Storks' nest switch jolts scientists

By Alex Kirby
BBC News Online environment correspondent in north-east Poland

Storks in Poland have set scientists an intriguing puzzle: to understand why they are changing their nesting habits.

Storks are moving from this...

Traditionally the birds used to nest on the roofs of buildings and in trees.

But over the last 25 years they have begun developing a marked preference for building their nests on electricity pylons instead.

Ornithologists think the change may be linked to greater breeding success.

Poland is one of the European strongholds of the white stork, with about a quarter of the continent's total population. The birds spend the summer there and winter in Africa.

In 1985 there were about 30,000 breeding pairs, and by 1995 around 40,000. The estimate for 2002 is 44,000 pairs.

Moving home

Dr Przemek Chylarecki, a biologist, is president of the Polish Society for the Protection of Birds (Otop).

...to this, with the power companies' help

He told BBC News Online: "In 1974 4% of Polish storks built their nests on electricity pylons, and by 1995 that had risen to 37%.

"We don't know why they prefer the pylons, but we suspect they may be breeding more successfully there - there is some slight evidence to support that. It does seem to be an evolutionary change."

The electricity companies have provided custom-built circular metal platforms that fit on top of the pylons, to accommodate the nests.

They began doing so only after they identified the storks' growing preference for the pylons, concluding it was cheaper to provide them with the platforms than to repair the frequent interruptions they caused to the power supply.

To have a stork nesting on your house is seen as a sign of God's blessing by the Poles, although some nests have been heavy enough to damage the houses beneath.

Weak to the wall

The birds are associated with fertility, as they are in many countries. They usually hatch a clutch of two or three eggs, and occasionally up to six.

Poland's stork numbers are on the up

In dry years they often throw the weakest chick from the nest, and one farmer said this year a pair on his land had thrown out two eggs, something unheard-of in his experience.

Despite the storks' current success, some ornithologists fear for their future after Poland joins the European Union in May 2004.

Pawel Sidlo of Otop told BBC News Online: "From examining their pellets we know the storks live mainly on rodents, reptiles, amphibians and insects, and in very wet areas on fish and water-snails as well. They'll even eat other birds' chicks.

Turning back the clock

"Small mixed farms are best for providing a diet like that, low in chemical use and relying on traditional land drainage methods.

"If many of those farms vanish when we become part of the EU's common agricultural policy, the storks could starve."

But a veterinary surgeon, Dr Janusz Gajewski, told BBC News Online: "The drainage system round here hasn't been well maintained, and we're beginning to see the regeneration of the river valleys.

"If nothing's done in the next decade the rivers could revert to the state they were in 50 years ago, so I'm not worried for the storks."