Feral Pigeons (Columba livia)
Feral pigeons (Columba livia), also called city doves, city pigeons or street pigeons, are derived from domestic pigeons that have returned to the wild.  The domestic pigeon was originally bred from the wild Rock Pigeon, which naturally inhabits sea-cliffs and mountains.  All three types readily interbreed. Feral pigeons find the ledges of buildings to be a substitute for sea cliffs, and have become adapted to urban life and are abundant in towns and cities throughout much of the world. 
Cities famous for pigeons
Many city squares are famous for their large pigeon populations, for example, the Piazza San Marco in Venice, and Trafalgar Square in London. For many years, the pigeons in Trafalgar Square were considered a tourist attraction, with street vendors selling packets of seeds for visitors to feed the pigeons. The feeding of the Trafalgar Square pigeons was controversially banned  in 2003 by London mayor Ken Livingstone. However, activist groups such as Save the Trafalgar Square Pigeons  flouted the ban, feeding the pigeons from a small part of the square that is under the control of Westminster City Council, not the mayor. The organisation has since come to an agreement to feed the pigeons only once a day, at 7:30 a.m. 
Pigeons breed when the food supply is good—for wild rock doves this might be seasonally so they usually breed once a year. In the wild they are often found in pairs in the breeding season but usually they are gregarious.  In the urban environment, because of their year-round food supply, feral pigeons will breed continuously, laying eggs up to six times a year.
Feral pigeons can be seen eating grass seeds and berries in parks and gardens in the spring, but there are plentiful sources throughout the year from scavenging (e.g., dropped fast-food cartons) and they will also take insects and spiders. Further food is also usually available from the disposing of stale bread in parks by restaurants and supermarkets, from tourists buying and distributing birdseed, etc. Pigeons tend to congregate in large, often thick flocks when going for discarded food, and many have been observed flying skilfully around trees, buildings, telephone poles and cables, and even moving traffic just to reach it.
As a result of the continuous food supply, pigeon courtship rituals can be observed in urban parks at any time of the year. Males on the ground initially puff up feathers at the nape of the neck to increase their apparent size and thereby impress or attract attention, then they single out a female in the vicinity and approach at a rapid walk while emitting repetitive quiet notes, often bowing and turning as they approach. Initially, females invariably walk away or fly short distances, the males follow them at each stage. Persistence by the male will usually eventually cause the female to tolerate his proximity, at which point he will continue the bowing motion and very often turn full- or half-pirouettes in front of the female. Subsequent mating when observed is very brief with the male flapping his wings to maintain balance on the female. Sometimes the male and female beaks are locked together.
Nests are rudimentary as for the wild doves and pigeons. Favourite nesting areas are in damaged property. Mass nesting is common with dozens of birds sharing a building. Loose tiles and broken windows give pigeons access; they are remarkably good at spotting when new access points become available, for example after strong winds cause property damage. Nests and droppings will quickly make a mess of any nesting area. Pigeons are particularly fond of roof spaces, many of which accommodate water tanks, though they frequently seem to fall into the tanks and drown. Any water tank or cistern in a roof space needs to have a secure lid for this reason. The popularity of a nesting area seems little affected if pigeons die or are killed there; corpses are seen among live birds, who seem unconcerned.
On undamaged property the gutters, window air conditioners (especially empty air conditioner containment boxes), chimney pots and external ledges will be used as nesting sites. Many building owners attempt to limit roosting by using bird control spikes and netting to cover ledges and resting places on the facades of buildings. These probably have little effect on the size of pigeon populations, but can help to reduce the accumulation of droppings on and around an individual building.
Only the larger and more wary Common Wood Pigeon (which often shares the same territory and food supply) will build a tree nest; for some reason it prefers trees close to roads.
Wendell Levi in his book The Pigeon describes the crowing (cooing) in pigeons as mostly being associated with strutting and fighting in cock (male) birds. Hens (females) will coo, but this is noticeably less guttural than the cock birds. Cooing is also more frequent at mating and nesting time between pairs. Both parents share the incubation of their eggs.
Feral pigeons often only have small populations within cities. For example, the breeding population of feral pigeons in Sheffield, England, has been estimated at only 12,130 individuals.  Despite this, feral pigeons usually reach their highest densities in the central portions of cities, so they are frequently encountered by people, which leads to conflict.
One of the difficulties of controlling pigeon populations is the common practice of feeding them, as here in New York. Feral pigeons are often considered a pest or even as vermin, owing to concerns that they spread disease  and are much maligned in the media for transmitting bird flu, but it has been shown pigeons do not carry the deadly H5N1 strain.   Also concerns of them damaging property, causing pollution with their excrement, and driving out other bird species. Some also consider pigeons an invasive species. 
While pest exterminators use poison, hawks and nets have also been employed at ground level to control urban pigeon populations, though this generally achieves only a limited, temporary effect.
Long-term reduction of feral pigeon populations can be achieved by restricting food supply, which in turn involves legislation and litter (garbage) control. Some cities have deliberately established favorable nesting places for pigeons – nesting places that can easily be reached by city workers who regularly remove eggs, thereby limiting their reproductive success. 
Peregrine Falcons which are also originally cliff dwellers have also adapted to the big cities, living on the window ledges of skyscrapers and often feeding exclusively on Rock Pigeons.  Some cities actively encourage this through falcon breeding programs. Projects include Unibase Falcon project and the Victorian Peregrine Project.
Larger birds of prey occasionally take advantage of this population as well. In New York City, the abundance of pigeons (and other vermin) has created such a conducive environment for predators that the Red-Tailed Hawk has begun to return in very small numbers, the most famous of which is Pale Male.
Due to their non-selective nature, most avian poisons have been banned. In the United States market only 4-aminopyridine (Avitrol) and DRC-1339 remained registered by EPA. DRC-1339 is limited to USDA use only while 4-AP is a restricted use pesticide, for use only by licensed applicators.
The use of poisons has been proven to be fairly ineffective, however, as pigeons can breed very quickly — up to six times a year — and their numbers are determined by how much food is available; that is, they breed more often when more food is provided to them. When pigeons are poisoned, surviving birds do not leave the area. On the contrary, they are left with more food per bird than before. This attracts pigeons from outside areas as well as encouraging more breeding, and populations are re-established quickly.  An additional problem with poisoning is that it also kills pigeon predators. Due to this, in cities with Peregrine Falcon programs it is typically illegal to poison pigeons. 
Reducing food supply
A more effective tactic to reduce the number of feral pigeons is deprivation.   Cities around the world have discovered that not feeding their local birds results in a safe population decrease in only a few years. Pigeons, however, will still pick at garbage bags containing discarded food or at leftovers carelessly dropped on the ground. Pigeon feeding is banned in parts of Venice, Italy. 
In 1998, in response to conservation groups and the public interest, the National Wildlife Research Center (NWRC), a USDA/APHIS laboratory in Fort Collins, Colorado, started work on nicarbazin, a promising compound for avian contraception. Originally developed for use in resident Canada geese, nicarbazin was introduced for use as a contraceptive for feral pigeons in 2007.
The active ingredient, nicarbazin, interferes with the viability of eggs by binding the ZP-3 sperm receptor site in the egg.  This unique contraceptive action is non-hormonal and fully reversible. 
Registered by the EPA as a pesticide (EPA Reg. No. 80224-1), "OvoControl P", brand of nicarbazin, is increasingly used in urban areas and industrial sites to control pigeon populations. Safe and humane, the new technology is environmentally benign  and does not represent a secondary toxicity hazard to raptors or scavengers. 
Avian contraception has the support of a range of animal welfare groups including the Humane Society of the United States(HSUS), the American Society for the Prevention of Cruelty to Animals (ASPCA) and People for the Ethical Treatment of Animals (PETA).
USDA continues to develop wildlife contraceptives, including the recently registered Gonadotropin-releasing hormone (GnRH) for deer and Diazacon for birds and small mammals. Further development and registrations of Porcine Zona Pellucida or PZP, another immunocontraceptive targeted for feral horses, is supported by HSUS. The new field of wildlife contraceptives is developing rapidly and promises a bright future for the safe, effective and humane management of animal populations.
Dummy egg nesting
Dummy egg nesting programs have been tested in some cities with mixed results. Nest or coop structures are erected and the eggs are removed and replaced with dummy eggs. The eggs are then disposed of to prevent the pigeons breeding. Such structures are being used in New York City and also the Melbourne city centre by the Melbourne City Council at Batman Park   The loft used in Melbourne is on stilts, with a cage door allowing access from beneath for accessing structure at night when the pigeons are asleep.
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