Towards a conceptual framework for explaining variation in nocturnal departure time of songbird migrants

Abstract

Most songbird migrants travel between their breeding areas and wintering grounds by a series of nocturnal flights. The exact nocturnal departure time for these flights varies considerably between individuals even of the same species. Although the basic circannual and circadian rhythms of songbirds, their adaptation to migration, and the factors influencing the birds' day-to-day departure decision are reasonably well studied, we do not understand how birds time their departures within the night. These decisions are crucial, because the nocturnal departure time defines the potential flight duration of the migratory night. The distances covered during the nocturnal migratory flights in the course of migration in turn directly affect the overall speed of migration. To understand the factors influencing the arrival of the birds in the breeding/wintering areas, we need to investigate the mechanisms that control nocturnal departure time. Here, we provide the first conceptual framework for explaining the variation commonly observed in this migratory trait. The basic schedule of nocturnal departure is likely regulated by both the circannual and circadian rhythms of the innate migration program. We postulate that the endogenously controlled schedule of nocturnal departures is modified by intrinsic and extrinsic factors. So far there is only correlative evidence that birds with a high fuel load or a considerable increase in fuel load and significant wind (flow) assistance towards their migratory goal depart early within the night. In contrast, birds migrating with little fuel and under unfavorable wind conditions show high variation in their nocturnal departure time. The latter may contain an unknown proportion of nocturnal movements not directly related to migratory flights. Excluding such movements is crucial to clearly identify the main drivers of the variation in nocturnal departure time. In general we assume that the observed variation in the nocturnal departure time is explained by individually different reactions norms of the innate migration program to both intrinsic and extrinsic factors.

Keywords: Circannual and circadian rhythms; Departure; Innate migration program; Migration; Night; Songbird; Time.

Fig. 1

Variation in the nocturnal departure time of different songbird species as obtained from different radio tracking studies. Bean plots illustrate variation in nocturnal departure timing in relation to sunset (left side) and the proportion of the night (right side). Extension of the bean plots represent kernel density estimations of the distribution of individual departures, given as small lines in the plot. Broad lines represent the median nocturnal departure time of the respective group. Gray bean plots: autumn migration; white bean plots: spring migration. European Robin (Erithacus rubecula) at Rybachy, Russia [39] and Falsterbo, Sweden [46]; Song Thrush (Turdus philomelos) at Falsterbo, Sweden [46]; Swainson’s Thrush (Catharus ustulatus) at Long Point, Ontario, Canada [49, 50]; Eurasian Reed Warbler (Acrocephalus scirpaceus) at Falsterbo, Sweden [37]; Willow Warbler (Phylloscopus trochilus) at Falsterbo, Sweden [46]; Garden Warbler (Sylvia borin) at Falsterbo, Sweden [46]; Black-throated Blue Warbler (Setophaga caerulescens) at Long Point, Ontario, Canada [49, 50]; Northern Wheatears (Oenanthe oenanthe) at Wales, Alaska, USA [44] and on Helgoland, Germany [42, 43]; Sedge Warbler (Acrocephalus schoenobaenus) at Rybachy, Russia [48]

Fig. 2

Schematic conceptual framework for the regulation of nocturnal departure time in songbird migrants. The innate program provides the circannual, circadian rhythms and spatiotemporal schedule of migration. The photoperiod is used to calibrate or reset the innate migration program. The seasonal variation in birds’ fuel load and the seasonal-specific sequence of molt are regulated among other traits by the innate rhythms (broken arrows). The realized nocturnal departure time represents the interplay of both the intrinsic and extrinsic factors modulating the endogenous stimuli

Fig. 3

Predictions about the potential effect of different intrinsic and extrinsic factors on nocturnal departure time of songbird migrants. Predicted effects on mean nocturnal departure time (solid and dashed line) and its variation (shaded light gray) refer to factors in the respective boxes (solid and dashed fringe)