The Swedish White Stork Reintroduction Program

"The goal of the program is to reintroduce the white stork as a natural part of the Swedish fauna. We consider that goal to be reached when there is a migrating population of 150 free-breeding pairs".

Background

The white stork was once very abundant in southernmost Sweden (mainly in Scania). As many as 5 000 pairs may have been breeding during the 18 th and early 19 th century. The decline of the species started in the middle of the 19 th century, coinciding with the onset of the dramatic change of the agricultural landscape. In 1917 the population was down to 34 breeding pairs. In the period 1927-1941 the population varied between 8 and 13 pairs. Finally, in 1954, the last breeding attempt was made, which however failed. Between 1955 and 1988 storks were observed annually, but no breeding attempts were made. The extensive changes in the agricultural landscape were the driving force behind the population decline. When wetlands were drained and became cultivated, streams straightened, and meadows and pasture turned into arable land or forest, the stork lost a great part of the habitat on which it depends. Today, only 5-10% of the wetland areas in Scania remain and this change has had dramatic consequences not only for the stork, but for many other species depending on wetlands. During the last decades, however, the work of restoring wetlands has intensified. In addition to the benefit of biodiversity, wetlands limit the amount of nutrition leakage from fields. The possibilities to reconstruct the “landscape of the stork” and thereby bringing back the stork, is a significant driving force in many ongoing restoration projects.

 

The reintroduction program

In 1989 a reintroduction program started through a joint effort of the Swedish Society for Nature Conservation (SSNC) in Scania and the Scanian Ornithological Society (SkOF) in the same region. An aviary was built in the area where the species prevailed longest, and the first pair was released. Since then, pairs have been released yearly and more aviaries have been built. In 2011 the free breeding population reached 29 pairs and 55 juveniles became fledged. The majority of these birds have been released from captivity, but during the 20-year-period of the program, approximately 15 wild storks have immigrated and started to breed with “project storks”. This shows the nature of the stork – it is a social animal that may breed in colonies. Therefore, the possibilities to spread the stork into desired areas hinges on the establishment of aviaries where captive birds are held as “magnets”. Released pairs are then likely to start breeding in the close surroundings.

The requirement for building an aviary is that the locality is surrounded by sufficient areas of suitable habitat for the storks. There may be no powerlines in the near surrounding that may cause death or injure the storks. We believe that for a maximum chance of successful establishment of a local population, aviaries should be located where storks have been breeding before. This long-term work of spreading the storks across Scania has resulted in free-breeding pairs at about 10 localities.

Apart from the local populations connected to the aviaries, pairs have spontaneously built nests and bred at about 5 places. The negative consequence of colony breeders is that it may generate fairly high densities at some localities. In 2008 such negative “density effect” was observed at some of the largest colonies, with reduced breeding success due to local crowding and interactions between couples. The size of the colonies was therefore reduced by catching some of the couples and letting them breed in avaiaries instead. Consequently, the breeding success of increased to a normal level.

The breeding program works like this: Immature storks are kept separately in a large aviary, and when they have reached sexual maturity (at the age of 2 to 5 years - usually 3 or 4) they are moved to the “marriage agency” to form pairs. Before the onset of the breeding season, the group is separated so that birds that are relatives are not able to form pair. When a pair has been breeding successfully at least once, it is qualified for release. It will then be translocated to a suitable aviary and released the subsequent spring. For many years the offspring of the free-breeding pairs was collected and put in captivity. This was done to maximize the efficiency of the breeding program. However, now the efficiency of the captive breeding has reached such levels that we can release more than 50 juveniles each year in addition to the onces born in freedom!

 

Migration

The white stork is a long-distance migrating bird, wintering in the Sub-Saharan Africa. The European population is divided in an easterly and a westerly population, migrating along different routes. It is believed that the former stork population in Sweden migrated along the eastern route, together with e.g., Danish, Polish and East-European storks. Yet, the migration behavior is flexible and relies on several factors (social interactions, geography, topography, weather and genetics).

 

The adult storks released to breed in Scania are not migratory and winter close to the breeding sites. This is mainly due to the "manipulation" of the storks (kept in captivity and fed by human) However, the offspring born in freedom migrate under favorable conditions. This means that the number needs to be above some critical level, and/or that wild, migrating storks that visit Sweden in the summer, takes the step to move south and “show” the first-year storks the way towards the continent. In 2011 more than 100 juveniles migrated, which is the largest number ever recorded and probably on a level similar to the 19 th century.

 

Interestingly, the migrating storks use both the eastern and the western route (in contrast to the former population that mainly moved along the eastern route). This shows the flexibility of the migration behavior but also reflect the altered distribution of breeding storks at the continent. Juveniles in migrating populations normally migrate some week before the parental birds. This explain how siblings may migrate in different directions depending on who they migrate with and when migration starts. To learn more about the migration of the Swedish storks, two juveniles were equipped with satellite transmitters, thereby allowing us to follow them en route. Study the individual movements of theese storks under the link “GPS-storkar”.

 

Organization and finances

The project is run with the two founding societies, the Swedish Society for Nature Conservation (SSNC) in Scania and the Scanian Ornithological Society (SkOF), as principals. They have established a managing group with three members from each society. A project leader is in charge of the operations. Besides some 25 people contribute on a voluntary basis to the handling of some of the aviaries. Major contributor to the project is the Swedish Environmental Protection Agency . The Animal Protection Fund of Lund has contributed to the acquisition and operation of two GPS- transmitters.