Nocturnal flight calling behaviour of thrushes in relation to artificial light at night
Author(s): Gillings, S. & Scott, C.
Published: May 2021
Digital Identifier No. (DOI): 10.1111/ibi.12955
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.
Migratory birds face many challenges during their annual movements between breeding and wintering areas. 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.
Migrating at night is a common migratory strategy. 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. 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. This problem is particularly associated with skyscrapers in North America, where mass mortality events occur each year. 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 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). 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. 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 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. 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. 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.
Migratory birds are subject to many pressures during their lifecycle and many are declining as a consequence. 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. Comparable evidence for species and urban areas in Europe is limited. This study aimed to measure the response of nocturnally migrating thrushes to artificial light at night in the UK. 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. 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.
Seasonal patterns of call rates were positively correlated across sites, but there were large differences in absolute rates between 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. 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. European cities are not blighted by mass mortality of migrants striking illuminated buildings but even so, these results show that nocturnal migrants are influenced by light pollution. Ascertaining whether this has fitness consequences is a priority so as to inform the design and illumination of future urban areas.
NotesThe authors thank the European NFC recorder community for sharing audio material for the training library. Thanks to Jon Heath for sharing his manually processed recording totals, and to the following who volunteered to host an AudioMoth: Louise Bacon, Guy Belcher, Chris Brown, Ian Burfield, Malcolm Busby, Richard Dale, Nicki Dibb, Andrew Dobson, Brendan Doe, Alan Fersht, Pamela Gatrell, John Harding, Ken Hook, Bob Jarman, Pam Landshoff, Adrian Lyszkowski , Sarah Mardon, Dick Newell, Pat Reynolds, John & Erica Roote, Howard Slatter, Brenda Smith, Richard Thomas and Martin Walters. Thanks also to Vince Lea and Clive Sinclair for arranging the original mailing out to CBC members. Philipp Boersch-Supan provided statistical advice and the paper benefited from comments from Rob Robinson and three reviewers. The cost of AudioMoths was covered by a grant from the British Birds Charitable Trust. Development of the CNNs and analyses were supported by the Esmée Fairbairn Foundation and BTO.
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