Family life is critical for migration and survival in young Bewick’s swans
By Hans Linssen, Emiel van Loon, Robert Wilson, Sarah Sonsthagen, Götz Eichhorn, Judy Shamoun-Baranes and Bart Nolet
Extended parental care is well known among swans and other long-lived migratory birds, but direct evidence of its importance for offspring migration and survival is scarce. Using GPS tracking and genetic data from Bewick’s swan families, we show that juveniles which lost their parents prematurely migrated later and survived far less often than those that stayed with their family.
Bewick’s swans (Cygnus columbianus bewickii). Photo by Frederick Fleet.
Bewick’s swan cygnets travel south with their parents and are generally assumed to stay with them through winter and into spring. Using genetic data to confirm true parent-offspring and sibling relationships among GPS-tracked birds, we found that families usually stay together remarkably long: most break-ups occurred well into the offspring’s first northward migration, when the young birds were nearly a year old. Many pairs even reunited in later years, sometimes migrating together again.
A severe disturbance event gave us an inadvertent way to test what happens when this bond breaks early. In December 2021, hunters entered a roost site shortly after we had tagged a group of swans there, killing several tagged birds and severely disturbing the rest, leaving ten GPS-tagged juveniles without parents from their first midwinter onwards. These orphaned juveniles behaved very differently from birds with intact families. Almost none formed new family bonds, despite plenty of unrelated swans nearby. Instead, they lingered at the wintering grounds long after the rest of the population had left, eventually migrating alone, on average a month later than juveniles that kept their parents. Only 4 of 10 (40%) survived to the next winter, much less compared with 12 of 14 (86%) among those with intact families.
These results provide rare, direct evidence that family life forms a critical pathway through which young swans learn to migrate successfully, and they underline the need to minimise disturbance of family groups during the sensitive winter period.
The full article can be found here
