Helm and Gwinner searched for signs of migratory behavior in two subspecies of stonechats, Saxicola torquata, comparing a migrant that breeds in Austria, S. t. rubicola, and its equatorial resident relative, S. t. axillaris. European stonechats are short-distance, nocturnal migrators that begin their journey when daylight lasts just over 12 hours. Since they would otherwise be sleeping at night, nocturnal activity can serve as a proxy for Zugunruhe. African stonechats are sedentary species that do not abandon their breeding grounds in Kenya. To investigate the presence of Zugunruhe in a resident species, the researchers raised and bred the offspring of Kenyan stonechats in their lab in Germany. One group of these birds was held for the duration of a migratory period under the nearly equal light and dark conditions of their native habitat, and a subset remained under these conditions for a year and a half. A control group was exposed to the natural seasonal light fluctuations of southern Germany. Helm and Gwinner recorded the birds' nocturnal movements with infrared motion sensors, and counted the number of movements within ten-minute intervals. If 20 or more movements were noted, the interval was considered "active."
Even though the African stonechats experienced no temporal cues--light levels remained constant--their nocturnal activity roughly tracked the season. The African birds' migratory restlessness, marked by repeated, spontaneous outbursts of nocturnal activity, echoed that seen in European stonechats, though it was less pronounced. The African birds also showed a telling relationship between hatching date and onset of nocturnal activity: just like their migratory counterparts, late-hatching birds became restless earlier and earlier, coinciding with the migratory season.
The African birds' behavior can be attributed only to Zugunruhe, the researchers concluded, suggesting the influence of an inborn, precisely timed migratory program. The presence of this program in both migrants and residents suggests that the urge to migrate may have evolved in their common ancestor. Helm and Gwinner propose that it may be a common avian feature. Given the proper environmental triggers, this innate migratory program might kick in to allow birds to escape deteriorating habitats caused by global climate changes or other ecological disturbances. With evidence that Zugunruhe exists in nonmigratory birds, researchers can continue exploring migratory behavior in any number of resident-migratory pairs to probe the many ways birds take flight to improve their chances of survival.
Citation: Helm B, Gwinner E (2006) Migratory restlessness in an equatorial nonmigratory bird. PLoS Biol 4(4): e110.
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