Stopover departure decisions in songbirds: do long-distance migrants depart earlier and more independently of weather conditions than medium-distance migrants?

Abstract

Background Songbirds following distinct migration strategies (e.g. long- vs. short- to medium-distance migrants) often differ in their speed of migration during autumn and, thus, are assumed to face different time constraints. During migration, most songbird species alternate migratory flights with stopover periods. Many of them restrict these migratory flights to the night, i.e., they are nocturnal migrants. At stopover, nocturnal migrants need to select a specific night (night-to-night decision) and time of night (within-night decision) to resume migration. These departure decisions, which largely determine the speed of migration, are jointly affected by a set of intrinsic and extrinsic factors, i.e., departure cues. Here we aim to assess whether the set of intrinsic and extrinsic factors and the magnitude of their respective effects on stopover departure decisions differs between nocturnally migrating songbird species, depending on their migration strategy and associated time constraints. Methods We radio-tracked migrating Northern Wheatears (Oenanthe oenanthe; long-distance migrant), European robins (Erithacus rubecula) and Common Blackbirds (Turdus merula; both medium-distance migrants) during autumn stopover and analysed their night-to-night and within-night departure timing in relation to intrinsic and extrinsic factors. Results Species generally differed in their departure timing on both temporal scales, with shortest stopovers and earliest nocturnal departures in the long-distance migrant. Some factors, such as day of year, fuel load, cloud cover and crosswind, had consistent effects on stopover departure decisions in all three species. However, species differed in the effects of tailwind assistance, change in atmospheric pressure and air temperature on their stopover departure decisions. Whereas night-to-night decisions were affected by these extrinsic factors in either both or one of the medium-distance migrants, such effects were not found in the long-distance migrant. Conclusions Our results suggest that the general timing of departures in songbirds is affected by the species-specific migration strategy and associated time constraints. Further, they imply that the assessment and usage of specific extrinsic factors, i.e., weather conditions, as departure cues is adjusted based on this migration strategy, with the long-distance migrants being least selective at departure. Other intrinsic and extrinsic factors, however, seem to be used as departure cues independent of migration strategy.

Winds at departure shape seasonal patterns of nocturnal bird migration over the North Sea

“…Yet, the occurrence of IMN in late autumn and early spring and the observed differences in airspeeds between IMN and WMN in autumn (Table 1) imply that different species might be involved in migration on different nights. IMN in our study may reflect predominantly short-distance migrants who have been shown to respond differently to weather conditions at departure than long-distance migrants in a comparative analysis of several songbird species (Packmor et al 2020). Currently, due to the limitations of the radar system and the similar distributions of airspeeds between seasons we cannot determine whether species composition differs between spring and autumn.…”

Quantifying year-round nocturnal bird migration with a fluid dynamics model

“…This feature may prove particularly useful in follow up studies that aim at linking movements and stop-overs to geographical features or short, intense weather events. In addition, our quantitative estimation of bird’s take-off would also nicely complement studies on species-specific variations in the timing of take-off as retrieved from individual tracking (Müller, Eikenaar, Crysler, Taylor, & Schmaljohann, 2018; Packmor, Klinner, Woodworth, Eikenaar, & Schmaljohann, 2020), and shed new light into the variation in landing behaviour.…”

Associations of Synoptic Weather Conditions With Nocturnal Bird Migration Over the North Sea

“…While we do not have information on the species in our study, the timing of these nights suggests that they may be generally representative of short to medium distance migrants in the region. Packmor et al (2020) for example showed that night to night departure decisions of a short and medium distance migratory passerine was influenced by tailwinds and change in barometric pressure, while these weather factors did not influence departure decisions of a long distance migrants. However, in order to determine whether differential response to weather among and perhaps even within species results in peak migration over the North Sea occurring predominantly in early spring or late autumn requires further research integrating information on fluxes, species composition and regional synoptic conditions.…”

“…For example, the passage of high-pressure systems can be related to opposing winds in autumn and supporting winds in spring (Dokter et al, 2013). Additionally, species with similar flight modes but contrasting migration strategies, for example short vs long distance passerines may respond differently to weather conditions at departure (Packmor et al, 2020). As many weather variables are closely intercorrelated, less influential variables such as the relative humidity will tend to vary in predictable ways as a function of the variables of higher importance, such as the pressure systems (Richardson, 1990), or precipitation, especially during relatively unstable atmospheric conditions.…”