A quasi-experimental approach using telemetry to assess migration-strategy-specific differences in the decision-making processes at stopover

Abstract

Background: Migrant birds travel between their breeding areas and wintering grounds by alternating energetically and physiologically demanding flights with periods of rest and fuelling, so-called stopovers. An important intrinsic factor influencing the decision to resume migration is the amount of energy stores available for the next flight. Correlative studies with free-flying birds and experimental studies with caged birds have shown that the amount of energy stores affects the day-today , within-day and the directional decision of departure. The methodological advantages of both the correlative and experimental approach are combined when radio-tagging many individuals on the same day and subsequently determining the departure decisions at a high spatiotemporal resolution. Making use of such a quasi-experimental approach with an automated radio-tracking system at stopover, we studied the effect of energy stores on departure decisions and whether they vary between species of different migration strategies experiencing contrasting time constraints. For this, we chose a long-distance migrant, the common redstart (Phoenicurus phoenicurus), and a medium-distance migrant, the European robin (Erithacus rubecula), because the former has to travel at relatively higher speed to reach its wintering ground in a reasonable time at the expense of relatively higher energetic costs for travelling than the latter. Results: Common redstarts with higher energy stores were more likely to resume migration than their conspecifics with lower energy stores, whereas this pattern was absent in the European robins. The amount of energy stores significantly affected the timing of departure within the day, with large energy stores yielding early departures in both species. Departure directions from the stopover site during the first night after capture were oriented towards the seasonally appropriate direction but were not affected by variation in energy stores. Conclusions: We demonstrate the importance of variation in energy stores on the departure decisions and that it may affect species with different migration strategies dissimilarly in autumn. Nevertheless, knowledge of other intrinsic factors, such as feeding conditions, health status and physiological consequences of previous flights, is additionally required to better understand the departure decisions of migrants, as this is the key to providing an overall assessment of the decision-making process.

Autumn migrating passerines at a desert edge: Do birds depart for migration after reaching a threshold fuel load or vary it according to the rate of fuel deposition?

“…Remarkably, even after long stopover periods and despite the increase in their fuel loads, some of the birds were still departing for migration with rather low departure fuel loads. Altogether, these findings suggest that both capture fuel load (Goymann et al, 2010;Schmaljohann and Eikenaar, 2017;Schmaljohann and Klinner, 2020) and fuel deposition rate (Lindström and Alerstam, 1992;Bayly, 2006;Delingat et al, 2008) have critical consequences for migration timing. These might translate to bird fitness by possibly affecting the chances to accomplish the cross-desert journey.…”

Understanding the ecological and evolutionary function of stopover in migrating birds

“…Bairlein, 1985; Loria & Moore, 1990; Goymann et al ., 2010; Cohen, Moore & Fischer, 2014a; Deppe et al ., 2015), there are also many studies that failed to find an association between fuel stores and departure probability (e.g. Salewski & Schaub, 2007; Tsvey, Bulyuk & Kosarev, 2007; Schaub, Jenni & Bairlein, 2008; Schmaljohann & Klinner, 2020). These empirically demonstrated deviations from the expected departure pattern strongly suggest that the current level of fuel stores was not decisive for the departure decision in those birds.…”

A magnetic pulse does not affect free-flight navigation behaviour of a medium-distance songbird migrant in spring

“…During spring migration, robins regularly stop over on the island, with about 200 individuals per day during peak passage from the end of March to the end of April (Dierschke et al 2011). Robins are solitary night migrants, so their migratory behaviour is independent of conspecifics (Schmaljohann and Klinner 2020;Packmor et al 2020;Dorka 1966). According to ring recoveries, robins passing Helgoland in spring migrate to northern Germany, southern Scandinavia and the Baltic region (Fig.…”