Blinded by the light: how light pollution lures birds into urban areas during autumn migration

Toronto skyline at night

From Rare Bird Alert

On their autumn migration south in the Northern Hemisphere, many birds are being lured by artificial light pollution into urban areas that may be an ecological trap, according to new research using weather surveillance radars from the northeastern United States over a seven-year period to map the distributions of migratory birds during their autumn stopovers.

Since most of the birds that migrate in the US are nocturnal and leave their stopover sites at night, the research group took snapshots of the birds as they departed.

Red-winged blackbird. Photo by Mick Dryden

“Shortly after sunset, at around civil twilight, they all take off in these well-synchronized flights that show up as a sudden bloom of reflectivity on the radar,” University of Delaware’s Jeff Buler said. “We take a snapshot of that, which allows us to map out where they were on the ground and at what densities. It basically gives us a picture of their distributions on the ground.”

The researchers were interested in seeing what factors shape the birds’ distributions and why they occur in certain areas.

“We think artificial light might be a mechanism of attraction because we know at a very small scale, birds are attracted to light,” Buler said. “Much like insects are drawn to a streetlight at night, birds are also drawn to places like lighthouses. Especially when visibility is poor, you can get these big fall-outs at lighthouses and sports complexes. Stadiums will have birds land in the stadium if it’s foggy at night and the lights are on.”

One hazard for birds attracted to city lights is death from flying into high buildings. Buler said that some cities such as Toronto have even gone so far as to institute ‘Lights Out’ programmes, turning off the lights in tall buildings to deter birds from colliding with them.


Sky Glow

The research team analysed the distributions of the birds in proximity to the brightest areas in the northeast such as Boston, New York, Philadelphia, Baltimore and Washington, D.C.

“These are super-bright, large metropolitan areas,” Buler said. “We found an increasing density of birds the closer you get to these cities. The effect goes out about 200 kilometres [about 125 miles]. We estimate that these flying birds can see a city on the horizon up to several hundred kilometres away. Essentially, there is no place in the northeastern United States where they can’t see the sky glow of a city.”

Parks and yards

The researchers also found that suburban areas, such as people’s backyards and city parks, such as Fairmount Park in Philadelphia, harbour some of the highest densities of birds in the northeast.

“Fairmount Park has higher densities of birds than at Cape May, New Jersey, which is where birders typically go to see birds concentrating during migration,” Buler said.

When they do get lured into cities, the birds seek out suitable habitat, which can cause concerns from a conservation standpoint as lots of birds pack into a small area with limited resources and higher mortality risks.

“One of the things we point out in this research is that there might be negative consequences for birds being drawn to urban cities. We know there’s risk of collision with buildings, collision with vehicles, and getting eaten by cats, which are a major predator,” Buler said.

“Domestic cats could be the largest anthropogenic source of mortality for birds. If birds are being drawn into these heavily developed areas, it may be increasing their risk of mortality from anthropogenic sources and it may also be that the resources in those habitats are going to be depleted much faster because of competition with other birds.”

Another concern: light pollution created in these cities has been increasing in recent years with the advent of LED lights, which are much brighter than the incandescent lights they replaced.

“The transition of street lighting from incandescent to LED continues to increase the amount of light pollution,” Buler said. “If you think about it from an evolutionary sense, for all wildlife really, mammals and insects and birds, they’ve only been exposed to this light pollution for less than 200 years. They’re still adapting to the light.”

Access the paper Artificial light at night confounds broad-scale habitat use by migrating birds here

6 thoughts on “Blinded by the light: how light pollution lures birds into urban areas during autumn migration

  1. Precisely when they are increasingly becoming susceptible to Falcon (even at night) or Hawk attacks as well. Our songbirds need urgent conservation efforts directed at them, the world over.

    Kind Regards

    Tony Powell and natuestimeline

  2. *when and where* they are increasingly becoming susceptible etc. Eg. in our towns and cities. Most large cities in the UK now have resident pairs of Peregrines around, for instance. For every bit of good news there are potential downsides I’m afraid.

    • Although, presumably, with millions killed striking buildings and rather large numbers of domestic cats lurking at ground level, the small number of peregrine pairs in big cities aren’t that big a threat to migratory songbirds. And don’t most town peregrines thrive on feral pigeons? St Helier has one peregrine pair and they do take small numbers of a variety of species but mostly they take pigeons.

      • Yes, true. But by merely being yet another successful increasing in abundance predator, specialising in passage migrants whether songbirds or waders, this is yet another problem the less successful species could perhaps do without. Of course, not much one can do other than to maybe have periods when the lights are off and to keep up the habitat creation and feeding of those increasingly threatened songbirds and wader types I mention. The rise of the generalists is a real one, and they are quick to adapt to new food sources when one meal type (feral pigeons die out, and nighttime migrants replace them) changes to another. The city Peregrine diet is a-changing, and there are more moonlit and streetlight-lit raids these days than ever before. Shame, the specialist insectivorous types don’t have much more us humans could offer them.

        • There is little evidence in Europe that the feral pigeon population is doing anything other than increasing or remaining stable, depending on which location we look at. In the UK and across Europe, however, the woodpigeon population is expanding exponentially. Whilst it is true that the peregrine population is increasing, it has not yet returned to a population size above that which existed before the historical persecution of that species, and as such peregrine predation is, if anything, no more significant than it has ever been regarding the general population of any species. ‘Night-time raids’ or roosting or migrating birds are less significant from peregrines than they are from owls or mammals. There is also the statistical proviso that less abundant species are less frequently encountered by peregrine, and are therefore predated less frequently than common species, although the significance of losing an individual from the population is obviously more significant for rarer species.

          Further, you seem to state that peregrines have ‘specialised’ in predating migratory species, when again this simply is not the case. Rather, Peregrine are opportunistic, and will take anything viable; they have been recorded taking a broad range of species from woodpeckers, through kingfishers and parakeets, to pigeons and ducks, and even other peregrine!

          As such, I am afraid that the biggest drivers behind quite simply have an anthropological origin: habitat loss and fragmentation, air pollution (birds are sensitive to air pollution at an order of magnitude several times greater than humans), changes in agriculture, and predation by introduced species (possums, cats, stoats, rats, etc, depending on location). Until these effects are mitigated against, worrying about the minuscule effect of natural predation on overall population levels is like urinating into a hurricane.

  3. Thanks, James. No urinating from me this end, only those in denial, perhaps. I agree with a lot of what you say, and we mustn’t make this into a Peregrine-hating post as that was clearly not my intention. I simply believe the pace of change is not one which people might imagine it to be; things are changing rapidly ecologically-speaking, especially when talking about our common and rarer bird populations. Your comment “as such peregrine predation is, if anything, no more significant than it has ever been” doesn’t hold true anymore when there are increasing pressures from other predators as well and additionally many external factors as well. Some species are being lost on a local or regional level when they were relatively abundant only ten years ago or so, with global ramifications felt a few years on from there. In other words, ecological arms races are affecting the least-adaptable species, and without interventions (certain ones potentially deemed unsavoury) only the generalists will remain until perhaps the Sixth Extinction Kills them too.

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