What causes bird-building collision risk? Seasonal dynamics and weather drivers

Abstract

Bird-building collisions are a major source of wild bird mortality, with hundreds of millions of fatalities each year in the United States and Canada alone. Here, we use two decades of daily citizen-science monitoring to characterize day-to-day variation in building collisions and determine the factors that predict the highest risk times in two North American cities. We use these analyses to evaluate three potential causes of increased collision risk: heightened migration traffic during benign weather, increased navigational and flight errors during inclement weather, and errors in response to reflective glass. The seasonal phenology of collisions was consistent across sites and years, with daily collision rates approximately two-fold higher in autumn as compared to spring. During autumn migration, day-to-day variation in collision rates is best explained by weather conditions around dawn with four major drivers: no precipitation, high atmospheric pressure, cold temperatures, and clear skies. In spring, collision risk was best predicted by either overnight or morning weather, driven by a lack of precipitation and warm temperatures. Based on these results, we hypothesize that collisions are influenced by two main weather-driven mechanisms. First, benign weather causes an increase in migration traffic in both spring and autumn, creating greater opportunity for collisions to occur. Second, cold clear conditions cause an additional increase in collision risk for autumnal migrants. These conditions may be particularly hazardous in autumn because of the high abundance of naive and diurnal migrants at that time of year. Our analysis also establishes that a relatively small proportion of days (15%) are responsible for 50% of the total collision mortality within a season, highlighting the importance of targeting mitigation strategies to the most hazardous times.

Heavy migration traffic and bad weather are a dangerous combination: Bird collisions in New York City

“…This difference stems from the presence of migrating juveniles in fall (Dokter et al., 2018) and the use of more easterly routes in fall compared to spring (La Sorte et al., 2014). As expected, and similar to studies in other cities in North America (Lao et al., 2022; Scott et al., 2023), we found that radar‐measured migration densities were generally higher in fall and that more collisions were recorded in NYC in fall than the spring.…”

“…Collision monitoring efforts provide a unique opportunity to study a major source of mortality for birds during their annual cycles, and each new study fills a gap in our still imperfect knowledge about the true magnitude of birds killed in collisions with buildings. The effects of weather conditions (Lao et al., 2022; Lin et al., 2019; Panuccio et al., 2019; Scott et al., 2023) and nocturnal migration traffic (Elmore, Riding, et al., 2021) have been identified individually as factors in bird–building collisions. Our study provides evidence that these factors should be considered in tandem to predict bird collisions and to provide more nuanced guidance for the timing of collision mitigation programs such as Lights Out initiatives.…”

“…Low cloud ceiling and fog also disorient birds by reflecting light and amplifying ALAN and skyglow (Panuccio et al., 2019). Unfavourable weather conditions may also lead to birds becoming grounded or making navigational or flight errors in close proximity to dangerous glass expanses (Lao et al., 2022; Scott et al., 2023).…”

“…We included polynomial (quadratic and cubic) terms for day of year because of known non‐linearity of collision rates across each season (e.g. Scott et al., 2023). We expected that migration traffic could affect how ceiling height, visibility, zonal wind and meridional wind influence collision rates.…”

“…However, little is known about the influence of weather conditions on bird collisions at the city scale (e.g. local scale, kilometres; Lao et al., 2022) and few studies have accounted for migration traffic when examining the relationships between weather and collisions (but see Scott et al., 2023). This is an essential consideration for separating weather conditions that encourage or support migration from those that increase an individual migrant's collision risk.…”