Nocturnal bird migration at Besh Barmag bottleneck in Azerbaijan as revealed by means of acoustic monitoring

Abstract

SummaryBird migration studies are sparse in the Caucasus region, but have received more interest in recent years. To date, these studies have focused on diurnal migration and no information about nocturnal bird migration is available from this region. Therefore, nocturnal bird migration in the Besh Barmag bottleneck (Azerbaijan) was acoustically analysed on the basis of 1,464 h 44 min of sound recordings cost-efficiently obtained with an autonomously operating recorder and an omnidirectional microphone between sunset and sunrise on 63 nights in autumn 2011 and 67 nights in spring 2012. In total, 88,455 calls of 106 migrating species were detected. Of these, 2,172 calls could not be identified due to recording deficiencies or imperfect familiarity with some of the vocalisations and may involve as many as 20 species. The calls and songs of another 13 non-migratory species were not counted. Due to organisational or technical constraints some nights in the study periods could not be analysed and so the ensuing data gaps were repaired by interpolation, resulting in an estimated total of 108,986 calls in autumn 2011 and 33,348 calls in spring 2012. In both seasons the most vocally productive and species-rich phase was civil morning twilight, containing as it does the onset of diurnal migration. In autumn 2011, 54.7% of the recorded calls occurred in civil evening and morning twilight and 68.8% in spring 2012. But species and call numbers were also high in the darkest twilight and night phases. The interpretation of the data is, however, partly conjectural and any future access to truly reliable information on migration densities is conceivable only through radar studies.

Monitoring bird migration using nocturnal flight calls on the East Asian–Australasian Flyway: a case from Beijing

“…In our study, only a small portion of the NFCs recorded were from the civil twilight hours. This is in contrast with previous findings that these hours contribute a major proportion of NFCs during migration seasons (Heiss 2018). This difference may be linked to three factors.…”

“…This difference is consistent with some similar studies from other flyways (e.g. Farnsworth 2004;Heiss 2018;Horton et al 2015a), and has four possible reasons. First, because of the many juveniles that undertake their first autumn migration but that may not survive that journey or their first winter (Latta and Faaborg 2002;Marra and Holmes 2001), the number of migratory individuals in the spring tends to be much lower than in the autumn, as evidenced by the 20-40% lower spring biomass of migratory birds estimated by radar imagery in North America (Dokter et al 2018).…”

“…For each species or species group during a given season, we then tallied the total number of calls, differentiating among the various phases of the night time as demarcated by civil dusk and civil dawn (solar elevation angle = -6°). This differentiation is in light of the increasing evidence from North America and Europe that, compared with sunset and sunrise, civil dusk and civil dawn better demarcate the temporal span of nocturnal migration for most species (Cooper et al 2023; Heiss 2018; Gillings et al 2018). Yet because of the lack of information from EAAF, it would be risky to discard data from the evening and morning civil twilight hours (i.e.…”

“…Therefore, relative to bona fide night and potentially strongly related to birds passing over, calls may be less concentrated in civil twilight hours that are potentially strongly related to bird departure or landing. Second, birds' flight altitudes may be lower at our study site than in the case of Heiss (2018), which would make night-time NFCs easier to capture at our study site. Third, there may be fewer diurnal migrants calling during civil twilight hours at our study site, related to possible differences in individual number, calling behaviour, and/or timing of departure/landing.…”

“…Farnsworth 2005), information that is often difficult to obtain by other means of remote sensing such as radar imagery (Guo et al 2023; Shi et al 2023). Whilst NFC monitoring has been increasingly used in North America and Europe over the last few years (Gillings and Scott 2021; Heiss 2018; Horton et al 2015a; Van Doren et al 2023; Watson et al 2016; Winger et al 2019), its application in other regions remains relatively limited. The opportunity for its application, however, is straightforward in these other regions, with many guides and databases having been developed to help the identification of NFCs (e.g.…”