Light Pollution on Bird Migration Behavior

By Lydia Bruno

As urban dwellers, we are only too familiar with the effects of light pollution. Rarely can we see stars at night and often we have to travel to remote locations to even glimpse the wondrous Milky Way. Add to that, studies show light pollution can disrupt our circadian rhythms, affecting our ability to get a good night’s rest.

A recent 2023 study published in Nature Communications, Artificial Light at Night is a Top Predictor of Bird Migration Stopover Density (Horton et al.),  outlined how light pollution also significantly impacts birds, especially during migration. This is particularly relevant to San Francisco as the Pacific Flyway, a major migratory route, includes the San Francisco Bay Area. An estimated billion birds travel through this flyway during spring and fall migratory seasons. The majority of these migrants are songbirds and they take flight primarily at night. Skyglow, or the artificial brightening of the night sky by light pollution, affects their nighttime migration by increasing stopover. 

What is stopover? Like it sounds, stopover is the act of stopping over a place, essentially a rest stop along the birds’ epic journey, a pause to refuel, rest or take refuge from adverse weather conditions. This is an important and necessary part of their journey. However, due to habitat changes, with many areas becoming drier, less forested and brighter at night, historic stopover locations are at risk. 

Along the Pacific Flyway, skyglow was found to be the top driver of stopover in an area. While this may at first seem positive, there are in fact, negative consequences depending on where the stopover occurs. Urban areas, like San Francisco, generate more skyglow than less populated areas and can become ecological traps, luring birds to areas where they are at greater risk of colliding with buildings,predation, and potentially throwing them off course, disconnecting them from their migratory path.. 

The next step for researchers is to determine whether stopover hotspots are truly important ecological locations, the effect of skyglow luring the birds to the area or a combination of both.

What we do know is, with light pollution increasing at a disconcerting 10% per year in North America, we need to invest in advocacy, collaboration and policy change to decrease this trend and reduce skyglow.

How can you help? There are simple actions we can all take, including turning off unnecessary indoor and outdoor lights at night, closing blinds/curtains after dark, using warm colored LED bulbs (as bluer colored light has larger reach and worsens skyglow) and using dimmers and timers to minimize light usage. You can learn more here.

Another action you can take is to contact your local representative(s), asking them to take steps to mitigate and reduce light pollution and support updates to bird safe building ordinances that cover light reduction. While all the lights of the city contribute to artificial skyglow, we should consider  the impacts of big projects like the Bay Bridge lights installation. This installation, while fun to look at and enjoy from an aesthetic viewpoint, can also potentially affect migrating birds by contributing to skyglow. 

There are some simple steps we can take to strategically mitigate the harm for birds. This includes asking the San Francisco Planning Commission to work with the operators and architects of the Bay Bridge lights project to do the following: 

  • Strategically choose light bulb color & brightness (warmer colors at dimmer levels less likely to disorient birds)
  • Use automatic controls (timers and/or dimmers) to turn on/off lights as needed
  • Turn off lighting during migration seasons (APR-MAY, AUG-OCT, especially on foggy nights)
  • Use timers to turn off lights at intervals for 15 to 20 minutes to allow birds trapped by lights to escape

With continued advocacy and corresponding policy change, we can help reduce skyglow for the wellbeing of ourselves and the birds.

Lydia Bruno is a resident of the Outer Sunset neighborhood in SF and enjoys connecting with community via birding, gardening and volunteering. One of her favorite activities is the monthly North Lake (Golden Gate Park) habitat restoration workday where she has met so many wonderful people and learned (and is continuing to learn) about native plants that support our local birds, bees and butterflies.