• Aeroecology
• Bird migration
• Environmental change
• Spatial ecology

Effects of light pollution on bird migration: a radar-based case study at the coast of Croatia

Most birds migrate at night. Migration in birds evolved over the last five million years, strongly linked to a global climate change that led to increasing seasonality in today’s temperate regions. Since then, migratory bird species have evolved traits that facilitate long navigational flights, such as multisensory orientation, wing- and body shapes and highly specialized metabolism. Especially long-distance migrants are perfectly equipped for a life on the wings. Since the beginning of the 20th century, fast human progress is challenging these long-developed behaviours and mechanisms in birds. It is no surprise that many bird populations are severely declining in numbers.

Light pollution has long been neglected in conservation measures. Migratory birds, though, are highly dependent on natural light cycles. Light and dark synchronize their annual timing, and the birds’ orientation during migration is often based on celestial cues such as the stars or the position of the sun at dusk. Light pollution disrupts natural cycles in birds and prevents an undisturbed use of nightly orientation cues. Consequentially, artificial nightlights are a stressor to birds, in addition to climate change and habitat fragmentation.

During my PhD, I study the effects of light pollution on birds on the coast of Croatia. Compared to other countries in southern Europe, Croatia’s coastline has stretches with an almost natural nightscape, alternating with stretches that are heavily affected by light pollution, making Croatia an outstanding study site. Together with the Swiss Ornithological Institute and the Croatian Institute of Ornithology, I will use data gathered by dedicated bird radar devices to monitor individual birds' altitude, air speed and flight direction. Two pairs of radars are installed, positioned to cover a maximum difference between high and low levels of light pollution. Additionally, weather radar data helps analyze movement patterns in a geographic context.

It is to be tested whether, on a regional scale, birds approaching the coastline are lured by light. On a local scale, birds are expected to show light-induced behavioural changes linked to disorientation. These can be increased variability in flight directions, lower air speeds and a change in flight altitudes. I aim to understand migratory birds' responses to artificial light, which is a first step towards mitigating harm from light pollution. After all, light pollution is simple to reduce compared to other anthropogenic emissions. So far, there is just a lack of motivation.

Figure 1: Study area at the Adriatic coast of Croatia. The map shows sky glow, the indirect light pollution as it is reflected by atmospheric scattering. The green triangle shows the expected migration direction of birds in the spring season, with birds moving towards north-east, approaching the coastline from the sea. The area marked as “weather radar” illustrates the surveyed area at the lowest radar elevation angle after considering topographic beam blockage. The two sets of BirdScan ornithological radars are placed inside this surveyed area to allow a combined usage of radar data.