Pigeons are prolific breeders
Pigeons are family birds. They form monogamous pairs, and together can raise up to 6 clutches of 2 eggs each year under good conditions. And cities generally provide such good conditions, with plenty of food all year round and roosting/nesting sites in buildings. Not surprisingly, pigeon colonies can include large numbers of birds. Mostly they do not move far from where they were born, although if the colony becomes crowded, young birds may join neighbouring colonies that are less crowded.
SA Health has a heading on its web page (1)1: “Feral pigeons can be health pests”, although it admits that pigeons are more likely to be a nuisance than a health risk. A thorough search of reports of disease caught from pigeons showed that there were 176 cases between 1941 and 2003. The researchers concluded that the health risks of pigeons to humans is very low, even for people whose occupations bring them in close contact with nesting sites (2).
However, the accumulation of faeces from large numbers of birds can be unpleasant, and can cause damage to buildings and monuments. Overpopulation and competition for resources is also not great for the health of the pigeons. So what can be done?
Lethal control
SA Health mentions non-lethal methods of controlling resources – restrict access to food and nesting/roosting sites, for example, by sealing gaps in rooves, installing spikes on ledges, etc. However, while useful, such restrictions are difficult to achieve over a large area.
SA Health then mentions lethal methods, such as the use of alpha-choralose mixed with food. The birds become comatose and can be picked up by licensed pest controllers for euthanasia. While this is not a painful toxin, it is also not a long-term solution for pigeon control. As SA Health states in relation to lethal control methods: “… this is usually only a short term solution as removed pigeons are quickly replaced by juvenile birds and the flock can become larger than the original size.”
The same point is also made by researchers (3, p. 226):
“… for monogamous species with high mortality rates and high productivity, such as feral pigeons, culling is likely to be less effective than the reduction of reproductive potential for controlling population. Actually, even though culling has been widely applied to feral pigeon populations in several cities in the past and is still used in several contexts, no scientific study has demonstrated the efficacy of this approach in significantly affecting population size. … Moreover, massive killing of pigeons is difficult to accept by many citizens, which determines further problems of ethical nature.“
It is well known that killing a few birds with a high reproductive rate is futile. So why do it?
Recent media reports in Adelaide in October described a shooter in suburban streets killing pigeons (4). Not only is shooting a few pigeons totally ineffective as population control, at this time of year birds also have dependent young who will suffer and die when parents are killed. So why are there no regulations to protect the welfare of pigeons?
Controlling breeding – egg removal
One option for limiting breeding is to set up pigeon houses as nesting sites and then remove the eggs. While apparently benign, this strategy has unforeseen side effects. Five pigeon houses were studied, four with egg removal and one with no interference (5). With egg removal, females laid eggs every 4 weeks, compared to every 11 weeks without egg removal. With egg removal, the percentage of yolk in the egg was significantly lower, showing that there was a high energy cost to females producing extra eggs. This cost could lead to poor condition in females, and greater susceptibility to disease and parasites.
Controlling breeding – reduced egg hatchability
The chemical nicarbazin is commercially available in Australia as OvoControl®P. Corn kernals treated with the chemical can be automatically scattered from a hopper early in the morning, at a rate of 10g for each bird in the colony. The drug does not affect the adult birds and does not affect fertility, but prevents the development of the egg. The egg yolk wall becomes permeable, so the yolk mixes with the albumin and prevents the embryo from developing (6).
This method of population control has been used in a number of countries, including Spain, Italy, the USA, Belgium, Portugal and Hungary (6). The table below describes investigations in some of these countries.
(6) | 24 towns and cities in Catalonia, Spain, 8 years | Steep decline in the number of pigeons in first 2 years, steady decline thereafter. Average 12% a year over 8 years. |
(7) | Barcelona, Spain, 34 colonies over 3 years | After 1 year, treated colonies declined by 22%, untreated colonies increased by 13%. Treated colonies declined by 55% after 3 years. |
(8) | Genoa, Italy, 4 feeding stations, 8 years | First 4 years showed declines of ~40%, further declines to 65% of original population after 8 years. Decline not seen in control station. |
(9) | Italian cities and towns, 552 colonies, data over 17 years | Colonies decreased from 28% – 71% from the first treatment, contols stayed the same or increased. |
(10) | San Diego, USA, 1 year | Treated colony declined by 53%, the untreated colony stayed the same. |
American researchers describe the use of nicarbazin as follows (11, p. 405):
“Since its introduction, OvoControl programs have been deployed at a wide range of commercial and industrial sites, including oil refineries, power plants, chemical manufacturing, steel mills, rail facilities, breweries, automobile assembly, college campuses, hangars, hospital complexes, hotels and other large industrial venues. These are most often facilities where moving the birds around with physical exclusion is not effective at reducing bird related damage.“
The final point is an important one: It is not always possible to restrict roosting/nesting sites and the availability of food, which is what some critics of nicarbazin advocate. These researchers (12) developed a model for the effect of fertility control on the pigeon population, and concluded that the 50% reduction over 5 years predicted by the model was not enough.
However, a halving of the population seems a considerable achievement. While restricting resources to avoid over-breeding is a good idea, it has its limitations. The additional use of fertility control is a good strategy to keep colonies healthy and in check.
Nicarbazin-treated corn has no unintended side effects. Corn kernels are too large for non-target species to eat, and fertility reduction is reversible if the treatment is withdrawn.
References
- https://www.sahealth.sa.gov.au/wps/wcm/connect/public+content/sa+health+internet/conditions/bites+stings+and+pests/feral+pigeons+management+and+control
Haag-Wackemagel, D. & Moch, H. (2004). Health hazards posed by feral pigeons. Journal of Infection, 48 (4) 307-313
Giunchi, D., Albores-Barajas, Y., Baldaccini, E., Vanni, L. & Soldatini, C. (2012). Feral pigeons: problems, dynamics and control methods. In Soloneski, S. & Larramendy, S. (eds). Integrated Pest Management and Pest Control – Current and Future Tactics. InTechOpen
Gichuhi, A. Alarm as gunman opens fire on pigeons. The Advertiser, 30/10/2024
Jacquin, L., Cazelles, B., Prevot-Juillard, A-C., Leboucher, G. & Gasparini, J. (2010). Reproduction management affects breeding ecology and reproduction costs in feral urban pigeons (Columba livia). Canadian Journal of Zoology, 88 781-787
Gonzalez-Crespo, C. (2024). Evaluation of 8 years of fertility control (nicarbazin) to manage urban pigeon populations. Wildlife Research, 51 WR22166
Gonzalez-Crespo, C. & Lavin, S. (2022). Use of fertility control (Nicarbazin) in Barcelona: an effective yet respectful method towards animal welfare for the management of conflictive feral pigeon colonies. Animals 12 856
Albonetti, P., Marletta, A., Repetto, I.& Sasso, E. (2015). Efficacy of nicarbazin (Ovistop®) in the containment and reduction of the populations of feral pigeons (Columba livia var. Domestica) in the city of Genoa, Italy: a retrospective evaluation. Veterinaria Italiana, 51 (1)
Ferri, M., Ferraresi, M., Gelati, A., Zannetti, G., Ubaldia, A., Contiero, B. & Bursi, E. (2009). Use of nicarbazinee in the control of urban pigeon colonies in Italy in 1990-2007. Annali della Facolta di Medicina Veterinaria Universita di Parma, 29 91-102
MacDonald, A. & Wolfe, E. (2009). The efficacy of OvoControl®P (nicarbazin) as a contraceptive for pigeons in urban IPM. Other Publications in Wildlife Management, 62
Wolf, E. & MacDonald, A. (2016). The efficacy of OvoControl® (0.5% nicarbazin) in the management of feral pigeons (Columba livia). In Timm, R. & Baldwin, R. (eds). Proceedings of the 27th Vertebrate Pest Conference, University of California Davis
Giunchi, D., Baldaccini, N., Sbragia, G. & Soldatini, C. (2007). On the use of pharmacological sterilisation to control feral pigeon populations. Wildlife Research, 34 306-318
Scholarship at its best.