Barrier crossing in small avian migrants: individual tracking reveals prolonged nocturnal flights into the day as a common migratory strategy

Abstract

Over decades it has been unclear how individual migratory songbirds cross large ecological barriers such as seas or deserts. By deploying light-level geolocators on four songbird species weighing only about 12 g, we found that these otherwise mainly nocturnal migrants seem to regularly extend their nocturnal flights into the day when crossing the Sahara Desert and the Mediterranean Sea. The proportion of the proposed diurnally flying birds gradually declined over the day with similar landing patterns in autumn and spring. The prolonged flights were slightly more frequent in spring than in autumn, suggesting tighter migratory schedules when returning to breeding sites. Often we found several patterns for barrier crossing for the same individual in autumn compared to the spring journey. As only a small proportion of the birds flew strictly during the night and even some individuals might have flown non-stop, we suggest that prolonged endurance flights are not an exception even in small migratory species. We emphasise an individual’s ability to perform both diurnal and nocturnal migration when facing the challenge of crossing a large ecological barrier to successfully complete a migratory journey.

Elemental composition in feathers of a migratory passerine for differentiation of sex, age, and molting areas

“…Recently, Adamík et al (2016) has reported on the prevalence of the Congo basin as wintering habitat of the collared flycatcher by using light-level geolocators. Interestingly, dissolution of trace elements (e.g.…”

“…As a model for our study, we have chosen the collared flycatcher (Ficedula albicollis) ( Figure 1S, online resources), a long-distance migratory, hole-nesting, insectivorous passerine that breeds mainly in deciduous woodlands of Central Europe and it winters in sub-Saharan Africa (Cramp andPerrins 1993, Hölzinger 1993;Adamík et al 2016). This species is ideal for longterm ecological studies, as its individuals can easily be captured in artificial nestboxes, and they have a high breeding-site fidelity (Hegyi et al 2013).…”

Sleeping on the wing

“…oceans), continuous flight can be inferred. In addition, in songbirds, variability in daytime light levels has been used to estimate when the birds are flying (low variability) and when they are on land foraging in foliage (high variability) [14,15]. The interpretation of flight behaviour from geolocator data can be strengthened further by the addition of devices that detect water immersion [16] or flapping (accelerometry) [11,12,17].…”

“…[68]. Finally, several European songbirds engage in non-stop flights lasting 1-3 days that include crossing the Mediterranean Sea and Sahara Desert [14,15].…”

Conditions at autumn stopover sites affect survival of a migratory passerine

“…They migrate by alternating short periods of flight with longer stopovers, but they gain mass mostly at stopover sites in the Mediterranean area, just before crossing the Sahara Jenni 2000, 2001;Rguibi-Idrissi et al 2003). Local observations and recent geolocator studies indicate that most birds begin the autumn migration in late July-August and reach their African wintering grounds in September-October (Andueza et al 2014;Adamik et al 2016;Araújo et al 2016). In spring they undertake the return migration in March-May and reach breeding grounds in late April-early June (Arizaga et al 2010).…”