Bright lights in the big cities: migratory birds’ exposure to artificial light

T of flying organisms (eg birds, bats, insects) occupy the airspace within the troposphere during different periods of their annual cycles (Diehl 2013). The recent recognition of airspace as vital habitat – one that is subject to increasing modification by humans – highlights the fundamental need to understand how organisms cope with such alterations (Lambertucci et al. 2015), which pose numerous challenges to airborne organisms during periods of transit, including nocturnally migrating birds. Of the nearly 630 terrestrial species of birds regularly occurring in North America, approximately 70% are considered migratory, and of these more than 80% migrate at night (WebTable 1). Yet most studies of associated risks have focused on terrestrial habitats, underscoring a fundamental knowledge gap that can be addressed with recent technological (including computational) advances. Light pollution of the airspace is a relatively recent but growing threat to nocturnally migrating birds (Longcore and Rich 2004; Van Doren et al. 2017; CabreraCruz et al. 2018). Increasing urbanization has greatly amplified the amount of artificial light at night (ALAN; Kyba et al. 2017), with almost onehalf of the contiguous US experiencing substantially photopolluted nights (Falchi et al. 2016). Light sources – including streetlights, safety lights, and extensively lit buildings – can disturb wildlife in a multitude of ways (Gauthreux and Belser 2005; Hölker et al. 2010; Rodríguez et al. 2017). Highpower light installations like lighthouses and communication towers are known to attract nocturnal migrants and are responsible for substantial mortality (Gauthreux and Belser 2005; Longcore et al. 2012). The numbers of birds attracted to or trapped by illumination depend on light wavelength (Poot et al. 2008) as well as weather factors such as fog and precipitation (Gauthreux and Belser 2005). Highpower light installations can even attract migrants in already heavily photopolluted areas and in skies with clear weather conditions (Van Doren et al. 2017). An increasing number of artificial structures are now present in the lowest reaches of the troposphere (Davy et al. 2017), and their continued expansion poses an everincreasing threat to wildlife. In the contiguous US, annual fatal bird collisions with buildings, communication towers, power lines, and wind turbines cumulatively number in the hundreds of millions (Loss et al. 2015). For nocturnally migrating birds, direct mortality as a result of collisions due to attraction to light (Gauthreux and Belser 2005) is the most obvious and direct effect of ALAN, but there are also more subtle effects, such as disrupted orientation (Poot et al. 2008) and changes in habitat selection (McLaren et al. 2018). There is also growing evidence that light pollution alters behavior at regional scales, with migrants occupying urban centers at higherthanexpected rates as a function of urban illumination (La Sorte et al. 2017). While ALAN acts as an attractant at both large (La Sorte et al. 2017) and local (Van Doren et al. 2017) scales, there is also evidence of migrating birds avoiding strongly lit areas when selecting critical resting sites needed to rebuild energy stores (McLaren et al. 2018).