Nocturnal departure timing in songbirds facing distinct migratory challenges

Abstract

Most migratory songbirds travel between their breeding areas and wintering grounds through a series of nocturnal flights. The timing of their departures defines the potential flight duration and thus the distance covered during a migratory night. Yet, migratory songbirds show substantial variation in their nocturnal departure timing. With this study, we aim to assess whether the respective challenges of the migration route, namely its distance and nature, help to explain this variation. At a stopover site, we caught Northern Wheatears (Oenanthe oenanthe) of two subspecies that differ in distance and nature of their onward migration route in spring, but not in autumn. We determined the start of their nocturnal migratory restlessness during short-term captivity, and radiotracked their nocturnal departure timing after release in both migration seasons. Northern Wheatears started their nocturnal migratory restlessness earlier when facing a long remaining migration distance and an extended sea barrier in spring. Individual departure directions generally affected the nocturnal departure timing with early departures being directed towards the respective migratory destination. In spring, this pattern was predominantly found in birds carrying relatively large fuel stores, but was absent in lean birds. At the same time, birds facing a short remaining migration distance and no extended sea barrier strongly reacted to relatively large fuel stores by an early start of nocturnal migratory behaviour (migratory restlessness and departure timing), whereas this reaction was not found in birds facing a long remaining migration distance and sea barrier. These results suggest that the basic diel schedule of birds' migratory activity is adapted to the onward migration route. Further, they suggest that birds adjust their behavioural response, that is start of nocturnal migratory behaviour, to fuel stores in relation to their impending migratory challenges. This is a substantial step in understanding variation of nocturnal departure timing and its adjustments in migratory songbirds. Further, it emphasizes the importance of interpreting birds' nocturnal migratory behaviour in the respective ecological context.

Endogenous Programs and Flexibility in Bird Migration

“…Within the time window set by the program for migration, decisions about its implementation are sensitive to a range of environmental factors that determine successful migration (Figure 4). Departure time may be adjusted in response to level of fuel reserves, and relative to the expected onward migration route, including distance of barrier crossings (Müller et al, 2018). Responses to these environmental factors are partly inherited, and therefore, we here expand on their effects.…”

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.…”

Potential age differences in the migratory behaviour of a nocturnal songbird migrant during autumn and spring

“…The time of highest signal strength during departure was used as bird's nocturnal departure timing, cf. Smolinsky et al (2013), Eikenaar et al (2017), and Müller et al (2018). This was used to calculate the nocturnal departure timing in relation to local sunset (minutes after sunset) and the sun's elevation at departure (° below horizon).…”

“…Considered detections of the different antennas detailing the directions of signals were weighted by their temporal proximity to the last detection. When antennas aligned to a direction close to the estimated departure direction failed recording signals during the departure event and/or the signal got lost shortly after birds' take-off (< 3 min), the corresponding departure directions were excluded, because they were probably imprecise, for details see Müller et al (2018).…”

“…We converted departure direction (circular variable) into a measure of deviance between observed (radio-tracked) departure direction and seasonally appropriate migration direction which enabled the inclusion of directional information in subsequent multivariate analyses, cf. Müller et al (2018). This measure of deviance, the resultant vector length ρ (rho; rho of departure direction hereafter), was calculated using the 'circ.summary' function of the R package 'circular' (Agostinelli and Lund 2017).…”