These results confirm that monitoring of flight calls can provide valuable information on the timing of nocturnal migration, but that the effects of artificial lighting must be taken into account in any comparisons of abundance across sites. Call rates were up to five times higher over the brightest urban areas compared to darker villages, suggesting a strong phototaxic effect of artificial light at night on migratory thrushes. Seasonal patterns of call rates were positively correlated across sites, but there were large differences in absolute rates between sites. We trained a convolutional neural network to automatically locate and identify thrush calls in the audio recordings, achieving AUC values in withheld validation data of 0.93–0.98, and recall on independent field data of 85–94%, depending on species. We used passive acoustic recorders deployed across a gradient of artificial lighting, to record the flight calls of three thrush species, with an expectation of greater call rates over brightly lit areas. This study aimed to measure the response of nocturnally migrating thrushes to artificial light at night in the UK. Comparable evidence for species and urban areas in Europe is limited. Evidence from North America shows that for species that migrate at night, bright artificial light sources associated with urban areas can disrupt natural movement patterns, leading to direct and indirect fitness consequences. Migratory birds are subject to many pressures during their lifecycle and many are declining as a consequence. This work highlights the need to better understand the effect of ALAN on birds and other wildlife, and also the potential for passive acoustic monitoring in helping to answer ecological questions. Although evidence from other studies, such as those involving radar, indicates that this result can be explained by birds being attracted to brightly lit areas, it is also possible that individual birds call more often than normal in such environments if they are disorientated. The results showed that thrush call rates were up to five times higher over the brightest urban areas than in the darkest villages, suggesting a strong effect of ALAN on these migratory species. The audio recordings were analysed using artificial neural networks, which had been trained to identify calls of the target species and differentiate them from other sources of nighttime noise. The selected birdwatchers’ gardens spanned a gradient of nocturnal illumination in Cambridgeshire - from the brightly lit city of Cambridge to the darker surrounding villages and countryside - and recordings were made during peak thrush migration, between late September and mid-November in 2019. This study used passive acoustic monitoring devices deployed in the gardens of local birdwatchers to record the calls of nocturnally migrating thrushes (Redwing, Blackbird and Song Thrush). While UK and other European cities tend to have fewer tall buildings, it is important to establish the possible impacts of ALAN on migrant birds on both sides of the Atlantic. This problem is particularly associated with skyscrapers in North America, where mass mortality events occur each year. Previous studies have shown that nocturnal migrants can be affected by ALAN, which can make them disoriented and cause them to collide with structures such as lighthouses, oil platforms and tall buildings. For example, 80% of UK summer migrants move after dark, and 40–150 million birds are estimated to cross the North Sea at night each autumn. Migrating at night is a common migratory strategy. New research from BTO has investigated whether artificial light at night (ALAN) could disrupt movement patterns of migrant species, thereby adding to the pressures they face throughout their lifecycle. Migratory birds face many challenges during their annual movements between breeding and wintering areas. New research from BTO has investigated the effect of artificial light at night on birds, indicating that nocturnal migrants are attracted to more brightly lit areas.
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