Low birth rates may spell black storks' disappearance from Estonia

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Black Stork in Estonia. Source: Tiit Jürisson

The black stork (Ciconia nigra), already a protected species in Estonia, is in danger of disappearing altogether, partly due to low rates of reproduction, ERR's science portal, Novaator, reports.

Far less common, and far more wary of humans, than the more familiar white storks (Ciconia ciconia), black storks are summer visitors to Estonia and do not winter here. They prefer to nest in wooded areas. Estonia is the limit of the species' northernmost extent in Europe.

Breeding is in April and May, with a typical clutch numbering two to five eggs.

A recent Environmental Board (Keskkonnaamet) study found high mercury levels in the bloodstream, which may be vectored via the food chain and may in turn affect both fertility rates and the health of such offspring as there are.

In 2020, only ten storklings went on to reach maturity, the board found. A subsequent study found surprisingly high levels of mercury, poisonous to birds and humans alike if in sufficient concentrations, in black storks found in Estonia.

Storks feed on fish which also have high levels of mercury, the result of pollutants which, while not at levels which would be hazardous to humans, given the storks dependence on fish, can pose a treat, though where this happens – black storks in Estonia primarily migrate to southeastern Europe in August and September – is not clear. Other bird life including birds of prey, such as eagle owls, have also been found to have unusually high levels of mercury in the system. Even more worryingly, eagle owls do not migrate.

Other issues include parasites and potential loss of habitat, though nesting sites are protected, and that protection is enforced – to the extent that, for instance, a planned wind-farm in Pärnu County has had to take a recently-found nest into consideration.

The original Novaator piece (in Estonian) is here.

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Editor: Andrew Whyte

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