Stone the crows?

The Indian house crow, described by scientists as an invasive pest, is living up to expectations in the Kenyan coastal city of Mombasa, grabbing food off tourists' plates and bullying other bird species out of town.

The crows are not indigenous to East Africa but there are lots of them in Mombasa.

When you look at the skies here, you will rarely see any other bird apart from the black and grey house crows.

As their full name suggests, their original home is India, but they often migrate, sometimes by stowing away on ships.

In East Africa though, researchers say they were introduced intentionally around 1890 in order to control rubbish.

In 1947 the house crow reached Kenya and since then its numbers have exploded, thanks to the growing human population and the accompanying mounds of garbage, which provide the birds' main source of food.

Nose for delicacies Years of poor rubbish management in Mombasa have meant dumping sites punctuate the landscape of residential areas. Crows are everywhere.

Unfortunately however, the crows do not eat just rubbish or leftovers - they also have a nose for delicacies.

They have become such a nuisance in eating places that some hotels have hired people specifically to chase crows away from diners.

"We have a lot of crows around, and our restaurant being an open restaurant, crows are a menace to the customers," says Donald Shipenzi, who manages a restaurant in a Mombasa beach resort.

"They jump into the restaurant, they climb on the food, then they do actually pass droppings on chairs, and this is actually a big problem for us."

Moving between rubbish dumps, human waste management sites and then restaurant tables has also made the birds a health hazard.

Scientists say there's a risk of them spreading a range of diseases to humans.

Driven out Environmentalists are also concerned about the impact on bird diversity in light of the crows' aggressive behaviour towards other bird species.

Conservationists say many other bird types, including indigenous species, are being driven out.

Smaller birds are not alone in being attacked. The crows have also targeted chickens, frustrating those who breed poultry at home with the intention of allowing their chickens to forage for food.

The aggressive crows compete for food with the chickens and attack and feed on their young.

Some breeders have tried unconventional measures to counter this, including by painting their chicks in bright colours - yellow, red, navy blue and green.

Agnes Kaloki, who has done this, says she thinks this tactic deceives the crows.

"They feed right here with the crows, which do not bother them until they become adult chickens," she says, with a tinge of pride.

So, can the menace of the crows be contained? It won't be easy.

A senior public health official in Mombasa has told me workers have trapped hundreds of birds in cages and killed thousands by suffocating them.

But he says the crows seem to be breeding faster than they can be killed.

Intelligent Between 1999 and 2005, A Rocha Kenya, a conservation and research organisation, used an avian poison called Starlicide to eliminate crows in Malindi, another town on the Kenyan coast.

Their records show they managed to reduce crows from hundreds to fewer than 50.

But the Kenya government later banned the importation of Starlicide and today the crows are back in Malindi in their thousands.

Trapping crows also has its difficulties: Experts say the crows are intelligent and can remember what traps look like and even the people who set them.

Conservationists argue that the best way to cut the crow population is to reduce the birds' access to food, first by trying to remove the rubbish, which provides their main food source.

The scale of the problem is substantial though.

The birds have spread as far north as Djibouti and as far south as Durban in South Africa.