Every year, as the seasons change, billions of animals embark on journeys to find food, to get to better habitats or to breed. They migrate in groups and as individuals, flying, swimming, crawling and walking across international borders and through habitats to survive, transporting seeds and nutrients.

A major new report by the United Nations finds that humans are not only making those journeys more difficult, but have put many migratory species in a perilous state.

Nearly half of the world's already threatened migratory species have declining populations, the first of its kind UN report found. More than a fifth of the nearly 1,200 migratory species monitored by the UN – whales, sea turtles, apes, songbirds and others – are threatened with extinction.

"These are magnificent species that take unbelievable journeys, in some cases, that are economically beneficial [for humans], as well as the stuff of poetry and song and cultural significance," said Amy Fraenkel, executive secretary of the UN Convention on the Conservation of Migratory Species of Wild Animals.

The report, compiled by conservation scientists, is the most comprehensive assessment of the world's migratory species ever carried out. It looked at 1,189 different species that are already protected by the Convention on Migratory Species — a 1979 treaty intended to conserve species that move across international borders — to see whether conservation efforts are working.

In some cases, they are. Wildlife crossings are helping animals traverse over roads and fences. Regulations are helping prevent poaching and overconsumption of some threatened fish and mammals. Habitat protections are giving species room to move and prosper.

To reverse population declines though, the report's authors said, those "efforts need to be strengthened and scaled up."

The publication is the latest global report to raise concerns about the planet's non-human inhabitants. A 2019 assessment on the world's biodiversity found that 1 million of the Earth's estimated 8 million species are at risk of extinction, many within decades, because of human activities like overconsumption, deforestation, pollution and development. A 2022 report by the World Wildlife Fund found that wildlife populations have declined by an average of 69% in the last 50 years.

For migratory species, the threats from human activities can be amplified. Protections for species vary from country to country. Enforcement of conservation laws can differ depending on locale.

Hunting and fishing – overexploitation – and habitat loss from human activities were identified as the two greatest threats to migratory species, according to the new report. Invasive species, pollution – including light and sound pollution – and climate change are also having profound impacts, the report found.

Many species migrate with the change of seasons. Human-caused climate change is altering seasons, lengthening summers, shortening winters and shifting the timing of spring and fall. Scientists have documented animals, like birds in North America, adjusting the timing of their migrations to match those shifts. Not all are keeping pace with the rate of change, leading to what scientists call phenological asynchrony.

World leaders from the 133 countries that have signed on to the Convention for Migratory Species are meeting this week in Uzbekistan to chart a path forward.

The new report, Fraenkel said, should give the parties a sense of urgency, but it should also be a guide for anyone "who wants to keep seeing the birds flying and the whales jumping in water," she said. "Look at this report and find something [you] can do to help these incredible species continue to survive."