Are movements of daytime and nighttime passerine migrants as different as day and night?

Abstract

Even after decades of research, the migration of songbirds still holds numerous secrets. Distinct stopover and routing behavior of diurnally and nocturnally migrating songbirds has been stated in the 1960s, but empirical confirmation is yet lacking widely. We studied the behavior of individual diurnally migrating dunnocks and nocturnally migrating blackcaps by means of large-scale automated radio-telemetry. Birds were radio-tagged during their stopover at the German North Sea coast. Our data indicate longer initial stopover duration in the diurnally migrating dunnocks, opposing the hypothesis of nocturnal migrants needing more time to recover due to their longer migratory flights. Nonetheless, dunnocks stopped over more often along their tracks as when compared to the nocturnally migrating blackcaps. Behavior en route did not differ as clearly between species challenging the general view of contrasting routings of diurnal and nocturnal migrants with regard to landscape and open water. Our results imply additional factors of relevance other than differences in species or daily migration timing per se. We discuss and highlight the need of detailed and individual based data to better understand stopover and routing behavior of songbirds in the environmental context.

Keywords: North Sea; blackcap; diurnal versus nocturnal migration; dunnock; radio‐telemetry; stopover.

Figure 1

Species distribution maps of dunnocks (left) and blackcaps (right). Maps are shown in Mercator projection. Arrows represent hypothetical migration routes of individuals migrating through our study area (box) as derived from recoveries of birds ringed on Helgoland (Dierschke et al., 2011). Species distribution data were kindly provided by BirdLife International (2019). Note that a small fraction of blackcaps also migrates until sub‐Saharan Africa (not shown)

Figure 2

German Bight (south‐eastern North Sea) showing (a) locations of Motus network receivers and (b) locations of tag deployment and the island of Helgoland (asterisk) as well as progress/detour bearings during either migration season (pie charts) and threshold latitude/longitude for offshore/alongshore flight classification (dashed line) as described in the methods section

Figure 3

Tukey style box and whisker plot of daily start times of sustained flights of dunnocks (gray boxes, n = 29) and blackcaps (white boxes, n = 19) recorded at the German Bight during spring and autumn (corrected for night length). Zero refers to sunset, one to sunrise

Figure 4

Hours (UTC) of flights recorded per species and season. Gray bars indicate times of sunrise and sunset during each season

Figure 5

Tukey style box and whisker plot of a) initial stopover duration, b) progress track length, and c) detour track length at the German Bight in dunnocks (gray boxes) and blackcaps (white boxes) during spring and autumn migration season. See Table 1 for statistics