How do fledgling migratory birds find their way to Africa during their first migratory trip, and do juvenile birds manage to migrate and survive if they are translocated? These were the main questions being addressed in the research, given that translocating birds is a method often used by conservationists to strengthen threatened populations. Translocation is particularly successful with regard to sedentary birds (non-migratory birds). 'Very little was known, however, about this regarding migratory birds', says Vansteelant.
Cains and Abels
Dr Bernd Meyburg (BirdLife Germany, NABU) is active in attempting to counter the population decline of lesser spotted eagles (Clanga pomarina) in Germany. Lesser spotted eagles lay clutches of two eggs, but usually only the eldest chick survives. The second chick is killed by its elder brother or sister, often even if there is enough food. This behaviour, which is also exhibited by other large eagle species, is referred to as cainism, after the biblical story about Cain and Abel.
Meyburg's team saves 'Abel' eagles from wild eagle nests in order to strengthen the German population. In 2009, very few lesser spotted eagles managed to breed in Germany, so the team took a dozen Abels from the Latvian population, which was much larger. These were translocated some 940 kilometres to Germany, where they were raised and released into the wild. Vansteelant: 'To discover whether these birds were able to find their way during the first autumn migration, we tracked twelve Abels (translocated from Latvia to Germany), eight Cains (raised in the wild in Germany) and five mature German lesser spotted eagles with satellite tags.'
Setting off too early
Most lesser spotted eagles who manage to complete the migratory trip to Africa avoid the Mediterranean Sea by flying to the south-east. They fly in the direction of Asia, over the Bosphorus, via Turkey, Syria, Lebanon, Israel and Egypt, further into Africa. Eagles from all over Europe converge along this route and every autumn the skies above Istanbul, Tel Aviv and the Suez Canal darken for a few days due to the large numbers of birds. However, the lesser spotted eagle study showed that most of the translocated eagles – as well as some of the juvenile eagles that grew up in the wild in Germany – were unable to find this route.
The Cains left the breeding grounds at about the same time as the mature birds, while the Abels had already left six days earlier. Vansteelant: 'Seven of the eight Cains managed to reach Africa. The Abels, however, left too early to be able to join more experienced birds en route. They instinctively flew south and thus arrived on the northern coast of the Mediterranean Sea. Many of these birds died while trying to find an overland route, probably starving to death. Three of the birds which were translocated from Latvia managed to find the route via the Bosphorus and Suez Canal. Two of these had begun migrating at the same time as the German eagles.'
Despite the fact that many of the Abels did not manage to reach Africa, the researchers still feel that it helps to save Abels from nests. 'Out of all the juveniles, after five years one of the Cains and one of the Abels still returned to Europe to breed. Every eagle counts in this shrinking population', says Vansteelant.
Meyburg, B.-U., Bergmanis, U., Langgemach, T., Graszynski, K., Hinz, A., Börner, I., Meyburg, C. and Vansteelant,W. M. G. (2017). Orientation of native versus translocated juvenile lesser spotted eagles (Clanga pomarina) on the first autumn migration. J. Exp. Biol. 220, 2765-2776.