio receivers spread across the Gulf that would passively intercept tagged birds,” says Mehlman.

Clear identification of these research gaps — and the technologies that can help fill them — can’t come soon enough. “We’ve known for many years that populations of neotropical migrants are declining,” says Mehlman, “but the challenge is that there are multiple things going on.”