Programmed and flexible: long‐term Zugunruhe data highlight the many axes of variation in avian migratory behaviour

Abstract

Studies of Zugunruhe – the ‘migratory restlessness’ behaviour of captive birds – have been integral to our understanding of animal migration, revealing an inherited propensity to migrate and an endogenous timing and navigation system. However, differences between Zugunruhe in captivity and migration in the wild call for more data, in particular on variation within and among taxa with diverse migration strategies. Here, we characterise Zugunruhe in a long‐term dataset of activity profiles from stonechats (genus Saxicola) with diverse migratory phenotypes (976 migration periods from 414 birds), using a flexible and consistent quantitative approach based on changepoint analysis. For east African, Austrian, Irish, and Siberian stonechats and hybrids, we report key inter‐population differences in the occurrence, timing, and intensity of Zugunruhe. In line with expectations, we found the highest Zugunruhe intensity in the longest‐distance migrants, more variable patterns in short‐distance migrants, and intermediate characteristics of hybrids relative to their parental groups. Inter‐population differences imply high evolutionary lability of Zugunruhe timing within a robustly structured annual cycle. However, counter to theory, Irish partial migrants showed no segregation between migrant and resident individuals, and previously reported nocturnal restlessness was confirmed for resident African stonechats. Further features of nocturnal restlessness that did not align with migratory behaviour of stonechats were juvenile nocturnal restlessness even prior to postjuvenile moult, and protandry in spring, although stonechats winter in heterosexual pairs. Importantly, Zugunruhe of all populations declined with age, and the intensity of an individual bird's Zugunruhe was correlated with activity levels during other parts of the annual cycle. Our results confirm endogenous, population‐specific migration programmes but also reveal apparent discrepancies between Zugunruhe and migration in the wild. We thus highlight both the continued potential of Zugunruhe study and the need for circumspect interpretation when using migratory restlessness to make inferences about migration in the wild.

Timing avian long-distance migration: from internal clock mechanisms to global flights

“…Overall, this comparison supports an interpretation of Zugunruhe as a proxy for a readiness of birds to migrate, but cautions against considering it a direct reflection of migration flight 16 .…”

“…Development of tracking technology provides ever more detail from free-living birds 17 , and overall, the data indicate a good match with carefully interpreted data from captive studies 16 . For example, in the study of Bäckman et al 21 (see Fig.…”

“…These observations prompted Kramer 8 to propose that the integration of an inherited sense of time with an inherited sense of direction could function as a "clock and compass" mechanism, which could guide inexperienced birds on their first journey before learning about the environment en route. Since then, captive studies of a wide range of migratory species, and systematic experimentation of the captive environment have identified detailed time programs and their interactions with selected environmental factors 4,10,13,14,16 . Many of the sophisticated adaptations of migratory programs can best be studied in individually migrating songbird species, especially in hand-raised individuals during their first journey when young birds are still naïve.…”

Broad‐scale patterns of the Afro‐Palaearctic landbird migration

“…Zugunruhe data also demonstrate seasonal differences in the length and intensity of migratory activity (Van Doren et al, ). We found that the migration window was shorter (therefore, the migration traffic was more synchronized) and the speed of travel faster in spring compared with autumn on both flyways.…”

“…Long‐distance migrants generally depart from their breeding sites well ahead of senescence of the environment in autumn (Briedis et al, ), suggesting that the timing of departure in autumn is probably controlled by mechanisms other than deteriorating habitat conditions. Experimental studies on stonechats, Saxicola torquatus , revealed that migratory restlessness ( Zugunruhe ) starts significantly earlier in Irish birds compared with their continental counterparts (Van Doren, Liedvogel, & Helm, ), suggesting endogenous control for the start of autumn migration (Pedersen, Jackson, Thorup, & Tøttrup, ).…”