Travel Fever
People once believed that swallows, storks or nightingales would bury themselves asleep in swamps in winter, only to rise like a phoenix from the mud in spring. Today, of course, almost everyone knows the truth, but the migration of birds still offers new surprises. When the weather becomes more uncomfortable, the sun sets earlier, the leaves fall from the trees and the temperatures plummet, many people would like to emulate the migratory birds and flee south. At least in the animals, these changes in nature trigger an inner impulse that ornithologists describe with the beautiful term train restlessness.
This restless behavior strikes captive birds at just those times of the year when they would either need to fly to their wintering grounds or return to their breeding grounds: the animals will hop or flutter restlessly about in their cages; if their accommodation is even a so-called orientation cage, their movement and even their corresponding energy expenditure are roughly aligned with the species-typical migratory line. From these discoveries, researchers ultimately concluded that a kind of annual program is genetically anchored in birds, which ultimately also determines migration.
The fact that this phenomenon occurs in migratory birds is probably completely understandable. But what about species that are closely related to those wanderers between worlds, but do not migrate because of their tropical – or at least warm – home, but remain largely stationary? That's what the two scientists Barbara Helm and the late Eberhard Gwinner from the Max Planck Institute for Ornithology in Andechs, Bavaria, wanted to know.
They therefore hand-reared young of the African stonechat subspecies (Saxicola torquata axillaris) and exposed them to different lighting conditions. One group then lived for a year and a half under African day lengths, while other conspecifics had to cope with the annual cycle that is typical for southern Germany. Their data was then compared with the behavior of the European subspecies Saxicola torquata rubicola, which flees in winter. Stonechats are also suitable for this study because they migrate in the dark, making it easier to distinguish the hyperactivity during migration from other nocturnal sleeping behavior.
And indeed, those African subjects who were kept under the constant light conditions of their Kenyan homeland repeatedly showed different phases of the urge to move at night - similar to their European cousins. However, these activities were widely distributed among the various Kenyan birds over the entire test period, albeit with certain focal points during the typical migration period in autumn and spring.
If, on the other hand, the Max Planck migratory birds of the subspecies axillaris lived under European light conditions, their nocturnal activities synchronized completely with the corresponding seasons: their movements were too nocturnal both during autumn migration and – even more clearly – during spring migration time very pronounced. And like their European relatives, this behavior was independent of hatch timing: late-borns simply started at a younger age, matching the same timeframe as older siblings.
This eliminates the day length as an external clock for train disturbances because of the apparently not completely decisive influence of lighting, according to the scientists. Instead, there is probably an internal, genetically controlled clock that gives the singers an innate signal for species-specific train restlessness. And since it's present in both Africans and Europeans, it likely goes back to a common ancestor- although their evolutionary paths diverged between one and three million years ago.
In the opinion of Helm and Gwinner, this program could even subtly slumber in many non-migratory bird species and, if necessary, be activated at any time in the event of serious external influences - which suddenly make periodic migrations necessary. Due to the current rapid global change and the associated, sometimes rapid environmental changes, the prospects could improve, at least for some resident birds: If the actual home becomes uncomfortable in sections, they could then simply flee it for a while.
