Determined to keep its metro free of pigeon droppings, city transport authorities in Poland's capital have tapped the "killer" instinct of birds of prey to deal with this universal urban scourge.

"Just the presence of a bird of prey inside or at the entrance of a station is enough to act as a deterrent," says Piotr Zadworny, a bird breeder whom Warsaw subway authorities hired to solve their pigeon woes.

His two Harris hawks Draka and Mietek, a variety native to Central America, do the trick.

"Pigeons are the natural prey of hawks. They know very well that the appearance of a hawk means their death," Mr Zadworny says.

Before he began making his rounds with Draka and Mietek, the Sluzew metro station in southern Warsaw was plagued by hundreds of pigeons. Within just days, the number dwindled to a handful.

"I couldn't stand it anymore! The pigeons weren't afraid of anything, they would just fly into my kiosk and start eating the pastries," said the owner of a small shop inside the metro station.

With Draka, a female hawk from Mexico perched on his arm clad in a thick leather glove, Mr Zadworny chases away the pesky birds.

Most make their nests between cables running along the ceiling and walls. Pigeon droppings mar walls, cover the metro's granite floors and plop onto passengers' heads.

The chirps of baby pigeons coming from a space between two boards draw Draka's attention. As soon as an adult pigeon appears, she takes flight in pursuit. Mr Zadworny lets her briefly chase the bird, then calls her back.

They may scare pigeons, but are they allowed to kill them?

"Absolutely not! First, it would be contrary to a law which protects even pigeons. Secondly, it would be too dangerous, my birds could die - urban pigeons carry many diseases," says Mr Zadworny .

"They're worse than rats - it's a veritable epidemiological bomb," he adds.

His birds eat only chick meat that is frozen immediately upon slaughter to kill any harmful microbes. "Yes, it's brutal," he admits.

But with birds of prey worth between €10,000 to €30,000, the risk of disease must be kept to a minimum.

Draka works about five hours a day, before she shows signs of boredom. Her mate Mietek takes over then, Mr Zadworny says.

The cost of their services is a confidential matter. "It won't take long to recoup our investment," says Warsaw metro spokesman Krzysztof Malawko.

"The huge costs of cleaning the stations, the maintenance products and, above all, renovation work on the walls and ceilings will drop dramatically," he adds.

"We battled against the pigeons for years, without any success. We tried everything."

At wits end, metro authorities decided to seek advice from ornithologists.

They now plan to remove all the nests and cover the nooks and crannies with wire mesh to prevent pigeons from returning.