It may seem silly: do pigeons represent a dangerous nuisance? The answer is: yes. Their dejections ruin monuments and statues. But it is more than that: they carry extremely virulent germs.
Since Medieval Ages, the pigeons started to inhabit the European cities, where they easily found food in wastes discarded by humans. They started to nest on roofs, bridges, walls, or statues. It makes sense: its wild ancestor, the rock pigeon (Columba livia) nests on rocks.
Today, pigeons are so well integrated in the urban landscape that we ignore them. And their whitish and acid dejections cover, in some city centers, from the edge of the windows to the statues. Pigeons brought their services to the humankind: 18,000 doves were used during the WWII for transmitting secret messages without being detected by the enemies.
15 years ago, Paris had a population of 80,000 pigeons. That was less than half, compared to the dove population of the '70s. A female pigeon lays 2 eggs on a clutch, and 3-5 clutches annually. Many chicks die before reaching maturity, otherwise their population would boom. In Venice, the pigeon dejections have been harming the palaces and statues of the cities more than the pollution produced by the nearby industrial zone of Marghera.
Because of the germs contained by the pigeon dejections, some called these birds "the rats of the sky". Pigeons spread bird pest, a disease that causes havoc not only in wild birds, but also in the poultry farms.
Some of these germs are harmful for the people also, and an example is the bacteria causing ornithosis, translated to lung issues in humans, or the fungus Cryptococcus, which inflicts deadly lung infections, meningitis and meningo-encephalitis in persons with suppressed immune system, like in AIDS.
An odd case of bird interrelation is signaled in New York. The monk parakeet (Myiopsitta monachus) originated in Argentina. Starting with the '60s, many of them escaped the pet trade and formed colonies in 15 US states. This bird is a temperate-zone parrot, standing New York City winter. Within Green-Wood Cemetery (Brooklyn), the keepers initially attempted to destroy the unsightly nests at the entrance gate, but they no longer do so, because the presence of the parrots has decreased the number of pigeons nesting within it. A comparative chemical analysis showed that while pigeon feces destroyed the brownstone structures, monk parakeet feces had no effect. The parrots were preserving this historic structure against the pigeon feces...