• Written by Frank La Sorte, Research Associate, Cornell Lab of Ornithology, Cornell University

Millions of birds travel between their breeding and wintering grounds during spring and autumn migration, creating one of the greatest spectacles of the natural world. These journeys often span incredible distances. For example, the Blackpoll warbler[1], which weighs less than half an ounce, may travel up to 1,500 miles between its nesting grounds in Canada and its wintering grounds in the Caribbean and South America.

Map showing Blackpoll warbler range Blackpoll warbler abundance in breeding, non-breeding and migration seasons. Cornell Lab of Ornithology, CC BY-ND[2][3]

For many species, these journeys take place at night, when skies typically are calmer and predators are less active. Scientists do not have a good understanding yet of how birds navigate effectively at night over long distances.

Blackpoll warbler Blackpoll warbler. PJTurgeon/Wikipedia[4]

We study bird migration and how it is being affected by factors ranging from climate change[5] to artificial light at night[6]. In a recent study, we used millions of bird observations by citizen scientists to document the occurrence of migratory bird species in 333 U.S. cities[7] during the winter, spring, summer and autumn.

We used this information to determine how the number of migratory bird species varies based on each city’s level of light pollution[8] – brightening of the night sky caused by artificial light sources, such as buildings and streetlights. We also explored how species numbers vary based on the quantity of tree canopy cover and impervious surface, such as concrete and asphalt, within each city. Our findings show that cities can help migrating birds by planting more trees and reducing light pollution, especially during spring and autumn migration.

Declining bird populations

Urban areas contain numerous dangers for migratory birds. The biggest threat is the risk of colliding with buildings or communication towers[9]. Many migratory bird populations have declined over the past 50 years[10], and it is possible that light pollution from cities is contributing to these losses.

Scientists widely agree that light pollution can severely disorient migratory birds[11] and make it hard for them to navigate. Studies have shown that birds will cluster around brightly lit structures, much like insects flying around a porch light at night. Cities are the primary source of light pollution for migratory birds[12], and these species tend to be more abundant within cities during migration[13], especially in city parks[14].

Satellite image of U.S. at night with cities brightly lit Composite image of the continental U.S. at night from satellite photos. NASA Earth Observatory images by Joshua Stevens, using Suomi NPP VIIRS data from Miguel Román, NASA's Goddard Space Flight Center[15]

The power of citizen science

It’s not easy to observe and document bird migration, especially for species that migrate at night. The main challenge is that many of these species are very small, which limits scientists’ ability to use electronic tracking devices.

With the growth of the internet and other information technologies, new data resources are becoming available that are making it possible to overcome some of these challenges. Citizen science initiatives[16] in which volunteers use online portals to enter their observations of the natural world have become an important resource for researchers.

One such initiative, eBird[17], allows bird-watchers around the globe to share their observations from any location and time. This has produced one of the largest ecological citizen-science databases in the world[18]. To date, eBird contains over 922 million bird observations compiled by over 617,000 participants.

Large clusters of birds (blue and green splotches) captured by weather radar during spring migration, April-May 2019.

Light pollution both attracts and repels migratory birds

Migratory bird species have evolved to use certain migration routes and types of habitat, such as forests, grasslands or marshes. While humans may enjoy seeing migratory birds appear in urban areas, it’s generally not good for bird populations. In addition to the many hazards that exist in urban areas, cities typically lack the food resources and cover that birds need during migration or when raising their young. As scientists, we’re concerned when we see evidence that migratory birds are being drawn away from their traditional migration routes and natural habitats.

Through our analysis of eBird data, we found that cities contained the greatest numbers of migratory bird species during spring and autumn migration. Higher levels of light pollution were associated with more species during migration – evidence that light pollution attracts migratory birds to cities across the U.S. This is cause for concern, as it shows that the influence of light pollution on migratory behavior is strong enough to increase the number of species that would normally be found in urban areas.

In contrast, we found that higher levels of light pollution were associated with fewer migratory bird species during the summer and winter. This is likely due to the scarcity of suitable habitat in cities, such as large forest patches, in combination with the adverse affects of light pollution on bird behavior and health. In addition, during these seasons, migratory birds are active only during the day and their populations are largely stationary, creating few opportunities for light pollution to attract them to urban areas.

Darkening skies at night during migration season makes it easier for birds to navigate.

Trees and pavement

We found that tree canopy cover was associated with more migratory bird species during spring migration and the summer. Trees provide important habitat for migratory birds during migration and the breeding season, so the presence of trees can have a strong effect on the number of migratory bird species that occur in cities.

Finally, we found that higher levels of impervious surface were associated with more migratory bird species during the winter. This result is somewhat surprising. It could be a product of the urban heat island effect[19] – the fact that structures and paved surfaces in cities absorb and reemit more of the sun’s heat than natural surfaces. Replacing vegetation with buildings, roads and parking lots can therefore make cities significantly warmer than surrounding lands. This effect could reduce cold stress on birds and increase food resources, such as insect populations, during the winter.

Our research adds to our understanding of how conditions in cities can both help and hurt migratory bird populations. We hope that our findings will inform urban planning initiatives and strategies to reduce the harmful effects of cities on migratory birds through such measures as planting more trees[20] and initiating lights-out programs[21]. Efforts to make it easier for migratory birds to complete their incredible journeys will help maintain their populations into the future.


  1. ^ Blackpoll warbler (
  2. ^ Cornell Lab of Ornithology (
  3. ^ CC BY-ND (
  4. ^ PJTurgeon/Wikipedia (
  5. ^ climate change (
  6. ^ artificial light at night (
  7. ^ occurrence of migratory bird species in 333 U.S. cities (
  8. ^ light pollution (
  9. ^ colliding with buildings or communication towers (
  10. ^ declined over the past 50 years (
  11. ^ severely disorient migratory birds (
  12. ^ primary source of light pollution for migratory birds (
  13. ^ during migration (
  14. ^ city parks (
  15. ^ NASA Earth Observatory images by Joshua Stevens, using Suomi NPP VIIRS data from Miguel Román, NASA's Goddard Space Flight Center (
  16. ^ Citizen science initiatives (
  17. ^ eBird (
  18. ^ largest ecological citizen-science databases in the world (
  19. ^ urban heat island effect (
  20. ^ planting more trees (
  21. ^ lights-out programs (

Authors: Frank La Sorte, Research Associate, Cornell Lab of Ornithology, Cornell University

Read more

Metropolitan republishes selected articles from The Conversation USA with permission

Visit The Conversation to see more

Entertainment News

Ny’a: My Musical Journey

It’s been such an amazing musical journey. Starting my career in the late 1990’s and selling over 500,000 copies of my debut album “Embrace” worldwide was the beginning of it all.  It was an incredible start of my career. It was a whirlwind for m...

Ny’a - avatar Ny’a

Pro Tips for Gambling Online

While online gambling sounds like an entertaining thing to spend your afternoons playing, it is also serious business for professional players. Those that take gambling seriously know all the tips and tricks that make a good game. Not only do the...

News Co - avatar News Co

The New Art: Balancing Human and Synth in Music Creation

It’s a great time to be alive for indie music makers! There are many of us, maybe including you, who are creating good music out there, armed with nothing but our own ingenuity and the few dollars in our shallow pockets. But what’s the best way to ...

Angela Predhomme - avatar Angela Predhomme

An unstoppable force in indie R&B music since her re-emergence in 2015, Ny’a releases “Waiting”

An unstoppable force in indie R&B music since her re-emergence in 2015, Ny’a has topped the World Indie and Euro Indie Charts countless times, hit the R&B Soul Chart and Club Music Chart Top 20 and became the first artist since Janet Jack...

News Co - avatar News Co

An Aussie Band Breaks Into The US Music Market

AN AUSTRALIAN BAND’S STEPS TAKEN TRYING TO BREAK INTO THE US MARKET By skinsNbones The steps skinsNbones have taken as a band trying to break into the music market haven’t been taken in the traditional Australian way. We didn’t have the opportun...

News Co Media - avatar News Co Media


After a difficult year fraught with fear, anxiety and personal and sociopolitical struggles, nothing has the power to soothe our collective souls – or coincides as perfectly with our renewed sense of hope – than a heartfelt, blissfully beautiful love...

News Co Media - avatar News Co Media

Metropolitan Business News

Essential Tips for Developing Corporate Websites

The corporate industry, just like the others, is experiencing a massive technological revolution. Organizations have changed their marketing criteria by embracing the internet. Therefore, do not be ...

News Co - avatar News Co

How to Make a Personal Injury Claim for Minors

As a parent, you want to make sure your child is always safe. But as much as you want to keep him under your watch, it is impossible to be there for him all the time. When he is out there attendin...

News Co - avatar News Co

Dress for Success - Style Essentials for Every Businessmen

Did you ever glance at a person in passing and instantly think, “Wow, they must be successful”? It seems like the rich and successful people always stand out from the crowd without even trying, ...

Diana Smith - avatar Diana Smith

Why use Google Adwords?

Google ads, previously known as Google AdWords is one of the best and most effective ways to advertise your business online. According to data released by Google, over 5.5 billion searches are made ...

News Co Media - avatar News Co Media

Perfecting Web Design For A Health-Based Website

These days, when it comes to understanding what we put into our bodies, we are more focused than ever. It seems that everywhere we look, there is another health benefit, product, or trend on the...

News Co Media - avatar News Co Media

3 Realistic Reasons Why Physical Offices Are (Almost) Dead

Nowadays, more and more businesses are trying to find alternatives to traditional offices. For many years, brick and mortar offices have been at the heart of a company’s life. But work environme...

News Co Media - avatar News Co Media

Writers Wanted

News Co Media

Content & Technology Connecting Global Audiences

More Information - Less Opinion