The key principle of natural selection is that only the fittest survive long enough to reproduce. Individual quality or acquired experience from very early in life are believed to predict an animal’s ability to cope with the dangers of wildlife. Although research has suggested that more active animals are often fitter, few studies had put the theory to the test by directly comparing the activity of juvenile animals and their subsequent survival in the wild. White storks engage in long-distance migratory flights to breed in Europe and overwinter in Africa. As youngsters face high mortality during their first migration journey, Shay Rotics at the Hebrew University of Jerusalem, Israel, together with an international team of experts realised that these birds were an excellent case study to test whether an active lifestyle in early life predicts subsequent survival.
Across four consecutive years (2011–2014), Rotics and colleagues headed out to farmlands across Saxony-Anhalt in Germany looking for nests of white storks. Sometimes, the researchers monitored the birds with the help of drones to estimate the age at which they would fledge in preparation for their first migratory flight. About 2 weeks before the youngsters fledged, the scientists deployed advanced solar-charged GPS trackers on a total of 93 birds. In addition, 83 of the GPS trackers were also equipped with accelerometers to capture the fine detail of the birds’ activities. The data captured by the GPS trackers covered the time the birds were still in their nests (known as the pre-fledging period) as well as the ∼15 day period after they had taken to the wing (known as post-fledging period), when they gradually became fully independent and ready to start their long-distance journey.
By analysing the early-life activity patterns recorded on the GPS body-acceleration trackers, Rotics and colleagues found that the youngsters’ daily activity levels increased as they were growing older. Importantly, the birds that were more active during the pre-fledging period were also most active during the post-fledging period. Even more interestingly, the birds that had a more active lifestyle during either the pre-fledging or post-fledging period had better chances of surviving their first year of life and of successfully completing their first migration. This effect was particularly pronounced in the birds that increased their daily activity faster during the post-fledging period. The researchers suggest that an active lifestyle might reflect that the youngsters are stronger, perhaps have a bolder personality, or are better skilled, which might in turn explain why these birds had better chances of survival later in life.
The team also discovered that the birds that were ‘too hasty’ or took too long to depart on their migration had lower chances of surviving than those birds that showed intermediate post-fledging durations – it seems that the case of the ‘happy medium’ is best for departing stork chicks. The researchers argued that the ‘hasty birds’ possibly did not acquire sufficient experience to prepare successfully for migration. In contrast, the birds that took too long to depart were also slower to increase their daily activity as they grew, possibly reflecting a delay in their development that left them unfit to face the risk of migration.
This study highlights that tracking is a powerful tool for capturing variation in the lifestyle of wild animals. These results clearly show that the differences that set successful youngsters apart from those that won't make the migration grade can manifest in early-life activity levels, serving to predict the fate of individuals as they depart on their first life-defining adventure.