High-intensity urban light installation dramatically alters nocturnal bird migration

Abstract

Billions of nocturnally migrating birds move through increasingly photopolluted skies, relying on cues for navigation and orientation that artificial light at night (ALAN) can impair. However, no studies have quantified avian responses to powerful ground-based light sources in urban areas. We studied effects of ALAN on migrating birds by monitoring the beams of the National September 11 Memorial & Museum's "Tribute in Light" in New York, quantifying behavioral responses with radar and acoustic sensors and modeling disorientation and attraction with simulations. This single light source induced significant behavioral alterations in birds, even in good visibility conditions, in this heavily photopolluted environment, and to altitudes up to 4 km. We estimate that the installation influenced ≈1.1 million birds during our study period of 7 d over 7 y. When the installation was illuminated, birds aggregated in high densities, decreased flight speeds, followed circular flight paths, and vocalized frequently. Simulations revealed a high probability of disorientation and subsequent attraction for nearby birds, and bird densities near the installation exceeded magnitudes 20 times greater than surrounding baseline densities during each year's observations. However, behavioral disruptions disappeared when lights were extinguished, suggesting that selective removal of light during nights with substantial bird migration is a viable strategy for minimizing potentially fatal interactions among ALAN, structures, and birds. Our results also highlight the value of additional studies describing behavioral patterns of nocturnally migrating birds in powerful lights in urban areas as well as conservation implications for such lighting installations.

Keywords: artificial light; flight calls; nocturnal migration; radar ornithology; remote sensing.

Fig. 1.

Tribute in Light site. Observations (in Coordinated Universal Time) from the September 11–12 2015 Tribute in Light depicting altered behaviors of nocturnally migrating birds. (A) Direct visual observation. (B) Radar observation without TiL illumination and (C) with TiL illumination.

Fig. 2.

Time series of radar and acoustic measures of Tribute in Light impact on migratory birds. Observations (in Coordinated Universal Time) from September 11–12, 2015 of (A) migration activity within 500 m of the installation, (B) radial velocity within 500 m of the installation, and (C) vocal activity during periods of TiL illumination. D–F show corresponding data with and without illumination. Density increase factor (D) is defined as the peak bird density near the installation divided by the mean density 2–20 km away.

Fig. 3.

Spatial and temporal influence of Tribute in Light on migratory birds. Migration activity (Left column) and radial velocity (Right column) at the installation pooled across years by distance from the study site (A and B) and activity as a function of time since TiL shutdown (C and D). To account for year-to-year variation, migration activity was normalized across years using a z-score standardization (values minus the nightly mean, divided by the nightly SD). Illumination represented by green and periods without illumination by gray. C and D include only measures ≤500 m from the installation. Data fit with generalized additive models (A and B: bs = “cs,” m = 2, k = 10; C and D: bs = “ds,” m = 2, k = 5) and weighted by migration activity for radial velocity models. Shading represents 95% confidence intervals.

Fig. 4.

Simulated bird concentrations over time at an ALAN source (solid lines). Vertical dashed lines indicate time to steady-state stabilization. Model parameters a and κ are described in SI Appendix, Eqs. S1 and S2, with parameter σ fixed at 1,500 m (SI Appendix, Table S1). Model 1 represents the best fit to the observed patterns at the installation, but this model is still conservative in that higher-than-predicted concentrations of birds occurred in certain periods. In general, bird concentrations at the installation could only be explained by including directed flight toward ALAN for disoriented birds (κ > 0). These results support our observations that birds were disoriented by and attracted to the installation.