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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 100 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 18th and early 19th
century. The decline of the species started in the middle of the 19th
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 almost certainly the driving force behind
the population decline. When wetlands were drained and became cultivated,
streams straightened, and meadows and pasture turned into forest and arable
land, 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 another six
aviaries have been built. In 2009 the free breeding population reached 37
pairs and 19 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 usually breeds 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. We believe that for a maximum chance of successful
establishment of a local population, it 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. Recently we have seen such a “density effect” with
reduced breeding success due to local crowding at some of the largest
colonies. That will happen in nature, and act as a stabilizing force to
limit the increase in number.
We have (autumn 2009) 172
storks in captivity distributed between five aviaries across Scania. In
addition, we collaborate with, and keep storks in two zoos in the south of
Sweden. 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. In most years, the
offspring of the free-breeding pairs has been collected and put in
captivity. This was done to maximize the efficiency of the breeding program.
However, in some years when a large number of offspring have become fledged,
the juveniles have been “permitted” to migrate. In some years this has
happened, and in other years it has not. In the latter case, non-migrating
young storks are caught and put in captivity.
The
genetic origin of the storks
The official Swedish
reintroduction program started with a group of storks imported from
Switzerland in 1979. It turned out that the Swiss project has imported their
birds from Algeria in the 1950s. Therefore, the storks in the Swedish
reintroduction program have mainly been of Algerian descent. However, there
have also been more than 10 spontaneously immigrating founding birds after
the reintroduction program started, and these birds have been considered
native to northern Europe. These birds have mostly bred together with
project storks, and have left quite a few descendants. During the period
1989 to 2005 almost one third of the breeding events were by pairs where at
least one of the partners had some native ancestry. In the period 2006-2009
on average two thirds of the pairs had some native ancestry. In
reintroduction programs it is desirable to use founders that are genetically
close to the extinct population. Hence, since Denmark, our closest neighbor,
since year 2008 no longer have any breeding pairs, we turned towards Poland.
In 2004 we imported 40 storks
and in 2009, 18 storks.
Furthermore,
a study shown that
the reproductive success of the storks in Scania is almost twice as high for
the storks of native ancestry as for those of Algerian ancestry. Depending
on the lower reproductive success of the Algerian storks, and on the fact
that they are an undesirable source population, a decision was made to
discontinue their reproduction altogether. However, the free breeding pairs
of pure (or mostly) Algerian origin are used as foster parents for eggs and
chicks from the imported Polish storks. We started this work in a small scale
during 2006, and in 2009 10 Algerian pairs had their eggs changed into
Polish ones.
In practice, we let both
the free Algerian and the captive Polish storks complete laying of their
clutches. Then we identify pairs with approximately matching laying dates,
and remove the eggs from the Polish pairs and put them into the Algerian
pairs’ nests. The eggs of the Algerian storks are destroyed. In most cases
the Polish pairs lay a replacement clutch after approximately two weeks,
which they are allowed to keep and raise themselves. The Algerian pairs
receive between 2-4 eggs each. Even though storks may lay up to 6 eggs, they
rarely manage to raise more than 2-3 offspring. In order to be able to keep
track of ancestry in the population we only put eggs from one Polish pair in
each nest.
The outcome of this
manipulation has varied and with only a few years experience it is difficult
to draw any general conclusions. However, in terms of relative breeding
success, the foster pairs seem to be almost as good parents as the
non-manipulated pairs, and in that way, despite their inappropriate genetic
origin, the Algerian pairs contribute to the success of the reintroduction
program.
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 both genetics and social interactions.
The storks released
within the breeding program 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 may
migrate under favorable conditions. This has happened only a couple of times
during the 20-year-history of the project. A study has found that Algerian
storks are less likely to migrate than birds with native ancestry. Storks
with some native ancestry were approximately four times as likely to
migrate, as birds of pure Algerian ancestry. Other studies (not on the
Swedish storks) have focused on the importance of social interactions in the
migration behavior. They found that juveniles migrate in the same direction
as the storks they encounter en route. Since the storks migrate in group,
some storks will initiate migration. Such birds can be visitors having spent
the summer in northern Sweden and migrating south across Scania. It can also
be juveniles born in Scania with a strong urge to migrate. If wild storks
are present in Scania during the time period when migration takes place
(August-September), the chance of successful migration increase. A large
number of juveniles will probably have a similar effect. Migration did
happen already in the 1990-ies, when groups of pure Algerian storks flew
south together with wild visitors triggering the young storks’ urge to
migrate. After that, there have been years when no migration has happened
and the juveniles have stayed in Scania. In 2007, at least 9 out of 40
juveniles migrated and in 2008 the number was at least 8 out of 21. Since
some storks may migrate unseen, the real number might be higher.
Interestingly, however, the storks have been migrating along both migration
routes (i.e., the westerly via Gibraltar and the easterly via Bosporus) as
shown from two reports from Spain and Turkey. Also, offspring from the same
pairs have been migrating both towards south-east and south-west. To learn
more about the migration of the Swedish storks, we are planning to equip
some juveniles with satellite transmitters, thereby allowing us to follow
them en route.
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 10 people
contribute on a voluntary basis to the handling of some of the aviaries.
Major contributors to the
project are
Region Skåne (the county council of Scania), the
Swedish Environmental Protection Agency and the
World Wildlife Fund (WWF). Other contributor´s are SSNC in Scania, SkOF,
Alvin´s fund and a group of so-called godparents. The Animal Protection Fund
of Lund has contributed to the acquisition and operation of two GPS-
transmitters.
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