Some species of migrating birds are increasingly attracted to life in the big city – but these unnatural environments can be deadly.

Ana Morales stepped through the shrubbery, scanning the airwaves with a radio receiver. The device had picked up a signal from a transmitter that she and colleagues had previously attached to a Swainson's thrush, a small brown and white speckled bird native to the Americas. The same signal had popped up on Morales' handheld receiver a few days earlier, emanating from exactly the same bush in a park on the edge of Montreal in Canada.

This was a worry. It seemed so unlikely that the transmitter remained attached to a live bird – more likely, it had fallen off and was hanging on a branch. Just to make sure, Morales, a graduate student at McGill University, gave the berry-laden shrub a gentle shake – and then a flutter of feathers among the branches and shadows caught her eye. To her surprise, the thrush, very much alive and healthy, was hopping about the bush in protest at having been disturbed. This little bird had hung around for longer than she expected.

Swainson's thrushes migrate from northern areas to Central and northern South America every autumn. But some make a "pit stop" in and around cities such as Montreal. For a study published last month, Morales and her colleagues had been researching how Swainson's thrushes balance the need to migrate quickly – so as to maximise its benefits – with the need to refuel, such as by stopping over in places like Montreal. They caught and radio tagged a total of nearly 80 of the birds.

Huge numbers of migrating birds visit cities all around the world on their extraordinary journeys, which often cover thousands of kilometres. It is not always obvious why they come to urban locations. Some appear to be attracted by light. Others, such as the Swainson's thrush in its bush full of berries, seem to enjoy the food on offer. But cities are not always friendly to outsiders.

The death toll, sadly, is staggering. Some migrating birds, for example, are killed by domestic cats while others collide with buildings. Many thousands of birds die every year in New York alone when they crash into the brightly lit windows of skyscrapers, a well-known problem in megacities (Read more from BBC Future about why this happens). More recently, a flock of blackbirds was filmed dropping from the sky onto a street in the city of Chihuahua in north-west Mexico, leaving many dead.

And yet, scientists are discovering that towns, while dangerous, can sometimes help to support migrating species. So how do we ensure that cities act more as travel lodges – and not death traps – for these species?

During their study, Morales and her colleagues found that Swainson's thrushes make surprisingly long stopovers in Montreal, where many of the birds moult – a process through which the birds shed and regrow some of their feathers. That helps to prepare them for the long migration. It's like putting a new set of tyres on your car.

"It's pretty amazing that this small green area can support a bird for 40 days," says Morales, marvelling at how contented the thrushes seemed. The birds might move to this urban green space precisely because it is rich in resources such as berries and water.

Any bird landing in a city park could find such rewards, if they are present, but what draws birds to a bustling metropolis in the first place? It could largely be to do with light, says Morales' co-author Barbara Frei at Environment and Climate Change Canada, a department of the Canadian government.

No one knows exactly why birds are attracted to artificial light at night but there is ample evidence for this effect. One possibility, Frei says, is that birds – which use starlight and other phenomena to navigate – are naturally enticed by points of light.

More than 100 years ago, the Irish anatomist and ornithologist Charles Patten was stationed at a lighthouse off the coast of Ireland where he observed this phenomenon first-hand. According to his reports, mind-boggling swarms of migrating birds would fly towards the lighthouse and, unfortunately for them, crash into its windows. Many of these birds died, which allowed Patten to collect and study them. But back then, sources of very bright, artificial light were uncommon whereas today, electric light is visible practically everywhere at night.

Many millions, possibly even billions, of migrating birds die as a result of this every year. The tallest, brightest buildings situated along known migratory flight paths are likely the most deadly. Research suggests the huge McCormick Place convention centre in Chicago, for instance, caused up to 11,567 bird deaths between 2000 and 2020.

"A lot of the light isn't really necessary," says Frank La Sorte, at Cornell University, when referring to urban lighting in general. "It's excessive."

Switching more lights off at night when birds are migrating could save thousands of avian lives, he says. A study published last year estimated that by turning out half of the lights in McCormack Place in the spring and autumn, bird mortality could be reduced by nearly 60%.

The pull that urban light has on migrating birds is unlikely to be exclusive to large cities. Simon Gillings at the British Trust for Ornithology lives in Cambridge in the UK, a city with a population of just 130,000. The tallest building, Cambridge University's library tower, stands at less than 50m (164ft). "It's hardly Manhattan," he admits.

Gillings wanted to find out whether even a small city like this would act as a significant light-emitting beacon, attracting migrating birds at night. He and several volunteers placed audio recorders in their gardens during the autumn of 2019 and together recorded many thousands of hours of night-time audio.

"Anybody that starts doing that suddenly realises there's this huge wealth of birds moving around," says Gillings, marvelling at the range of bird calls captured by the recordings.

To comb through the data, he used an artificial intelligence system that automatically counted the number of calls in the recordings that were made by three migratory species – redwings, song thrushes and blackbirds.

This revealed a clear correlation. A greater number of calls from these species were picked up in gardens in densely populated, brightly lit urban areas. Gillings cautions that it is possible the number of birds flying over gardens in the study simply made more noise when flying near to bright light. However, it is at least some evidence that there is increased activity from migrating birds in locations with heightened illumination.

Gillings says that our first port of call should be to reduce the light emitted in urban areas in order to avoid unnecessarily attracting species such as redwings and song thrushes into towns and cities during their migration. People might ensure that security lights point downwards, for example, or that cycle paths are illuminated with low-level lighting only, he suggests. Other research indicates that red light may be less attractive to birds when they are migrating.

Among the many small birds that migrate huge distances every year in the Americas are warbler species such as the yellow-rumped warbler and the Nashville warbler. Some birds of these species travel all the way from North America to Mexico.

When the researcher Jorge Schondube and doctoral student Rodrigo Pacheco-Muñoz, both studying ecology at the National Autonomous University of Mexico, began tagging and recording these warblers at various locations in and near to the city of Morelia, they quickly found something unexpected. The birds that they captured in green spaces within the city were just as healthy in terms of body mass, feather condition and number of parasites as birds of the same species captured at non-urban sites, including a national park outside Morelia.

Schondube says he was surprised how well the warblers in built-up areas were faring: "Most of the time, I thought of those birds as birds that were going to die." But this proved not to be the case.

He also notes that 10 years ago there were no records of the Nashville warbler inside Morelia. But it seems to have taken to relying on well-watered, mature trees in urban green spaces more recently. Well-developed trees can harbour hundreds of insect species and can therefore support warblers, which are insectivores.

Because the birds seemed so content among city trees, the researchers argue that providing adequate tree cover could help to accommodate migrating birds should they choose to enter cities. The pair note that they rarely if ever find these warblers in city spaces that aren't so green, which may also indicate the importance of parks as a foothold.

Footholds can, though, become pitfalls, warns Frances Bonier at Queen's University in Ontario. An urban green space might attract birds but not actually provide all the resources that they need when they are breeding, for instance. This is known as an ecological trap.

And Gillings adds that, while a smattering of trees and green roofs in a city will be of benefit to some birds, others require wetlands or large, dense forests to really thrive. Still more rely on pristine coastal areas and so on. It would be a mistake to think we can compensate for the destruction of such habitats by making cities a bit greener. At the end of the day, those specialist species that rely on the world's rich diversity of habitats are the ones most at risk.

With that important caveat in mind, it remains important to ensure that cities are accommodating for wildlife, says Frei. Bird migration routes just happen to bring them close to many cities all around the world, and our bright lights draw them in. Frei proposes that urban planning take this into account. The designers of every new park or housing estate could include some vegetation suitable for birds and other species, for instance.

"We should plan it for all different things together – it's good for people, it's good for the planet, it's good for the wildlife," she asserts.

Pacheco-Muñoz agrees. Cities seem like the antithesis of nature but they really don't have to be because, in reality, they are already full of it. And, as is now clear, they draw many migrating species towards them.

"We need to think of cities as ecosystems," says Pacheco-Muñoz. "If we think about it, we are the masters of this ecosystem – and we can decide how to manage this place."

More about: