Less lighting and more trees instead of concrete: so cities can save migratory birds

Less lighting and more trees instead of concrete: so cities can save migratory birds

Many birds migrate at night, so cities could help them by planting more trees and decreasing light pollution

During their spring and autumn migrations, millions of birds travel between breeding and wintering sites, creating a magnificent natural spectacle. Often the migratory paths of birds cover enormous distances.





For example, Blackpoll's parula, a small passerine bird weighing only 12 grams, native to North America, is able to travel up to 1.500 miles (over 2.400 km) between its nesting grounds in Canada and its wintering grounds in Canada. Caribbean and South America.

Most bird species migrate at night, when the skies are typically clear and safe, as predators are less active. It is still unclear to scientists how birds are actually able to travel such long distances at night.

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The "science of ecological citizens"

The Cornell Lab of Ornithology of Cornell University (Ithaca, New York, USA) deals with bird migration and, in particular, has analyzed to what extent migratory phenomena are influenced by factors such as climate change and the presence of artificial light in night hours. In a recent study, Cornell researchers had access to millions of bird observations made by CDs. “Citizen scientists” to map the presence of migratory bird species in 333 cities in the United States, in the four seasons (winter, spring, summer and autumn).

Given that many of the species considered are small and migrate at night, the ability of scientists to use electronic tracking devices is severely limited. However, the Internet and new information technologies have spurred several Citizen Science initiatives. Volunteer citizens log into specialized online portals to post their observations of the natural world. One such initiative, eBird, allows bird watchers to share their observations with the rest of the world, from anywhere, at any time. This has produced one of the world's largest “green citizen science” databases.


To date, eBird contains over 922 million bird observations from over 617.000 participants. The information collected by the researchers was processed to determine the variation in the number of migratory bird species in relation to the level of light pollution in each city.


The damage of light pollution

Migratory bird species have evolved to use certain migration routes and particular habitat types, such as forests, grasslands or swamps. While humans may appreciate the visible presence of migratory birds in urban areas, bird populations do not benefit from it.

Urban areas harbor numerous dangers for migratory birds. Furthermore, they cannot offer adequate food resources or safe shelter that birds need during the migration phase or when their young are growing up. Cornell University ornithologists are quite concerned about migratory birds progressively moving away from traditional migration routes and their natural habitats. From the analysis of eBird data it emerged that it is precisely the cities that host most of the species of migratory birds in the spring and autumn seasons.

The brightness of the night sky in cities is mostly generated by artificial light sources, such as buildings and street lamps. The rise in the level of light pollution has been associated with the increase in bird species involved in migration; this phenomenon is due to the great attractiveness of the artificial lighting sources of the American metropolises, which guide the migratory behavior of birds, which tend to concentrate in urban areas. Conversely, migratory bird species are less numerous during the summer and winter months, due to the inhospitable habitats provided by the cities, lacking large wooded areas, and the negative impact of light pollution on the behavior and health of people. birds. Furthermore, since in summer and winter migratory birds are active only during the day and the populations are mainly sedentary, urban areas are not ideal destinations for migration.


Urban enemies of migratory birds

Urban areas are undoubtedly risky places for migratory birds. The main threat is the risk of collision with buildings or pylons. Over the past 50 years, populations of migratory birds have drastically reduced and it is likely that light pollution in metropolises has contributed to their decimation. Scientists have highlighted the negative effects of light pollution on migratory birds, which are disoriented by artificial lights to the point of hindering their movement. Birds cluster around light sources, just as insects are attracted to the artificial lights of a porch at night. In summary, cities are the main source of light pollution for migratory birds. The latter take refuge in the city during their migration and tend to stay in city parks.


Less lighting, more trees and less concrete

Cornell University scientists also examined the correlation between the variation in the number of species observed in each city and two elements of urban space: the abundance of vegetation in the canopy of trees and the presence of impermeable surfaces (concrete and asphalt). The results of the aforementioned study show that the migration of birds to urban centers would be facilitated by possible reforestation works and, as already noted, by urban projects to reduce light pollution, especially in spring and autumn. Climate is a further determining factor for migration. On the one hand, due to the "heat island effect", the structures and paved surfaces of cities absorb and release more heat than natural surfaces, making the former less hospitable than the latter. On the other hand, if buildings, roads and parking lots have almost entirely replaced the green areas of the cities, the urbanized areas are necessarily warmer than the surrounding peripheral areas. In winter, however, birds benefit from being less cold and more likely to find insects.

Fonti: The Cornell Lab of Ornithology/Environmental Pollution Elsevier/Science/eBird/National Geographic

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