Migratory restlessness in captive individuals predicts actual departure in the wild

Abstract

In captivity, migratory birds show increased activity during the time that they would normally migrate. The phenology and intensity of such 'migratory restlessness' has been shown to mirror species-and population-specific migration patterns observed in the wild and has consequently been used as a proxy for the motivation to migrate. Many studies doing so, however, were aiming to explain among-individual variation in migratory behaviour or traits, and not species-or population-specific traits. These studies thus assumed that, also at the level of the individual, migratory restlessness is an accurate proxy for the motivation to migrate. We tested this assumption for the first time and found that it holds; individuals showing very little migratory restlessness remained at stopover for longer than one night, whereas most individuals showing more restlessness departed sooner. This finding validates the use of migratory restlessness as a proxy for the motivation to migrate, thereby justifying the conclusions made in a large body of research on avian migration.

Sleeping on the wing

“…At the time of year when their wild conspecifics are migrating, captive songbirds exhibit migratory restlessness, or Zugunruhe, consisting of hopping around their cage and fluttering their wings at night [58]. This behaviour is thought to reflect the birds' endogenous urge to migrate at certain times of the year [59], and therefore provides an opportunity to examine seasonal changes in sleep in captivity. When compared with the non-migratory season, white-crowned sparrows (Zonotrichia leucophrys gambelii) and Swainson's thrushes (Catharus ustulatus) exhibiting migratory restlessness reduced the time spent sleeping at night by two-thirds [60 -62].…”

Start of nocturnal migratory restlessness in captive birds predicts nocturnal departure time in free-flying birds

“…Only the number of active 15-min intervals of the entire night did not predict sun's elevation at departure (Table 1). Possibly, the activity displayed outside the 1-5.5 h time period after lights off, i.e., the time span in which free-flying northern wheatear set off from Helgoland (Schmaljohann et al 2013), is not indicative of the motivation to depart, as already assumed Eikenaar et al (2014). Also, in our study, northern wheatears set off between 38 and 346 min after sunset (n = 21), cf.…”

“…1). As most individuals show at least some restlessness at night, also for birds not motivated to resume migration (Eikenaar et al 2014), both approximations estimate a Bstartô f nocturnal migratory restlessness. To exclude such individuals, we suggest tracking all birds the night after release.…”

“…(c) Total number of 15-min intervals in which a bird showed at least five activity counts for the time frame (1-5.5 h after lights off) corresponding to the time period when radio-tagged northern wheatears left Helgoland in other studies (1-5.5 h after sunset) (Schmaljohann et al 2013), cf. Eikenaar et al (2014); and (d) total number of 15-min intervals in which a bird showed at least five activity counts for the entire night, cf. Maggini and Bairlein (2010) and Eikenaar and Schläfke (2013).…”

“…In course of the season, the amount of nocturnal migratory restlessness varies among and within species: the longer the migration route, the more caged birds are active over the season (Berthold and Querner 1981). Recently, it was demonstrated that the amount of nocturnal migratory restlessness as displayed at a certain night is a good approximation for the motivation of that individual bird to resume migration, i.e., to set off from a stopover site on that night (Eikenaar et al 2014). Thus, nocturnal migratory restlessness integrated over season or night describes the migration behavior a bird would show under free-flying conditions.…”