Geographic barriers and season shape the nightly timing of avian migration

Aim: Millions 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.

Location: North western Europe: United Kingdom, Germany, Belgium, the Netherlands, France, Sweden and Finland.

Time Period: 2014–2020.

Major Taxa Studied: Aves, nocturnal migrants, predominantly passerines.

Methods: We 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.

Results: Most 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.

Conclusions: Our 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.