Lights Out for Spring Migrants

Window collision victims

As spring migrants begin to make their way through Ohio (see the migration forecast for Columbus here), it’s time to remember to turn off unnecessary outdoor lights overnight (and to close curtains to keep indoor lights inside). By doing so, we can help to prevent nocturnally migrating birds from becoming disoriented and drawn into urban areas where they are at risk of window collisions and other threats. 

During last fall’s migration season, I wrote an article for Song Sparrow on the impact of light pollution on birds and the corresponding importance of programs such as Ohio Lights Out (part of Audubon’s nationwide Lights Out program), which strive to reduce urban light pollution during the critical migration seasons. In the months since this article was published, additional research has been published that has shed more light (so to speak) on the wide-ranging, often detrimental impact of artificial night lighting on birds and other wildlife.

Most significantly, perhaps, a meta-analysis of over 100 studies on artificial night lighting was published in Nature. The authors concluded that, due to its disruptive ecological consequences, outdoor lighting should be used only when necessary and regulated like any other pollutant.  

Other recent studies have looked specifically at the impact of night lighting on birds. For example, research conducted on purple martins indicates that light pollution not only distracts and disorients migratory birds but also impacts the timing of migration itself. When martins were exposed to high levels of artificial night lighting, they migrated an average of eight days earlier during breeding season. This difference is significant given that purple martins and other insectivores need to time their spring migration to coincide with the emergence of insects they depend upon for food. If they arrive at their breeding grounds too early, they risk finding too little to eat. 

Purple martin eating yellowjacket – Photo Andrew Reding

Also published last fall, a comprehensive study of North American birds found that light pollution affects the timing of reproduction in numerous species. This study examined citizen science data, specifically data from the Cornell Lab’s NestWatch program, to track the impact of manmade light and noise on 58,506 unique nests from 142 different bird species across the continent. When exposed to artificial light at night, birds nested up to one month earlier than normal in open environments and 18 days earlier in forested environments. As with early migration, this raises worries about potential lack of adequate nutrition, if chicks hatch before their food sources are available. In both cases, however, the relationship is complicated due to the independent effects of climate change on seasonal timings.

As last year’s meta-study reveals, light pollution can also indirectly impact birds through its effects on the functioning of entire ecosystems. This is corroborated by a study published just this March, which demonstrates that artificial lighting can have impacts on pollinator activity that even extend into the daytime. The full ecological consequences of this finding are still uncertain, but it may have implications for which plants are able to propagate to provide food and shelter to birds (or to host insects and caterpillars that birds eat and feed their young). 

Columbus at night, CC BY-SA 2.0 Jsjessee

Artificial night lighting is typically associated with streetlights. However, another study published last fall used satellite imagery to show that streetlights contribute less to urban light pollution than expected. The researchers examined Tucson, Arizona, which uses “smart street lights” that permitted city officials to experimentally manipulate their brightness. Satellite imagery revealed that, on an average night, only about 20 percent of Tucson’s upward-directed light came from street lighting. The majority of the light pollution resulted from decorative and facade lighting, advertisements, sports lighting, brightly lit windows, and other sources. Although the exact proportion is likely to vary from city to city, the study confirms that mitigating light pollution is not solely the responsibility of public utility departments. When it comes to reducing urban skyglow, and thus helping migratory birds to stay out of harm’s way, the sum of the individual contributions of building owners matters.