Every year, billions of birds undertake journeys that stitch together continents, oceans, and hemispheres — flying from breeding grounds in the Arctic to wintering habitats in the tropics, threading through a patchwork of countries, ecosystems, and legal jurisdictions along the way. That interconnected life history has always made migratory birds uniquely difficult to protect. A single species might depend on wetlands in Canada, stopover forests in Central America, and coastal mudflats in South America — and a threat to any one of those links can undo conservation gains made everywhere else. A sweeping new global review, published in Nature Reviews Biodiversity, puts a number on just how badly that fragmented approach has failed: roughly 51% of the world’s migratory bird species are now declining.

The review, led by researchers from Georgetown University and the Smithsonian’s National Zoo and Conservation Biology Institute, working with a global collaboration of ornithologists, represents the most comprehensive assessment yet of migratory bird populations, the threats driving their declines, and the conservation strategies available to reverse them. It draws together decades of research, international monitoring programs, global biodiversity databases, and existing conservation agreements — an effort to establish not just what scientists already know, but where the biggest gaps in that knowledge remain.

Nearly 300 migratory species are already classified as globally threatened, but the review’s authors emphasize that the crisis extends well beyond that list. Many birds still considered common and widespread are also losing ground, a sign that the threats facing migratory species are broad and systemic rather than confined to a handful of already-endangered populations.

“The scale of migratory bird declines is both alarming and sobering,” said Peter P. Marra, dean of Georgetown University’s Earth Commons Institute and co-lead author of the study. “What we’re seeing is the predictable consequence of decades of environmental degradation. Yet these declines are not inevitable. This review goes beyond documenting the crisis by identifying the major threats, highlighting the critical knowledge gaps, and providing a science-based framework for reversing declines before more species are pushed toward extinction.”

Part of what makes migratory birds so ecologically valuable is exactly what makes them hard to protect: their movement across vast distances links ecosystems together in ways few other animals do. They help control insect populations, disperse seeds, pollinate plants, and support food webs that span entire continents. They’re also one of the most visible entry points through which ordinary people engage with wildlife — birdwatchers and community scientists worldwide feed observations into global databases that have become indispensable tools for tracking population trends and shaping conservation policy.

Even so, basic questions about migratory birds remain surprisingly unsettled. Of the roughly 11,000 bird species worldwide, scientists don’t have full agreement on which should even be classified as migratory, let alone precise data on where and why specific populations are shrinking. Newer tools are starting to close that gap: tracking devices small enough to fit on songbirds, community science platforms like eBird, AI-powered acoustic monitoring that can identify species from their calls, and increasingly sophisticated population models are together allowing researchers to map migration routes, link populations across seasons, and pinpoint the combination of habitat loss, climate change, invasive species, building collisions, disease, and pesticide exposure driving specific declines.

“Previous analyses have brought global attention to the fact that North American and European migratory birds are declining at alarming rates,” said Nathan W. Cooper of the Smithsonian’s National Zoo and Conservation Biology Institute and the study’s other co-lead author. “But, until now, we did not have a good assessment about how birds are doing across all of the global terrestrial and marine flyways.”

That global gap is central to the review’s argument. The authors contend that conservation strategies aimed at single sites or single countries are structurally mismatched to a problem that plays out across entire hemispheres and multiple governance systems at once. Their proposed framework calls for expanding monitoring capacity, strengthening the social science side of conservation work, protecting critical habitat, tackling major threats directly, and deepening international cooperation — treating a migratory species’ entire annual cycle, from breeding ground to migratory stopover to wintering habitat, as a single connected system rather than a series of disconnected local problems.

One example the authors point to is Road to Recovery, a science-based initiative Marra co-founded that is now based at Georgetown University. The program coordinates conservation efforts across the Western Hemisphere, aiming to identify the specific factors limiting a species’ population and act on them before that species slides toward threatened status — combining scientific and technological tools with attention to the human dimensions of conservation.

Despite the grim topline number, the review’s authors are careful to frame the findings as a call to action rather than a verdict. Species-specific conservation campaigns have, in the past, pulled waterfowl, cranes, shorebirds, and raptors back from the edge of extinction. The challenge now, according to the researchers, is scaling those individual successes up to match the geographic sprawl and interconnected complexity of migratory birds as a whole.

“These declines are not inevitable,” Marra said. “We know conservation works. This review shows that we have the scientific knowledge needed to reverse many of these declines. The challenge now is bringing together the partnerships, resources, and political and public commitment to implement those solutions at the scale migratory birds require.”

Endnotes

1. “Status, threats and conservation of Earth’s migratory birds.” Nature Reviews Biodiversity (2026). DOI: 10.1038/s44358-026-00177-7

2. Georgetown University Medical Center press release, July 23, 2026, via EurekAlert!: https://www.eurekalert.org/news-releases/1137405

3. Road to Recovery initiative: r2rbirds.org