Geographic barriers and season shape the nightly timing of avian migration

Abstract

AimMillions of birds take to the air for nocturnal migrations. Although it is widely recognized that migrants generally depart after sunset, nightly migration timing and their dependence on geographic features are hardly known at a continental scale, yet highly important for the mitigation of human‐wildlife conflicts. Using weather radars, we investigate barrier and seasonal effects on the timing of nocturnal bird migration.LocationNorth western Europe: United Kingdom, Germany, Belgium, the Netherlands, France, Sweden and Finland.Time Period2014–2020.Major Taxa StudiedAves, nocturnal migrants, predominantly passerines.MethodsWe use nocturnal bird migration distributions extracted from 55 weather radars. The variation between these temporal distributions is captured using a principal component analysis, barrier effects and seasonal differences are investigated with a general linear model.ResultsMost variation in nightly migration timing can be explained by a univariate axis that distinguished a more evenly spread migration from a skewed migration. We found migration to be more evenly spread in spring and to have a clear peak early in the night in fall. Furthermore, migration is more peaked early in the night on locations close to or just upstream of major geographic barriers.ConclusionsOur study shows that migration fluxes tend to be more skewed during the night along coastlines and more uniform inland, far from water barriers. Regional and seasonal differences in nocturnal timing can provide vital information for adjusting the timing of wind park curtailment, lights‐out initiatives or other conflicts between migratory birds and human activities.

Forecasting nocturnal bird migration for dynamic aeroconservation: The value of short‐term datasets

“…In autumn, the high importance of diurnal phenology might reflect different migration cohorts that can cross the radar in autumn. One cohort consists of birds leaving directly after sunset from the Dutch coast, ca 23 km from the radar, and thus their daily timing would be relatively predictable, as also shown for other coastal areas around Europe (Kranstauber et al, 2023). The other cohort is migrants from Scandinavia and Northwestern Germany (Bradarić et al, 2020), whose timing may vary depending on wind conditions experienced en route.…”

“…We also use the models to study the importance of different environmental variables in shaping seasonal migration dynamics in the North Sea basin. As model input, we use the day of year and time of day to describe migratory birds' mean circannual and circadian dynamics (Gwinner & Helm, 2003;Kranstauber et al, 2023).…”

“…We also use the models to study the importance of different environmental variables in shaping seasonal migration dynamics in the North Sea basin. As model input, we use the day of year and time of day to describe migratory birds' mean circannual and circadian dynamics (Gwinner & Helm, 2003; Kranstauber et al., 2023). Furthermore, we include wind assistance, air pressure, temperature and precipitation from the radar location at sea as well as from inferred departure locations on land, all of which have been observed to influence migration intensity (Bradarić et al., 2020; Kemp et al., 2013; Richardson, 1990).…”