En analyse af gylp fra hvid stork Ciconia ciconia i Danmark

Summary
Food contents in pellets
of White Stork Ciconia
ciconia in Denmark
1976-2003
Natural History Museum in Aarhus has
investigated 362 pellets from Danish
White Stork, collected during the period
1976-2003 by Hans Skov, especially from
Southern Jutland (several locations),
NW-Jutland (Vesløs, Vang) and NE-
Jutland (Vegger) . All of the 362 pellets
were fragmented and analyzed for the
content of vertebrate bones, earthworm
bristles, sand and pebbles, whereas a
representative sample, consisting of 123
pellets from eight main locations, was
further analyzed for remains of insects
and other small organisms .
The analysis showed that earthworms,
estimated from both bristle and sand
content, constituted a very large
amount of the remains in the pellets,
calculated as fresh weight on average
approx . 77% of the biomass in the
analyzed sample . Mammals and birds,
especially mole (Talpa europaea), field
vole and common vole (Microtus agrestis,
M. arvalis), were few in numbers, but
averaged approx . 16% of the calculated
biomass, while a large number of insects
(and a few mollusks), especially Carabidae,
Dytiscidae, Silphidae, Geotrupidae,
Aschnidae, Acrididae and Tipulidae, averaged
only approx . 7% of the calculated
biomass (fig . 12, p . 19) .
Amphibians, often considered important
food items of the White Stork, could
hardly be traced, probably because their
bones are dissolved by the stomach acid
of the stork . However, the remains of
very small insects, especially numerous
in pellets from Vesløs, indicate that such
storks have consumed amphibians,
assuming that the small insects were
primarily ingested by these amphibians .
In case a stork predominantly feeds on
amphibians fewer and smaller pellets
will be produced, and urea and the microscopic
bristles will to a higher extent
be excreted by the vent (Cloaca) . Sand,
primarily from digested earthworms, will
still be disgorged and thus occur in a
higher ratio compared to bristles than in
other pellets . This was actually the case
in Vesløs, but still it is difficult to estimate
the amphibian biomass on this basis .
Given the very irregular sampling
practices it is difficult to draw definitive
conclusions, but compared to previous
Danish studies (Skovgaard, 1920) it seems
evident that the White Storks used
to consume more large insects (Carabus,
Dytiscus, Geotrupes) than recently, when
predominantly medium sized insects
(Pterostichus, Silpha) were taken, presumably
because several of the larger
insect species have declined or become
locally extinct . Also, amphibians and
small mammals have become scarce
in the cultivated Danish landscape and
the large proportion of earthworms
in the analyzed pellets indicate that
earthworms gradually became the main
food ressource of the Danish storks .
Recent studies in Germany and Poland
5
(Dziewiaty & Schulz, 1998; Tryjanowski
& al ., 2006) suggest that these populations
to a much higher degree prey on
mammals, amphibians or insects, as the
habitats still provide these food items .
Following several decades of decline
the White Stork ceased breeding in Denmark
around the turn of the millennium .
One of the last nesting sites was Vesløs,
still a good habitat, but now extremely
isolated from the closest breeding
population . The last breeding site was
Ribe in SW-Jutland, where the habitat
was gradually spoiled, but the storks
survived from fishing in trout farms .
However, since then a few breeding
attempts have been made on Zealand
by semi-domesticated storks originating
from Sweden . Moreover, many White
Storks still breed in NW-Germany, close
to the Danish border, thus making it
likely that the species could return as a
breeding species if habitat conditions
improve .