[By Andrew Fisher & Gabriele James]

Summary 

This is the second post on research exploring interventions to improve the lives of urban wild vertebrate animals in South African cities. We are grateful for the support of a grant from EA Animal Welfare Fund. The aim of this post is to catalogue existing methods of controlling urban pigeon populations, with a focus on welfare impacts for pigeons. Where appropriate, we indicate which existing methods might be prioritised or modified, to improve pigeons’ welfare. Since pigeons are common in many cities worldwide, many of the insights here could be generalized to other cities with large-scale pigeon populations. 

Like rats, feral pigeons are numerous in large South African cities (and in most cities globally), yet a relatively neglected subset of urban wild vertebrate animals. Limited evidence from a staff survey at a large South African university campus suggests that people do not have strongly negative attitudes towards pigeons, compared to other pest wild animals e.g. rats, which (if true) bodes well for efforts to improve pigeons' welfare. 

Generalisable insights include:

• The need for “pigeon control” should not be assumed; rather, need should be justified based on clear evidence of e.g. economic damage, and take into account citizens’ perceptions of target pigeon populations through public participation processes.
• Research on overall welfare impacts of existing “control” methods is scarce compared to studies of their effectiveness, highlighting a need for more of the former type of research.
• Pigeon population estimation and monitoring is often overlooked, but pigeon control programs should be based on unbiased estimates of target pigeon populations.
• Control programs should be based on (and tailored according to) a sound understanding of local pigeon population dynamics and ecology, in order to improve effectiveness and humaneness.

Where pigeon control is justified on evidence-backed grounds, control programs should focus on reducing carrying capacity by e.g. pigeon-proofing target areas (which is likely to sustainably reduce pigeon populations), and do this as humanely as possible. Regarding existing methods of pigeon-proofing urban habitats:

• removal of existing eggs or nests is not recommended as it can be counterproductive and reduce female pigeons’ health;
• where deterrents are used to reduce access to nesting/roosting sites (rather than bird-friendly screens/mesh/nets, or pre-emptive building design), the most humane effective deterrents should be used;
• studies of a pigeon control program on a large South African university campus (in an urban area) suggest that:
  • stainless steel spikes are the most effective when used alone, but the combination of spikes with other visual deterrents (e.g. light reflectors) is likely to be more effective than spikes alone;
  • (i) recorded audio of raptors and distress calls of other prey birds, and (ii) visual presence of raptors (i.e. falconry) are ineffective at reducing pigeons in the long-term, but could temporarily scare pigeons away from the target area;
• there is inconclusive evidence (from studies not specific to South Africa) for the effectiveness of high-frequency audio, presence of fake owls, recorded raptor sounds, or reflector lights;
• a recent study (conducted in Italy) suggests that Bird Free® gel deterrent is an effective method for discouraging pigeons from specific nesting/roosting sites, and has prima facie acceptable welfare effects; however its effect is limited to the treated area such that it is not suitable for large-scale management aimed at reducing the carrying capacity of the urban environment;
• Avitrol (which causes seizures in affected birds in order to deter flocks from target areas) does not appear to be widely used in South Africa, but where used, should be reduced or eliminated due to significant welfare concerns.

Reducing pigeons’ food supply is plausibly less humane than other non-lethal methods of reducing carrying capacity, e.g. if pigeons (especially juveniles) suffer more from starvation due to increased competition or scarcer food. The overall welfare impact of sudden reductions in food supply seems sensitive to local conditions, and should be better understood through more research. Apart from welfare concerns, effectiveness of food reduction appears to vary depending on local conditions (e.g. likely to be most effective where most of the flock rely heavily on a primary food source that can reliably be controlled/reduced). 

Pigeon control programs that focus on humanely reducing carrying capacity in the long-term could be complemented by fertility control (e.g. nicarbazin), under appropriate conditions. Inconclusive evidence of nicarbazin’s long-term efficacy in reducing abundance of free-ranging pigeons suggests that efficacy is sensitive to local population size, dynamics and ecology. If so, field studies of nicarbazin for specific free-ranging pigeon populations are indicated. Subject to further research on its overall long-term welfare effects (including side-effects), fertility control is promising from a welfare perspective to the extent it could non-lethally and sustainably increase resources available for individual pigeons in a population by reducing (especially juvenile) mortality rates. 

Lethal methods to reduce pigeon populations include the use of avicides (i.e. lethal bird poisons), controlled hunting, and cage-trapping for subsequent culling. Lethal methods do not seem to be widespread in South African cities, but their use should be reduced or eliminated, since (in addition to negative welfare impacts) they are likely ineffective in the long-term if not combined with reducing the carrying capacity of the target area. Cage-trapping (for subsequent killing) could be effective in specific contexts viz isolated residential areas with low building density, where isolated pigeon populations have not yet reached “high density”. However, cage trapping is not likely to be effective where most indicated, viz in high-density urban areas where pigeons are already abundant. Where use of cage traps cannot be eliminated, adverse welfare effects should be minimised by e.g. ensuring cage traps meet internationally-recognised welfare standards; checking traps at least four times a week; and killing pigeons as humanely as possible after trapping. 

Introduction

This is the second post in a series focusing on ways to improve the lives of urban wild vertebrate animals in South African cities. We are grateful for the support of an EA Animal Welfare Fund grant. Like rats, (i) large-scale populations of feral pigeons live in most South African cities (and globally); (ii) urban pigeons are often considered to be pests given their potential to transmit zoonoses and cause economic damage to buildings, and are targeted by “pigeon control” programs to reduce their numbers. 

However, in South Africa pigeons (although an invasive species and often considered a "pest") do not seem to be perceived in a strongly negative light by comparison to e.g. rats. For example, a survey of staff at the University of South Africa (Pretoria campus) suggested that many staff did not have a negative perception of pigeons on campus, were opposed to inhumane pigeon control methods, and felt that human–pigeon interactions and the viewing of squabs in nests contributed positively to their work environment (Harris et al, 2016). Further, staff indicated that, should control be imposed, humane, non-lethal measures were preferred over eradication (Harris et al, 2017). Implications include that the need for “pigeon control” in South African cities should not be assumed; rather, justification of pigeon control programs should be sensitive to human perceptions of interactions with pigeons in the putative target areas, arising from a public participation process (Harris et al, 2017).

It is plausible that modifying existing “pigeon control” programs to make them more humane (from a pigeon welfare perspective) is a sensible starting point for improving urban pigeons’ welfare. The aim of this post is to catalog existing methods of controlling urban pigeon populations, with a focus on welfare impacts for pigeons. Where appropriate, we indicate which existing methods might be prioritized or modified, to improve pigeons’ welfare. 

We focus on urban pigeon populations in major South African cities; however, as urban pigeons are common in many cities worldwide, many of the insights here could be generalized to other cities with large-scale pigeon populations. 

General insights 

Require evidence-based justification of the need for “pigeon control” in the first place

The typical rationale for control of urban wild animals considered “pests” (including pigeons) is that they cause economic damage or other harms to humans (Giunchi et al, 2012). However, many control programs lack appropriate estimation of damage caused by target pigeon populations, making their justification unclear (Giunchi et al, 2012). Need for “pigeon control” should not be assumed; rather, control programs should be based on clear evidence of need for control, i.e. unacceptably high risk to human health, or economic damage. The need for control should ideally also consider affected humans’ perceptions through a public participation process (Harris et al, 2017).Where need for control can be justified on objective damage-based grounds, this should be balanced against likely overall welfare impacts for pigeons and non-target species. Only the minimum required level of control should be used to reduce damage to within acceptable levels, considering pigeons’ welfare.

Need for more research on overall welfare impacts of existing “control” methods

There seems to be a lack of research on welfare effects of "pigeon control" measures, compared to research on the effectiveness of such methods, indicating a need for more research on (direct and indirect) welfare impacts of existing control methods on pigeons and non-target species. Quality evidence of effectiveness remains important, however, because the most humane effective methods should be prioritised over ineffective methods.

Improve estimation and monitoring of pigeon populations

Attaining reasonable and unbiased estimates of target pigeon populations is often overlooked, but is an important component of an effective pigeon control program because it allows objective evaluation of programs’ effectiveness (Giunchi et al, 2012). Estimates of pigeon populations in South African cities are difficult to find, suggesting that population estimation and monitoring are inadequate. Where cities (or other public or private actors) have formal pigeon control programs in place, these should be informed by the best available size-estimates of target populations and studies of target population dynamics and ecology.

Tailor control programs to local pigeon population and ecology

An adequate understanding of target pigeon population size, distribution, dynamics, and resources (including food) is likely to improve the effectiveness of any control program (Stukenholtz et al, 2019; Giunchi et al, 2012). Plausibly, improved understanding of local populations is also likely to improve the welfare impacts of control programs (for pigeons and non-target urban wild animals).

Reducing carrying capacity 

Where pigeon control is justified on evidence-backed grounds, control programs should focus on sustainably reducing carrying capacity, and do this as humanely as possible.

Reducing the carrying capacity of a particular urban area with a large-scale pigeon population is likely to be effective in sustainably reducing the population (Stukenholtz et al, 2019; Senar et al, 2017; Giunchi et al, 2012). Carrying capacity can be reduced in the long-term by making the urban habitat less attractive for pigeons. Common existing approaches include reducing pigeons’ access to nesting or roosting sites; and reducing pigeons’ primary food supply. 

Based on studies we reviewed, we think controlling nesting/roosting sites (with the most humane means available) is plausibly better from a pigeon welfare perspective than reducing food supply (which could displace pigeons to areas with less food or more competition from other birds, possibly leading to suffering and death from starvation). However, more evidence on overall welfare effects across methods is needed; in particular (i) localized welfare impacts likely depend on local population size, dynamics and ecology; (ii) all methods that displace pigeons from a target site could potentially cause indirect starvation, depending on various factors including displaced pigeons’ access to resources in their new territory. 

Reducing access to nesting/roosting sites

Excluding access to nesting sites (e.g. with netting or screens) can reduce reproductive output and abundance in a particular location under certain conditions (Stukenholtz et al, 2019). 

However, removing, puncturing (or otherwise destroying) already laid eggs, or using dummy eggs, are likely to be ineffective (Giunchi et al, 2012). Removing eggs can prompt pigeons to increase their reproductive efforts (and so be counterproductive from a population control perspective), and cause poorer physical condition in female birds (raising welfare concerns) (Jacquin et al, 2010). This suggests that egg removal could have negative welfare consequences for pigeons’ health, including parasite resistance (which might have knock-on effects on human beings or other wild animal species).

Attractive roosting sites can also be excluded by netting or mesh screens. Preventatively, new buildings can be designed to avoid attractive nesting or roosting sites. Removing roosting sites is plausibly as humane as excluding access to nesting sites (without removing existing eggs or nests), and more humane than reducing food supply (see below). However, the overall welfare effect of excluding access to nesting/roosting sites will depend partly on whether displaced pigeons can move to alternative nesting/roosting sites in nearby non-urban areas (i.e. outside the target area) without significant adverse welfare effects. 

Deterrents

Many commercially available deterrents are commonly used to deter pigeons from otherwise attractive nesting or roosting sites. These include stainless steel spikes, chemical/gel repellants, high-frequency noise-making devices, light reflectors, recorded sounds of predators (e.g. raptors), presence of actual predators (i.e. falconry), and bird shocking devices (though these latter seem scarcely used in South African cities, since they are more expensive than e.g. light reflectors and spikes). Even though producers claim non-lethal bird shocking devices are humane, they plausibly cause more suffering (by delivering an electric shock) than other deterrents, though studies of comparative welfare impacts would be needed to determine this more accurately. 

In general, where deterrents are used, (i) there should be adequate evidence of their effectiveness, and (ii) the most humane of the effective deterrents should be used. Whereas many existing studies investigated comparative effectiveness, more research is needed to determine the relative humaneness of effective deterrents. 

Harris et al (2016) studied the effectiveness of visual deterrents and physical barriers to pigeons on the University of South Africa campus in Pretoria (a large South African city) over two years. The study found that:

• visual deterrent rotating prisms that reflect ultraviolet light to interfere with pigeon sight line and flight pattern (e.g. Eagle Eyes™) reduced the pigeon population index by nearly 40%;
• gold and silver plastic flags that move with the wind and give the impression of fire and danger (Fire Flags), reduced population index by approx. 33%;
• stainless steel bird spikes to prevent pigeons from perching, reduced population index by nearly 70%;
• the combination of all the above deterrents was associated with a reduction in the population index by approx. 45%.

Spikes were the most effective intervention when used alone (in this local context), but the combination was expected to be the most effective if a site offered alternative perches rendering the spikes less effective (Harris et al, 2016). 

There is inconclusive evidence of effectiveness of high-frequency audio deterrents: a study of pigeon management in Ljubljana suggested ultrasound deterrents were ineffective because pigeons became habituated to the sound (Dobeic et al, 2011).

It is also unclear whether (presence of) fake owls, raptor sounds, and reflector lights are effective deterrents: Stukenholtz et al (2017) suggest that the balance of evidence indicates they are ineffective. 

Harris et al (2020) conducted a recent study of the impact of using the sound of raptors and bird distress sounds, as well as the visual presence of raptors, on the behaviour of feral pigeons at the University of South Africa campus in Pretoria. Recorded sounds of raptors and alarm/distress calls of other birds (not pigeons) were played near pigeon populated sites in the morning and evening, and separately, (using a falconer) raptors were flown over these sites. The study found no significant long-term reduction in pigeon numbers between the control, audio and raptor scare groups (possibly due to a limited sample size); however pigeon numbers significantly decreased within the 30 minute period post-scare at dawn, suggesting audio and visual deterrents caused short-term temporary behavioural changes (Harris et al, 2020). The authors note that pigeons do not have a distress call, which might limit the efficacy of the tested audio deterrents (Harris et al, 2020). 

Despite inconclusive evidence of its long-term efficacy in this study, the authors recommend falconry (i.e. controlled presence of birds of prey e.g. raptors) as a humane non-lethal control method and suggest that it could be effective if trained raptors are flown frequently over a longer period to associate the site with danger (Harris et al, 2020). Other anecdotal evidence suggests that falconry can be useful in reducing abundance in the absence of pigeon-proofing the target area (which is preferable in terms of efficacy) (Stukenholtz et al, 2017 generally; and here in South African cities). However, evidence on long-term efficacy of falconry in reducing pigeon populations appears to be inconclusive. The controlled presence of raptors can also be a relatively expensive and time-consuming pigeon control method (Harris et al, 2020). From a welfare perspective, although some commercial bird deterrent companies claim that falconry is a humane non-lethal control method, more empirical research is needed on the overall welfare effects of “raptor presence” on pigeons (and non-target species). For example, as with other “scare” methods that displace pigeons from a particular area (assuming they are effective), indirect welfare effects might include starvation due to increased competition or less food in new sites to which affected pigeons are displaced. 

Regarding chemical/gel repellants, a recent study by Gagliardo et al (2020) tested the effectiveness of a gel repellent (Bird Free®) in deterring pigeons from night and day roosting areas, as well as from sites where pigeons tended to build nests. The study showed that pigeons were effectively deterred from both night and day roosting sites in treated areas (for one year in both cases), and were discouraged from building nests in treated sites for at least three months (Gagliardo et al, 2020). Due to the local efficacy of the treatment (pigeons are only repelled from the treated area), it is effective for discouraging pigeons from specific sites, but not suitable for large-scale management aimed at reducing the carrying capacity of the urban environment by reducing access to nesting/roosting sites (Gagliardo et al, 2020). Gel repellents have been on the market for over seven years, however there are some concerns about their use because previously studied products are very sticky (posing danger of gluing pigeons or non-target wildlife, with associated death or distress) (Gagliardo et al, 2020). Bird Free® seems to be an improvement on previous gel repellents from a welfare perspective: due to its high viscosity it is not sticky enough to glue feathers (Gagliardo et al, 2020). The immediate welfare effects on pigeons seem acceptable (unless perhaps used in areas where pigeons are already nesting, in which case the disruption to nesting pigeons might cause significant distress), but further research is required to evaluate its overall welfare effects. 

Avitrol (and similar seizure-inducing compounds aimed at scaring birds away from a target area) do not appear to be widely used (or even commercially available) in South African cities. We therefore do not discuss it in detail here, even though Avitrol is used in some foreign jurisdictions (e.g. some states in the USA) and its negative welfare effects for pigeons are concerning. 

As mentioned before, field studies considering local characteristics of the target pigeon population and ecology are needed, to inform specific combinations of deterrents such that negative welfare impacts are minimized. 

Controlling the food supply 

Reducing pigeons’ food supply (e.g. by public education campaigns against feeding pigeons) can increase intrapopulation competition for scarce food resources, decrease the probability of juveniles entering the population, and decrease reproductive success due to unfavourable conditions (Stukenholtz et al, 2019). An experimental study in Barcelona showed that an education campaign aimed at limiting the amount of food people make available to pigeons, succeeded in reducing both food available and feral pigeon abundance (Senar et al, 2017). The intervention reduced the target population by approx. 40% during the study period, and the population took months to recover after the education campaign ended (Senar et al, 2017). This is supported by a recent study of the effect of different stages of lockdowns in Singapore on feral pigeons, which found that pigeon abundance decreased significantly in open food centres and feeding hotspots, likely due to less feeding of pigeons and food litter by humans (Soh et al, 2021). Pigeons did not significantly return to these areas when the lockdown was eased, probably due to them still foraging in adjacent green areas where pigeon abundance had increased during the lockdown (Soh et al, 2021).

A lesson from the above studies seems to be that reducing human-caused food availability could effectively reduce pigeon population (depending on local population dynamics and ecology). Whilst it seems sensible that naturally available food for urban pigeons should not be supplemented by humans (either intentionally or as human food waste) (Stock & Haag‐Wackernagel, 2016), sudden reductions in food supply could have a substantial adverse effect on pigeons’ welfare. More research is needed on its (direct and indirect) welfare effects on pigeons. For example, in the Barcelona study, population reduction due to reduced food supply changed population composition: in particular, skull and wing length of remaining pigeons increased, and their body mass and body condition decreased (Senar et al, 2017). The authors suggest this is because remaining birds had high competence to access reduced food resources, which was associated with reduced body condition of these birds (Senar et al, 2017). If it takes more energy to access scarcer food resources such that remaining pigeons have reduced body condition, this is a negative welfare effect on remaining pigeons. In addition, indirect effects on displaced pigeons (or pre-existing pigeons in areas to which displaced pigeons move) are often overlooked. As with other methods which (effectively) displace pigeons from a target area, displaced pigeons might not be able to access sufficient food in new areas if they are displaced to areas where there is more competition with other pigeons for scarcer food (and other resources). If so, they may experience suffering from starvation, making reduction of food supply less acceptable from a welfare perspective. 

Welfare concerns about reducing pigeons’ food supply are supported by evidence that a sudden reduction in food supply is likely to adversely affect juvenile pigeons. Stock and Haag‐Wackernagel (2016) acknowledged that reduced food supply negatively impacts pigeons’ reproduction, and further investigated at what stage(s) of the reproductive cycle such negative impacts occur. They found that a sudden food shortage reduced breeding couples’ attempts to breed, and reduced the number of hatchlings that survived to fledgelings (as reduced food supply made it more difficult for parents to meet hatchlings’ increased energy needs) (Stock & Haag‐Wackernagel, 2016). In the face of increased food-stress, breeding couples do not reduce clutch size from two eggs to one egg, because a single hatchling is not able stay warm enough without a sibling, such that a sudden reduction in food supply is likely to reduce the ability of nestlings to fledge (and therefore likely to increase mortality of juveniles) (Stock & Haag‐Wackernagel, 2016). 

In sum, from a pigeon welfare perspective, reducing food supply is plausibly less humane than other non-lethal methods of reducing the carrying capacity (by urban habitat modification). This is partly because pigeons (especially juveniles) are likely to suffer more from the effects of increased competition or scarcer food e.g. starvation (though actual overall welfare impact seems highly sensitive to local conditions which should be better understood). 

Apart from welfare concerns, the effectiveness of reducing food supply appears to vary depending on local conditions (Stukenholtz et al, 2019; Giunchi et al, 2012). For example, pigeons can adaptively exploit a variety of food sources, so if most individuals in a flock are able to compensate by flying to areas providing alternative food sources, then food reduction is less likely to reduce the target population. Conversely, if most of the flock rely heavily on a food source that can reliably be controlled/reduced, then the method is more likely to be effective (Stukenholtz et al, 2019; Giunchi et al, 2012). In the South African context, there are agricultural areas nearby the largest South African cities (viz Johannesburg, Pretoria, Cape Town, Durban), such that pigeons displaced from urban areas could plausibly compensate by flying to adjacent agricultural areas and foraging there (and then return to nest/roost in urban areas). If this is the case, then reducing food supply in South African cities is likely to be less effective (subject to empirical investigation).

Fertility control 

Where justified, pigeon control programs that  focus on sustainably and humanely reducing carrying capacity could be complemented by fertility control (under  appropriate conditions).

Methods of fertility control have been used to reduce pigeon abundance. A recent example is the use of nicarbazin-based compounds in baited feeders. Several studies support the efficacy of nicarbazin bait to reduce hatchlings in feral pigeons (e.g. Avery et al, 2008). However, overall effectiveness in reducing abundance of free-ranging pigeons is unclear. On the one hand, there is some evidence of its long-term efficacy to reduce pigeon populations (Albonetti et al, 2015; Wolf & MacDonald, 2016). On the other hand, a one-year study in Barcelona (conducted in 2017) found that nicarbazin has no effect on reducing pigeon populations there, and highlighted possible unforeseen ecological effects of nicarbazin if consumed by non-target species, such that the authors recommended against the use of nicarbazin to control populations, in favour of reduction of carrying capacity by urban habitat modification (Senar et al, 2021). The observed overall increase in pigeon population in Barcelona was probably because the carrying capacity of the city had not yet been attained, food was abundant (often supplied by the public), and the population readily backfilled by incoming pigeons from other regions (Senar et al, 2021). 

The inconclusive evidence from the above-mentioned studies suggests that efficacy of nicarbazin is sensitive to local population size, dynamics and ecology. If so, field studies of nicarbazin for specific pigeon populations are indicated, to help determine local efficacy as well as overall welfare impacts. From a welfare perspective, fertility control is a potentially more humane method to reduce pigeon abundance, subject to further research on its overall welfare effects for pigeons and non-target species. A plausible rationale for fertility control from a pigeon welfare perspective is that it could non-lethally and sustainably increase resources available for each individual in a population by reducing mortality rates, particularly amongst juveniles. 

However, some studies suggest we need better data on long-term effects and side-effects of contraceptives on pigeons and non-target species (Gray & Cameron, 2010; Giunchi et al, 2012). Pending a better understanding of long-term safety, efficacy, and welfare impacts (including side-effects), fertility control (e.g. nicarbazin) should be used with caution to complement efforts to sustainably reduce carrying capacity (see above). The addition of fertility control should preferably be based on simulation models of local population and ecology indicating that fertility control would be effective at the margin (e.g. fertility control is likely to be additive if the relevant compound is easy to distribute to the population because the population is relatively small and food is relatively scarce) (Giunchi et al, 2012). Where fertility control is used, target population and ecology should be well-understood in order to maximise efficacy and minimise potential adverse welfare impacts (e.g. place baited feeders such that exposure to non-target birds and other species is minimised) (Stukenholtz et al, 2019). 

Reduce lethal methods 

Lethal methods to reduce pigeon populations include the use of avicides (i.e. lethal bird poisons), controlled hunting, and cage-trapping for subsequent culling. [Cage-trapping followed by relocation without killing is a non-lethal method, but is likely to be ineffective at reducing target populations due to pigeons’ remarkable homing ability which enables them to return to the target site after relocation (Stukenholtz et al, 2019)]. 

The extent of use of lethal methods in South African cities is unclear. However, it is plausible that lethal methods (including avicides) are not widespread, since most leading South African pest control companies offering pigeon management do not mention or offer lethal methods. If the use of lethal methods is limited in South Africa, then reducing lethal methods would be of lower priority than some other interventions to improve pigeons’ welfare mentioned in this post. 

To the extent that avicides (and other lethal methods) are used, they should be reduced or eliminated. In addition to their negative welfare impacts, they are likely less effective than reducing carrying capacity or fertility control (Giunchi et al, 2012). Evidence suggests lethal methods might temporarily decrease a pigeon population, but do not cause sustainable long-term reductions (due in part to feral pigeons’ year-round breeding capacity which allows populations to rebound quickly in the absence of other control methods) (Giunchi et al, 2012; Stukenholtz et al, 2019). Senar et al (2017) suggest that culling is ineffective in the long-term (from a pigeon management perspective) if not combined with reducing the carrying capacity of the target area (by e.g. reducing food supply), partly because pigeons from adjacent areas quickly refill the target area in the absence of simultaneous reduction in carrying capacity. 

Cage-trapping (and subsequent killing of pigeons) has been used as a control strategy; however it is likely to be effective only under certain conditions. A study conducted in Spain over a seven-year period (2003-2010) investigated the efficacy of such cage-trapping as a population control mechanism in the context of isolated residential areas with low building density, where isolated pigeon populations had not yet reached “high density” (Farfan et al, 2019). In this specific context, trapping was an effective way to reduce feral pigeon populations within a relatively short period, partly because the relative isolation of the studied pigeons prevented (i) immigration of fledglings from adjacent high-density colonies, and (ii) rapid population recovery from frequent breeding typical of large high-density populations (Farfan et al, 2019). Trapping (and subsequent killing) is not likely to be effective in reducing populations in high-density urban areas where pigeons are already abundant, for the same reason lethal methods in general are ineffective in these contexts (viz high reproductive rates leading to population recovery, population backfill from adjacent flocks etc.) (Farfan et al, 2019). However it is suggested that trapping could be a valuable method for pigeon control in conjunction with long-term reduction of carrying capacity by habitat modification and/or fertility control (Farfan et al, 2019). 

If cage traps are used at all, they should be used only under conditions for effectiveness highlighted by Farfan et al (2019). Care should be taken to minimise adverse welfare effects on pigeons and non-target species, e.g. cage traps should meet internationally-recognised welfare standards; traps should be checked often enough to minimise injury and mortality of target and non-target species caught in traps (Farfan et al (2019) recommend checking traps at least four times a week); and if pigeons are killed after being trapped, they should be euthanised as painlessly as possible rather than e.g. relocated to a site where they are later used as game for hunters (as in the Farfan et al (2019) study. 

References

Albonetti, P., Marletta, A., Repetto, I., & Sasso, E. A. (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), 63-72.

Avery, M. L., Keacher, K. L., & Tillman, E. A. (2008). Nicarbazin bait reduces reproduction by pigeons (Columba livia). Wildlife research, 35(1), 80-85.

Dobeic, M., Pintarič, Š., Vlahović, K., & Dovč, A. (2011). Feral pigeon (Columba livia) population management in Ljubljana. Vet Arhiv, 81(2), 285-298.

Farfán, M. Á., Díaz-Ruiz, F., Duarte, J., & Real, R. (2019). Feral pigeon (Columba livia var. domestica) management in low-density urban areas: prevention is better than cure. Urban Ecosystems, 22(6), 1027-1035.

Gagliardo, A., Pollonara, E., Vanni, L., & Giunchi, D. (2020). An experimental study on the effectiveness of a gel repellent on feral pigeons. European Journal of Wildlife Research, 66(2), 1-8.

Giunchi D, Albores-Barajas Y, Baldaccini NE, Vanni L, Soldatini C. (2012). Feral pigeons: problems, dynamics and control methods. In: Soloneski S, Larramendy ML (eds) Integrated pest management and pest control - current and future tactics. InTechOpen, Rijeka, pp 215–240.

Gray, M. E., & Cameron, E. Z. (2010). Does contraceptive treatment in wildlife result in side effects? A review of quantitative and anecdotal evidence. Reproduction, 139(1), 45–55. 

Harris, E., de Crom, E. P., Fouche, J., & Wilson, A. (2020). Comparative study on the short-term effects of audio and visual raptor presence on a pigeon population, with a view towards pest control. International Journal of Pest Management, 66(1), 31-39.

Harris, E., De Crom, E. P., Labuschagne, J., & Wilson, A. (2016). Visual deterrents and physical barriers as non-lethal pigeon control on University of South Africa’s Muckleneuk campus. SpringerPlus, 5(1), 1-16.

Harris, E., De Crom, E. P., & Wilson, A. (2017). Staff perceptions on pigeon control strategies on the University of South Africa’s Muckleneuk campus. TD: The Journal for Transdisciplinary Research in Southern Africa, 13(1), 1-8.

Harris, E., De Crom, E. P., & Wilson, A. (2016). Pigeons and people: mortal enemies or lifelong companions? A case study on staff perceptions of the pigeons on the University of South Africa, Muckleneuk campus. Journal of Public Affairs, 16(4), 331-340.

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