I’ve never ever expected to see the word “Zugunruhe” in New York Times! But here it is. It is one of my most favourite words of all times (right after “elusive”), and is even described pretty accurately:
Zugunruhe brooks no confusion. It has a Germanic certainty, and there can be no doubt what it means, once you know what it means. I confess that I only learned the word this week. If I understand the paper about it by Barbara Helm of the Max Planck Institute for Ornithology in Andechs, Germany, and the late Eberhard Gwinner in the Public Library of Science (www.plos.org), I have often experienced zugunruhe but didn’t know it.
The two researchers studied African stonechats. Stonechats are in the thrush family, and many breed in Europe and fly south in winter. The birds that the researchers studied were residents in Africa, and were thought not to have a genetic predisposition to migrate. What the researchers were looking for was evidence of zugunruhe in the resident stonechats, which they raised in the lab at the Max Planck Institute, under a variety of circumstances.
The scientists did not expect to find it in the resident birds. But they did. The stay-at-home stonechats exhibited the same sort of nocturnal restlessness as their migrating relatives. The conclusion: some level of zugunruhe may be common even in birds that don’t migrate.
Yup, it is usually translated as “migratory restlesness” and describes a behavior, usually at night, usually in birds, of increased activity, orienting towards the flight destination (e.g., towards the north in spring), practice flights, etc. Since most birds are diurnal (day-active), an increase (or even appearance) of activity during the night is a pretty good behavioral indicator that the animals are experiencing the urge to migrate.
Update: The paper  that prompted this article has just been posted online on PLoS: Migratory Restlessness in an Equatorial Nonmigratory Bird
European (migratory) and Kenyan (non-migratory) stonechats were kept in the laboratory and their locomotor activity was monitored continuously. Some of the birds were kept in a constant photoperiod (light/dark 12.25:11.75 h) often seen in Kenya. Other birds were exposed to naturally changing photoperiod in Southern Germany (40 degrees North). In both treatments, both the European and the Kenyan birds showed increased nocturnal activity (zugunruhe). In constant photoperiods, each bird exhibited its own circannual rhythm, independent of the activity of its neighbors. In natural photoperiods, all the birds exhibited nocturnal pre-migratory restlesness at the appropriate season. The activity of the migratory species was much more pronounced, but the non-migrants (who vigorously defend their territories all year round in Kenya) also exhibited substantial levels of nocturnal activity:
From the paper’s conclusion:
Our study thus demonstrates common, complex features of Zugunruhe in resident and migrant birds, suggestive of ancestral patterns. However, Zugunruhe programs of African residents are unexpectedly precise, given an estimated divergence from a common ancestor 1 million to 3 million years ago [23,24]. Furthermore, evolutionary rates and heritabilities of migratory traits are reportedly high [8,11,19]. This suggests that Zugunruhe programs of African residents may either be adaptive or maintained by stabilizing selection . Assuming that Zugunruhe indicates time windows during which movements can easily be released or inhibited [1–5], several selective advantages are conceivable. Persistent Zugunruhe windows could enhance and accelerate adjustments to changing conditions [8,18,20]. Intratropical movements are common in birds and could occur at times in African stonechat populations [9,18,23,28,30]. Southern African stonechat subspecies are thought to be partial altitudinal migrants . The Kenyan population is distinct [24,31] but also inhabits high altitudes. It is conceivable that periodically, altitudinal or other seasonal migrations, for example, related to drought, become necessary. Furthermore, the maintenance of Zugunruhe could be favored by related behaviors, for instance, dispersal [8,17,18,20].
 Barbara Helm, Eberhard Gwinner, (2006) Migratory Restlessness in an Equatorial Nonmigratory Bird. PLoS Biol 4(4): e110