Artificial light at night (ALAN) has a negative impact on natural systems worldwide. ALAN causes changes in physiology and behavior in organisms, which can have an impact on populations, communities, and ecosystems. ALAN’s confusing effect on nocturnal migration is one of the most serious consequences for birds.
Image Credit: Albert Beukhof/Shutterstock.com
During the migratory flight, nocturnally migrating birds are drawn to ALAN on an individual level. During the stopover, populations of nocturnally migratory birds have been shown to be closer to ALAN, and species numbers have been demonstrated to be connected with ALAN sources in urban areas. This article will look at seasonal associations with light pollution trends and their effect on nocturnally migrating bird populations. The research was published in Ecosphere.
Outside of seasonal migration, ALAN can have a negative impact on nocturnally migratory birds. Urban sources of ALAN are related to decreased abundance and fewer nocturnally migratory species at the population level during both breeding and non-breeding seasons. ALAN may affect migratory and resident animals’ circadian rhythms, behavior, and physiology at the individual level.
ALAN is often treated as a static source of pollution in studies on the ecological impacts of ALAN on migrating birds. ALAN, on the other hand, is a dynamic phenomenon influenced by urban expansion and degradation, as well as technological advancements in lighting.
As a result, documenting the effects of ALAN requires using a whole annual cycle viewpoint. However, the spatial link between nocturnally migratory bird populations’ seasonal distributions and ALAN trends has not been investigated.
The goal of this research is to document how correlations with ALAN annual trends are defined across the entire annual cycle for nocturnally migrating birds, with the objective of enhancing baseline information on the regions and seasons where mitigation efforts like Lights Out programs would have the greatest impact.
Experts show how populations of nocturnally migratory bird species that nest in North America and travel throughout the Western Hemisphere are linked to ALAN patterns throughout the course of the year.
For the combined period 2005–2020, researchers compare weekly estimations of relative abundance for 42 nocturnally migrating passerine (NMP) bird species obtained from data from the eBird community science initiative with yearly estimates of ALAN for the period 1992–2013.
As a result, researchers anticipate that the 42 NMP species will be linked to favorable ALAN trends during the majority of their yearly life cycles.
Their goal is to inform ALAN mitigation efforts and increase the understanding of the ecological implications of various types of environmental pollution for birds and other species by validating these predictions.
Researchers assessed the four seasons of the annual cycle (nonbreeding, spring migration, breeding, and fall migration) for the 42 NMP species using the following technique to support the interpretation seen in Figure 1.
Figure 1. The great-circle (geodesic) distance between weekly centroids of occurrence weighted by relative abundance for 42 nocturnally migrating passerine bird species. The fitted black line and 95% confidence band are from a generalized additive mixed model (GAMM) with species included as a random effect. The vertical polygons demarcate spring migration (15 March–17 May) and autumn migration (10 August–19 October) as delineated by the inflection points in the fitted GAMM line. Image Credit: La Sorte, et al., 2022
Researchers estimated ALAN by year for the period 1992–2013 in the Western Hemisphere using the harmonized global nighttime light dataset normalized using stepwise calibration (see Figure 2).
Figure 2. (a) Average artificial light at night (ALAN) and (b) the trend in ALAN during the period 1992–2013 within the Western Hemisphere. The ALAN data are gridded at a 30-arcsecond spatial resolution (ca. 1 km at the equator), and the units are digital numbers (DNs; range = 0–63). The trend analysis was implemented using ordinary least-squares regression. The data are displayed using a Mollweide equal-area projection. Image Credit: La Sorte, et al., 2022
The 42 NMP species had different associations with ALAN annual trends depending on the week and the species, as depicted in Figure 3.
Figure 3. Weekly associations with trends in artificial light at night (ALAN) during the period 1992–2013 for 42 nocturnally migrating passerine bird species. Image Credit: La Sorte, et al., 2022
Figure 4 shows three significant clusters comprising 19, 15, and 8 species, respectively, discovered via hierarchical cluster analysis based on a minimum cluster size of eight species.
Figure 4. Dendrogram from a hierarchical cluster analysis of weekly associations with trends in artificial light at night for 42 nocturnally migrating passerine (NMP) bird species. The dendrogram labels are the common name alpha codes for the 42 NMP species. The colored annotations below the dendrogram identify species grouped into three clusters using an adaptive branch pruning technique. Image Credit: La Sorte, et al., 2022
In Figure 5, species in Clusters 1 and 3 were linked with low ALAN levels and positive ALAN trends during the non-breeding season, whereas species in Cluster 2 were associated with somewhat higher ALAN levels and greater positive ALAN trends.
Figure 5. Weekly associations with trends in artificial light at night (ALAN) averaged across 42 nocturnally migrating passerine (NMP) bird species in three clusters (see Figure 4). The size of the circles corresponds to average ALAN. The sample sizes are 19, 15, and eight species, respectively. The color ramp is migration speed (see Figure 1) averaged across the 42 NMP species (blue = slow, green = intermediate, and red = fast). The ALAN units are digital numbers (DNs; range = 0–63). Image Credit: La Sorte, et al., 2022
Figure 6 shows the seasonal distributions of species in the three groups in the Western Hemisphere.
Figure 6. The seasonal distributions within the Western Hemisphere of 42 nocturnally migrating passerine bird species grouped into three clusters (n = 19, 15, and 8, respectively) based on their weekly associations with trends in artificial light at night (see Figure 3). The maps show the proportion of each season species occur in the grid cells averaged across species in each cluster. The data are displayed using a Mollweide equal-area projection. Image Credit: La Sorte, et al., 2022
This research revealed three distinct clusters of NMP species, each of which had differing connections with ALAN trends based on weekly patterns of relative abundance in the Western Hemisphere. During the breeding season, two clusters of species were found in western and northern North America.
The species in these clusters had moderate levels of ALAN and somewhat negative ALAN trends. Species in these clusters were related to low ALAN levels and positive ALAN trends during the non-breeding season. Scientists discovered the third cluster of species whose positive ALAN trends persisted throughout the yearly cycle, peaking during migration, particularly in the spring.
During migratory and the non-breeding season, NMP species experience high ALAN levels and favorable ALAN trends in Central America, according to study findings. Central America’s unique topography necessitates large-scale migration methods inside the area.
Research data revealed that during the breeding season, southern North America had the greatest ALAN levels and strongest positive ALAN trends, whereas, during the non-breeding season, Central America had the highest ALAN levels and strongest positive ALAN trends.
Changes in lighting technology have traditionally influenced ALAN dynamics. This is now predicated on the shift to LED technology, which has resulted in increased ALAN emissions and changes in ALAN spectral composition in some areas. Depending on the scenario, switching to LED technology can either worsen or mitigate ALAN’s negative effects on birds.
It would be useful to analyze how LED technology, which has advanced since 2013, is affecting the ALAN patterns observed in this study, as well as the ramifications for the region’s nocturnally migratory bird species.
For many species, researchers summarized range-wide relationships with ALAN trends by week across the yearly cycle in this study. Exploring species-specific, local-scale connections with ALAN trends in data-poor locations, on the other hand, might be difficult. Efforts to improve the coverage of eBird data in under-sampled parts of the world might be beneficial in improving the spatial quality of these types of analyses.
Within the Western Hemisphere, research findings pinpoint the places and seasons when ALAN mitigation initiatives are most likely to provide the greatest benefits. These findings also lay the groundwork for further research into the impact of ALAN in recent bird population decreases in North America.
During migration, scientists identified Central America as a significant zone where reversing ALAN trends will likely benefit most individuals of the most species, particularly during spring migration.
Outside of migration, researchers found that reversing ALAN trends would likely have the greatest advantages in southern North America during the breeding season and Central America during the non-breeding season.
Because of urbanization and changes in lighting technology, the problems posed by ALAN for birds and other species will continue to develop, underlining the necessity of documenting ALAN relationships and their consequences at the individual and population levels across locations and seasons.
La Sorte, F.A., Horton, K.G., Johnston, A., Fink, D. and Auer, T. (2022) Seasonal associations with light pollution trends for nocturnally migrating bird populations. Ecosphere, 13(3), p.e3994. Available Online: https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecs2.3994.
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