Young, naïve starlings are looking for their wintering grounds independently of experienced conspecifics. Starlings are highly social birds throughout the year, but this does not mean that they copy the migration route from each other. By revisiting a classic ‘displacement’ experiment and by adding new data, a team of researchers at the Netherlands Institute of Ecology (NIOO-KNAW) and the Swiss Ornithological Institute (Vogelwarte Sempach) have settled a long-lasting debate. Their findings are now published in the scientific journal Biology Letters.
The question of how migratory birds locate their migration routes has intrigued humankind for centuries. Biologist Albert Perdeck from the Netherlands aimed to find answers when he displaced thousands of migrating starlings by plane from the Netherlands to Switzerland and Spain in the 1950’s and 1960’s. This experiment has become a classic study on the migratory orientation of birds. Now, seventy years later, colleagues have confirmed his findings and were able to solve a long-lasting scientific debate using this historical dataset.
Young vs. adult
The birds were individually recognisable using light-weight metal leg rings with a unique code — a method used by the Dutch Centre for Avian Migration and Demography, Vogelwarte Sempach and European partners until this day. Ring recoveries indicated that relocated young and adult starlings used different strategies to reach the winter destinations in the British Isles and France. “Adult starlings were aware of this move and adjusted their migratory orientation to reach their normal wintering areas,” according to Morrison Pot at the NIOO-KNAW. “Young starlings continued in a south-westerly direction — the direction they would have chosen when departing from the Netherlands — and reached ‘wrong’ destinations in southern France and Spain.”
New friends?
Over the years, experts in the field of avian migration have been divided about the interpretation of Perdeck’s results. Pot: “Starlings are highly social animals and, according to some experts, the relocated young starlings may just as well have joined a flock of local conspecifics.” The relocated starlings would have copied the migratory behaviour of their new friends showing them where to go. “If true, the migratory route is largely learned instead of inherited.” A major difference.
The team of researchers retrieved the historical data of Perdeck’s displacement experiments in the paper archives of the Dutch Centre for Avian Migration and Demography and compared the migratory orientation with the migratory behaviour of local Swiss and Spanish starlings. “The latter data were retrieved from institutional archives, but were unavailable in Perdeck’s days.”
Social migrants
By re-analysing this historical dataset, the team showed that the migratory orientation of the relocated starlings differed from the local conspecifics. Starlings are thus no social migrants or ‘copycats’. The alternative social explanation of Perdeck’s results has thus been debunked. As explained by Pot: “Starlings travel independently and decisions about where to go are not overruled by the migratory behaviour of others.” Recently, a study in collaboration with Vogelwarte Sempach showed that starlings migrate at night. This is in line with the 70-year-old findings, because how would you follow someone in the pitch darkness of the night?
Times of change
Learned or inherited behaviour, why does it matter? “In times of rapid changes in global climate and land-use, it is of great importance to understand whether migratory behaviour is largely inherited or learned,” says lead scientist and head of the Dutch Centre for Avian Migration and Demography Henk van der Jeugd. Inherited behaviours are less flexible to rapid change. “Although starlings are numerous and widespread birds that have adjusted to human dominated landscapes, their migratory behaviour is likely less flexible.”