Daniel Haag-Wackernagel

Feral Pigeon Management

Regulation von Strassentaubenpopulationen

 

Feral Pigeon Management

 

Increase of Mortality

 
Populations increase by adding new individuals through birth and immigration and they decrease by subtracting individuals through death and emigration. The resources dictate whether populations increase, stabilize, or decrease. In many European cities governments try to solve the pigeons problem by killing as many pigeons as possible. Feral pigeons in most cases are killed by shooting, trapping and poisoning. Poisons which were used are the stupefying bait alpha-chloralose (Murton et al. 1972, Morris 1969, Vater 2000), HCN hydrogen cyanide (Geisthardt 1977, Dimigen 1986, Schuster et al. 1989, Vater 2000) and strychnine (Weber 1979).
Toxic perches (Rid-a-Bird) are treated with the contact toxicants Endrin and Fenthion. The dermal contact toxicants are picked up by the bird’s feet while sitting on the perch and the pigeons usually succumb in 24 hours (Weber 1979, Martin & Martin 1982).
Avitrol (4-Aminopyridin) is used as a chemical frightening agent to remove pest birds from a given location. Birds eating the treated bait will be affected in a manner that will artificially cause them to emit distress and alarm cries and visual displays used by their species. This will frighten the flock and cause them to leave the site (Berndt 1970, Weber 1979, Martin & Martin 1982, Avitrol 2000). Generally, the use of poisons to eliminate nuisance birds has many repugnant if not hazardous aspects, particularly in cities. It is opposed by the public and, as other methods of killing, simply makes room for more birds to move in and occupy the same territories.
Several studies could demonstrate that increasing the mortality has no long-lasting effect on the population size (Murton et al. 1972, Haag 1984, Kautz 1985, 1990, Kautz & Malecki 1990, Sol & Senar 1995, Schnitzler 1999). In Basel we reduced several flocks to 80% of their original number (Haag 1984). After a few weeks, the flocks reached again their earlier size or were even larger than before. The vacancies left by the removed individuals are immediately refilled by juvenile birds and provoke immigration from neighboring flocks. A breeding pair under optimal conditions can produce up to 12 young/year (Haag 1987). The adult mortality of about 11% (Haag 1990) is relatively low, so there is strong competition for feeding and breeding resources. Dead individuals are replaced by young pigeons within a few weeks. Killing adult pigeons in a feeding flock only favors young pigeons which would otherwise have no chance to survive. Killing pigeons therefore only rejuvenates the flock and has no long-term effect upon the population size. Murton et al. (1972) had to kill 9000 pigeons over 3 years in order to reduce a population of 2600 birds down to 1300 individuals. They concluded their excperience as follows: “Our specific recommendation is that money should not be wasted on expensive campaigns to kill pigeons in the docks. Instead, we recommend a concerted attempt to clean up spillage in the area, so preventing birds from gaining access to food sources.” Similar results were found by Kautz & Malecki (1990) in New York, Kösters et al. (1994) in Munich and Sol & Senar (1995) in Barcelona.
Several cities as Berlin, Stettin, Prague and Warshaw tried to control their feral pigeon populations by promotion of enemies as e.g. Falco peregrinus or Accipiter gentilis (Vater 2000). Till today no provable results could be reached with the recolonisation of birds of prey. A recolonisation is very difficult and expensive. Birds of prey have very large territories (e.g. Accipiter gentilis with territory sizes of around 3 km2/pair) which prevent the high densities necessary for an effective decimation of the feral pigeon population. For example, in a territory of 10 km2 only 2–3 pairs of Accipiter gentilis can survive (Vater 2000). Additionally birds of prey take the risk of lead intoxication by feeding on feral pigeons, which show high lead levels (De Ment & Chisolm 1986, De Ment 1987, Haag-Wackernagel et al. 1998).
The increase of mortality by killing can not be recommended as a long-term method to reduce a feral pigeon population because of compensation of the killed birds by compensatory natality, immigration and density dependent increase of natality (Kautz & Malecki 1990, Johnston & Janiga 1995). Additionally, killing adult pigeons causes starving of their nestlings due to the all-year breeding of feral pigeons. One third of Central European cities questioned by Vater (1999, 2000) use trapping and killing without having solved their feral pigeon problem. Even very high harvest rates (28–45 % of the population) did not lead to a long-term reduction of the feral pigeon population. In consequence, Cologne, Nurenberg and authorities from other cities decided to stop trying to solve their pigeon problem by killing.
 

Reduction of Natality

 
Several methods are used to suppress the reproduction of the feral pigeon. All methods used till now seem to be ineffective due to compensation by compensatory natality and immigration – if there could be achieved a provable effect anyway.
The collection of eggs and nestlings is simple but ineffective and therefore not recommended (Rösener 1999). As well, the surgical sterilization (vasectomia bilaterale) seems not to be an adaptable method since it is very expensive and it is difficult to sterilize enough males (Dinetti & Gallo-Orsi 1998).
The use of oral contraceptives was applied with different kinds of chemosterilants and hormones (overviews in Johnston & Janiga 1995, Haag-Wackernagel 1997c, Rösener 1999). Chemosterilants in contrast to hormones are very toxic and suspected to effect more by their toxicity than by a reduction of fertility (Haag-Wackernagel 1997c, Dinetti & Gallo-Orsi 1998, Rösener 1999).
 

Chemosterilants

 
In Germany the chemosterilant Busulfan (1,4-bis-methyl-sulfonyl-oxy-butan), a cytostatic, was used over years because of its total inhibition of spermatogenesis for about 6 months following a single dose. Hoerschelmann et al. (1981) could demonstrate that Busulfan has unacceptable negative effects. 30% of the treated pigeons died and mortality of young which were fed by treated adults increased respectively. The birds showed severe pathological symptoms. Heinzelmann et al. (1989) proved that the decrease of the feral pigeon population in Munich was less due to an inhibition of the gonades by Busulfan than to casualtities caused by a delayed immunosuppression. Salmonellosis, yersiniosis, and other possible infections have been provoked by the immunosuppression and aggravated the municipal hygienic situation. Additionally, negative effects to the urban food chain as accumulation in predatory animals are unclear. Subsequently the use of Busulfan was dropped in Germany.
Ornitrol (20,25-diazacholesterol dihydrochloride, SC-12937) is a hypocholesterolemic agent which blocks the synthesis of Cholesterol resulting in a block of egg-laying for several months (Elder 1964, Arbeiter et al. 1975). Becker (1966) and Murton et al. (1972) demonstrated problems with the high toxicity of Ornitrol. Treated pigeons showed severe symptoms of intoxication as e.g. paralysis of their legs, tremor and vomiting. The birds were lying on their breasts and could not fly up. Besides, it was difficult to feed the pigeons because they reacted in an idiosyncratic way to the treated food. Therefore Ornitrol is not recommended for the control of feral pigeons.
Nicarbazine (4, 4’ dinitrocarbanilide 2 hydroxy-4, 6 dimethylpyrimidine) was originally used for controlling coccidiosis in chickens. Elder (1964) found no lasting effect on pigeons even when they received an approximate LD50 dose. Martelli et al. (1993) contrarily found a good inhibition of fertility with 400 ppm and no toxical effects on the pigeons. Dell’Omo et al. (1998) studied the intake of nicarbazine treated food and found that only 55% of the marked pigeons were recorded again at the feeding place. Nicarbazine must further be discussed.
 

Hormones

 
In order to limit the reproduction of feral pigeons by temporary sterilization, different hormones were tested, but only few were used on a large scale (Elder 1964, Rösener 1999).
Progesterone is a naturally occuring hormone with an antigonadotropic effect when used in high doses of 2–10 mg/bird/day (Elder 1964, Neubauer 1994). The progesterone treated maize is sold under the name “Ornisteril” and was used in different Swiss cities such as Lausanne and Geneva (Noetzli 1991), Bolzano (Baldaccini et al. 1994) and Rennes (Schnitzler 1999). The effect of Ornisteril is lost as soon as its administration is stopped (Elder 1964). Consequently, the treatment of a population demands a regular daily intake of about 30 g of treated food. Experiences with Ornisteril from Lausanne (Noetzli 1991) and Rennes (Schnitzler 1999) did not result in decreased laying of eggs. On the contrary, Baldaccini et al. (1994) reported a population limitation for Bolzano.
A pill consisting of Levonorgestrel and 17alpha-ethinylestradiol mixed with methylmethacrylate was developped at the University of Hanover in order to control fertility of the feral pigeons (Hennig 1993, Neubauer 1994, Kummerfeld et al. 1996). The pill slowly desintegrates in the stomach of the pigeon, maintaining a continuous release of the substances over a prolonged period of time. It causes infertility by significant regression of the gonads. The pills are described to having a suitable form and size in order to guarantee spontaneous taking up. They show good compatibilty. The ongoing field studies will have to prove the applicability and effectiveness of this hormone pill.
Problems occuring with the use of sterilants and hormones as reproduction inhibitors are caused by the necessity of treating the whole feral pigeon population. If this is not possible, the untreated birds may show compensatory natality and a possible decrease of natality can be compensated by immigration of young from non-treated flocks. Dominant individuals can eat more of the treated food, provoking a negative selection on healthy and strong individuals. The effects on the urban food chain have not been studied with most of the substances used and generally it seems questionable to deposit toxic and biologically effective agents in the human environment.
 

Resource Management

 
A control program should aim to reduce or eliminate the resources which are the crucial factors for survival in the feral pigeon’s urban ecosystem. The most important resources for a feral pigeon are food for survival, growth, moult and reproduction and space for feeding, breeding, loafing and roosting.
 

Reduction of Breeding Sites

 
To breed successfully, a feral pigeon needs a safe and sheltered breeding site. Columba livia is a hollow-breeder, which normally prefers dark and high-situated breeding sites. Suitable nesting sites are rare in Basel. Most are closed by netting or protected by pigeon deterring systems, to prevent pollution and deterioration. When a pair wants to breed, it has two options: First, the birds can bear high densities at suitable breeding sites. This leads to poor, slum-like hygienic conditions causing density-dependent high losses of eggs and nestlings. This results in a low natality (Haag 1991). The second choice is to accept an uncharacteristic nesting place. Feral pigeons have developed an enourmous ability in accepting untypical breeding sites e.g. under bright neon signs or on trees (Haag 1984). The latter means to get over an inborn search image of the specific nesting place. Therefore, to our opinion it is not possible to reduce a pigeon population exclusively by limiting the number of breeding sites. However, breeding site restriction in combination with other control methods as demanded by Vater (1999) seems to be a promising strategy to solve the pigeon problem. Baldaccini & Mongini (1991) could effect a decrease of 40 % of the feral pigeon population of Bolzano by a combination of breeding site reduction, catches and progesterone treatment.
 

Reduction of Food

 
The urban environment demands a distinct adaptability, either by learning or by genetic adaptation. Whereas feral pigeons get along with almost every situation, with respect to food this is not possible. Therefore, the most promising strategy to reduce a feral pigeon population is to limit food supply. Urban food sources can be classified into three categories:
1. Deliberate feeding by pigeon friends: active feeding by pleasure or religious reasons.
2. Waste: refuse tips, discarded food
3. Processing: spills by grain stores, manufacturing, distribution
The most simple way is to remove food directly from places and streets by cleaning or by preventing spillage and refuse. Feeding prohibitions try to prevent feeding. In a questioning of 54 cities of Central Europe 53% state to have a feeding prohibition (Vater 1999), but in many of them it is not possible to enforce the decrees. Feeding prohibitions are very difficult to control and in many cities a strong public opposition exists against these feeding bans. In Basel, a prohibition of pigeon feeding, issued by the city authorities in 1978, failed due to public protest and simple ignorance (Haag-Wackernagel 1995).
Our city pigeons depend above all on persons who feed them (Haag 1984). These pigeon friends feed regularly at the same time at the same places. The pigeons appear on time, so they do not have to waste any time waiting and searching for food. They can save their energy for rearing their young. Because of the continuous availability of food, regulation of the numbers of pigeons by seasonal fluctuation and shortage of food in winter is eliminated or insignificant. Additionally, the pigeons no longer need to undertake dangerous flights into the countryside, thus evading their hereditary enemies such as Falco peregrinus and Accipiter gentilis. Consequently artificial feeding by man allows the pigeons to withdraw partly from their natural regulation system. This implies all the consequences that occur when a species evades its natural selection pressure.
The artificial feeding by man seems on the other hand to be a very important selective factor that favours birds with a high behavioural adaptability, i.e. pigeons can learn a special begging behaviour (Weber et al. 1994).
 

The Pigeon Action of Basel

 
In 1988 the “Pigeon Action of Basel“ was founded as an interdisciplinary project of the University of Basel, the government and the Society for the Protection of Animals of Basel. By means of large information campaigns we tried to influence pigeon enthusiasts to stop or to limit their feeding activities. Our intention was to reverse attitudes toward pigeon feeding, convincing the public, that feeding ultimately harms the pigeons and is counterproductive. We drastically demonstrated the negative effects of feeding e.g. by pamphlets and posters showing shocking pictures of baby pigeons infected by diseases and parasites. We tried to explain the complicated relationship between feeding and overcrowding and the density-dependent causes of the poor living conditions. Our answer to the argument that the feral pigeons would starve with the decline of food was the elimination of pigeons by Game-inspectors of Basel who regularly removed birds to adapt the population to the reduced food supply initiated by the public restriction of feeding. These yearly catches amounted to less than 20% of the population and did not, in our regard, by themselves affect the population size.
In some official buildings, we built supervised pigeon lofts to house a small but healthy population as an alternative for the pigeon friends between 1988 and 1991. In 1996 we removed 1656 kg faeces and 2596 eggs from our 8 lofts where about 500 feral pigeon are housed. These lofts prove that we do not intend to exterminate the pigeons at all, but we want to develop a small but healthy pigeon stock. After the start of our campaign, part of the food had vanished by limitation of its source or because the feeders gave up. Dead individuals seemed no longer to be replaced by young because competition for food had been intensified and allowed fewer of the inexperienced animals to feed successfully. We also suppose that the food shortage led to a decline in breeding success resulting in a smaller juvenile population to replace losses. The population size of 13 weekly counted control flocks decreased from 1988 to 1992 to about half indicating a total population reduction from about 20 000 pigeon to 10 000 within 50 months. With the decrease in population size, damage caused by the pigeons was reduced accordingly. This method of our pigeon population regulation by public education combined with catches can be adapted for use in other cities. Yet for each case, a careful scientific evaluation of the local situation is very important.
 

Conclusions

 
Table 1 gives a survey of the discussed pigeon control strategies trying to give a personal assessment of the different measures. Exclusions and pigeon deterring systems generally move pigeon concentrations from one building to another. These measures simply treat the symptoms and not the cause.
Our experiences in Basel demonstrated that a population of free-living birds can be influenced only after a serious analysis of the ecological situation. Increasing the mortality or reducing natality without trying to reduce food supply will not result in a long-term reduction of the population size. When a human activity such as feeding pigeons is the reason for an eco-pathological effect, a solution has to be realized with man and not with the concerned ecosystem alone.
 

Pigeon Control Strategy

Assessment

Increase of Mortality  
Killing
shooting, trapping, poisoning
not recommend
no long-lasting effect on population size, high natality and immigration allows quick replacement of losses, ethical problems (cruelty to animals, starving of nestlings)
Promotion of enemies
birds of prey (Falco peregrinus, Accipiter gentilis)
not recommended
no effect on population size, recolonisation of birds of prey very difficult (large territories), risk of lead intoxication

Reduction of Natality

 
Collection of eggs  and nestlings
Surgical sterilisation
not recommended
ineffective, compensation by compensatory natality and immigration, expensive
Oral contraceptives
chemosterilants (Busulfan, Ornitrol, Nicarbazine)
hormones (Progesterone, Levonorgestrel-ethinylestradiol-pill)
not recommended
expensive, difficult to dose, unpredictable effects on urban ecosystems and non target birds, chemosterilants hazardous by their toxicity

Reduction of Food

 
removing of food
feeding prohibition
public education
recommended
removing food is expensive, feeding prohibition is difficult to control, public education is the most efficient strategy

Table 1 Survey and assessment of pigeon control strategies.

 

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Publikationen

 

Haag-Wackernagel, D. (1997) Bestandesregulierung bei Strassentauben. In: H. H. Sambraus und A. Steiger (Hrsg.) Das Buch vom Tierschutz, Ferdinand Enke Verlag, Stuttgart, 770–779.

Haag-Wackernagel, D. (2003)Die Strassentaube: Geschichte – Probleme – Lösungen. Der Ornithologische Beobachter 100: 33–57.

Blechman, AD. (2006) Pigeons. The fascinating saga of the world's most revered and reviled bird. Grove Press, New York, 239 pp.

Mooallem, J. (2006). Pigeon Wars. The New York Times Magazine, Oct. 15: 53–59.

Weissenborn, S.R. (2007) Fliegende Ratten. Welt Online, 6.11.2007

Crossland, D. (2007). Spiegel Online International 6.11.2007

 
© Universität Basel Anatomisches Institut Research Group Integrative Biology Daniel Haag-Wackernagel