Let there be Night: How light pollution harms migrating birds

If last month’s solar eclipse taught me anything, it’s that we all still love seeing cool stuff in the sky. I don’t think we realize how fast astronomical wonders are fading out of sight: studies show that our night skies grow about 10% brighter every year, and the number of visible stars plummets as a result. At this rate, someone born 18 years ago to a sky with 250 visible stars would now find only 100 remaining.

Vanishing stars may feel like just a poetic tragedy, but as I crouch over yet another dead Wood Thrush on my morning commute, the consequences of light pollution feel very real. Wincing, I snap a photo of the tawny feathers splayed around his broken neck on the asphalt.

It’s not the only such photo I’ll take this year. The building whose towering windows took this thrush’s life is infamous; like many other passersby, I pay close attention to the ground around such spots to record any victims. We upload our morbid photography to iNaturalist, an app usually reserved for more cheerful records and identifications of flora and fauna, where they automatically join several citizen science projects focused on bird-window collisions.

Why the macabre gallery? These collections provide concrete evidence of just how many birds windows kill. Ideally, that information encourages tactics to reduce casualties, like lobbying homeowners and institutions to add decals or screens to their glass.

Though such measures are critical for limiting window strikes during the day, we often overlook the damage our windows cause at night. Remarkably, most bird migration actually occurs by moonlight; if you stand under the stars and listen during migration — which began in earnest last week — an audible chorus of chirps and buzzes overhead indicates a sky full of birds.

Birds evolved to fly under the cover of darkness for many reasons. Nightly travel means they can spend the bright, bug-filled days gathering food. Darkness helps migrants avoid predators, and the lower turbulence and cooler temperatures of nighttime skies allows for more energetically efficient flights.

Though the specifics of nocturnal navigation are a longstanding ornithological mystery, that navigation is undoubtedly disrupted by the growing brightness of the night sky. Bright lights disorient the birds’ sense of direction and attract them towards the light sources themselves. In that state of confusion, nocturnal window strikes skyrocket.

In the U.S., one billion birds die each year by flying into windows. Distinct behavioral changes around brightly lit structures are the main cause of mortality: the usual conversation of nocturnal flight calls grows into a confused cacophony. Those that avoid collisions face different risks: the energy they waste investigating unfamiliar lights means they’ll have a harder time avoiding predators, catching enough food, and reaching their final destinations.

The harm caused by light pollution is not limited to birds. Even in rural areas, many nocturnal animals struggle to adapt to brightening nighttime environments: bats can’t fly efficiently, amphibians can’t reproduce normally, and insects can’t forage effectively.

Humans aren’t immune either. Brighter nights disrupt our sleep cycles — something I can personally attest to, as someone who grew up in Kent and now struggles nightly with the brightness of New Haven — and are associated with a slew of negative health effects. And though people widely associate more lights with greater safety, evidence of this is mixed.

Despite the repercussions, the reach and severity of light pollution keeps growing. Electricity usage in the U.S. is on the rise again; while much of that energy has more intensive uses, the U.S. Energy Information Administration estimates that 6% of residential and 17% of commercial energy is all for lighting.

As disheartened as I am by increasing energy usage and vanishing stars, by unnecessary lights glaring from empty sports fields or barren storage facilities, and by dead songbirds on my morning commute, there is a silver lining to this story. Fixing most pollution problems — microplastics, oil spills, atmospheric CO2 — requires complex, intractable solutions. Light pollution, on the other hand, is completely, immediately, effectively reversible.

This is a critical time of year to make that change. The transition from April to May brings spring rains, bursting tree buds, and a deluge of migrating birds: cheerful songs of Yellow Warblers, gleaming feathers of Scarlet Tanagers, playful flights of American Redstarts. You can watch these migrants gather above your home like a rising storm using BirdCast, Cornell Lab of Ornithology’s avian version of a weather radar.

The closer that storm comes, the more important it is to flip those light switches off. Simple steps, like limiting the use of aesthetic lighting, keeping the beams of necessary lights targeted, dim, and warmly colored, or curtaining bright windows, make all the difference to our birds. Participating in darkening the skies this spring will help keep these little travelers aloft — and maybe bring some stars back into view, too.

Alison Robey is a writer for the Kent Land Trust, a volunteer at the Sharon Audubon Center, and a third-year PhD candidate in Ecology & Evolutionary Biology at Yale University.