Information

Urban gulls and their management in Scotland: review

This report looks at the ecology of urban gulls in Scotland, the problems they cause, and management solutions.


6 Methods for controlling urban gull populations

Human intervention to limit gull populations and individual colonies has been undertaken with some, although not universal, success (see Thomas 1972, Calladine & Wernham 1996, Belant 1997, Owen et al. 2001 for general reviews). Many of the studies available for information have not been carried out in urban environments, however, and many have included only limited monitoring of their success in limiting gull numbers. The various methods that have been tried are described below, along with any evidence for their success or failure and the practicalities of applying them within urban environments. The legality of using the different methods in the Scottish context is covered in Chapter 7.

6.1 Non-lethal disturbance methods

Once adult gulls have selected a site on which to nest, they can show marked attachment to it regardless of disturbance ( e.g. Southern et al. 1985). Therefore, non-lethal disturbance methods may prove ineffective for deterring breeding birds. However, even if the survival rate of breeding adult gulls in urban colonies is very high (perhaps around 90% per annum), there will still be a proportion of birds (perhaps 10%) that will be breeding for the first time each year. If pre-breeders roosting in or close to urban colonies are subjected to suitable disturbance methods, then it may be possible to deter these more mobile birds from 'selecting' the urban colony in which to breed and use this as a long-term strategy for reducing the size of urban colonies.

Site tenacity probably also varies between species ( e.g. Wanless et al. 1996), and individual nesting areas. The latter effect might be a function of the size and age of the colony, with larger and established colonies perhaps being frequented by birds that show higher tenacity and are less affected by such disturbance techniques. Therefore, the overall effectiveness of disturbance methods on breeding gulls might also be expected to vary between species and also between nesting sites. For example, if Lesser Black-backed Gulls have a generally lower tenacity to breeding sites than Herring Gulls (after Wanless et al. 1996), then Lesser Black-backed Gulls may be the more likely of the two species to be deterred from breeding sites by such methods.

For deterrence through disturbance to be successful, intensive application over a prolonged period of time is generally thought to be required, particularly if the aim is to deter birds from breeding. The effectiveness of scaring techniques is also likely to lessen with prolonged use, however, as gulls become habituated to them (below), such that the application method might need to be varied through time to minimise the latter problem.

A five year study completed recently looked at a range of disturbance techniques aimed at deterring birds from landfill sites including: falcons, hawks, static and manually operated distress calls, the firing of blanks, rope bangers, lethal control (shotgun), pyrotechnics, bird scaring kites and automated sound generators (Baxter, 2004). Each technique was tested individually and in combination to determine the most effective method of deterring scavenging birds from the sites and the factors leading to the failure of any system. Some techniques were found to be highly effective over the short term but rarely maintained this effectiveness over longer periods. Combining techniques led to greater success than when individual techniques were used alone (Baxter, 2004).

6.1.1 Broadcasting sounds

The playing of the recorded distress calls of gulls has been used successfully to reduce the numbers roosting on open water: the playing of the distress calls of Black-headed Gulls discouraged the birds from roosting on the Milngavie Reservoirs (Benton et al. 1983), and the playing of a combination of distress calls of Black-headed Gulls and Lesser Black-backed Gulls, supplemented with the firing of bird-scare cartridges, proved successful in Oxfordshire (Gosler et al. 1995). In North America, breeding Ring-billed Gulls were deterred by a combination of broadcasting distress calls, using shell crackers and also tethered raptors (Morris et al. 1992). Such scaring techniques have not been universally successful, however, with the intensive use of pyrotechnics (including shell crackers and sirens) not deterring gulls from some breeding colonies in North America (Olijnyk & Brown 1999, Brown et al. 2001).

The broadcasting of other sonic deterrents, such as bangs, is considered less effective in that gulls more readily habituate to these (Bomford & O'Brien 1990, Belant 1997), although frequent repositioning, alteration of the type of noise and the timing of activation may potentially improve effectiveness (Belant 1997). The two effective studies from Britain, for which the results have been published, both used the broadcasting of species-specific distress calls, both were carried out outside of the breeding season, and both were in situations where alternative roosting sites were readily available. Within urban areas, methods that involve the broadcasting of distress calls or use of pyrotechnics (if the latter were a safe option at all) could prove to be as disturbing to local residents as the gulls themselves.

6.1.2 Use of birds of prey

The flying of falcons has been used to scare gulls from sites such as airport runways, although this is generally only considered an effective deterrent to gulls if they are flown at least daily (Belant 1997). The flying of hawks appeared to reduced the proportion of gulls (predominantly Lesser Black-backed Gulls) present in Dumfries that actually bred in 1999 but the effect, if this was actually due to the hawks, was not long lasting, suggesting that the birds became habituated (Coulson & Coulson 2000), or that some other factor was responsible for the lower proportion of birds breeding in 1999. Other scaring devises such as flags and using effigies of predators ( e.g. model owls and balloons with 'exaggerated' large eyes) have been considered ineffective in that gulls can readily become habituated to their presence (Belant 1997, Rock in prep., Wellpark Action Group, pers. comm.).

Some falconers that use birds of prey regularly for bird control work suggest that, in order to disturb gulls in an effective manner, a bird of prey that can be trained to look as though it is hunting but not to kill birds, and that flies high, is likely to be most effective, as such a bird can avoid the aggressive 'mobbing' behaviour that gulls direct at predators and also deter gulls from a wider area (I. Whittaker, pers. comm.). On these criteria, it is suggested that falcons are likely to more effective than hawks, and Peregrine/Saker hybrids have been suggested as one of the most effective type of birds to use (I. Whittaker, pers. comm.).

The study of disturbance techniques at landfill sites (Section 6.1 above; Baxter 2004) allowed a comparison of the effectiveness of falcons and hawks to be made. For falcons, the percentage reduction in bird numbers feeding at landfills sites varied between 70% and 86%, depending on whether the deterrence was in place during operational hours only or continuously from dawn to dusk (15 trials in total). For hawks, the different timing scenarios appeared to make little difference to the percentage decrease in bird numbers, which varied between 43% and 47% (12 trials in total). The study concluded that the use of falcons contributed significantly in reducing the need for other systems and also reduced the risks of habituation. The limited number of failures in deterrence occurred when falcons strayed off-site or could not be flown during poor weather (Baxter, 2004).

In general, all those with whom we communicated advocated that birds of prey must be flown daily, over a prolonged period of the breeding season (to deter breeding birds) or a large part of the year (to deter birds at landfills), and that further work is required in subsequent years to produce a long-term effect but none of these suggestions has been tested rigorously via suitably designed studies. The use of Harris Hawks has been reported as being effective in flushing birds from buildings, which can then be secured or protected (I. Whittaker; S. Towell pers comm.).

In 2004, some deterrence work with falcons was carried out for the first time at Tarnbrook gull colony in Lancashire, the site of long-term control work on the breeding gulls and at which rigorous counts of breeding numbers have been made on an annual basis ( e.g. Coulson et al. 1991; Sowter 1999 - 2004). Visual observations suggested that the number of breeding gulls present was reduced when falcons were operational and that the density of nests was lower in those areas where falcons were flown (D. Sowter, pers. comm.). It is not clear at present whether the use of falcons actually reduced the number of nests in these areas, however, because some nest removal work was conducted simultaneously over the same areas. Nevertheless, the use of falcons is seen as a promising deterrence technique at this gull colony and further research is planned for future years (D. Sowter, pers. comm.).

From our searches of the literature, discussions with Local Authority representatives in Scotland, and some of their falconer contacts, there appear to be no rigorous studies of the true effects of the flying of hawks or falcons on breeding gulls or gulls using urban areas. Practical considerations when considering using falcons to deter gulls in urban environments will include the public perceptions of (and any real) risk of injury to the bird of prey or the target gulls, the availability of suitable elevated locations from which to fly the birds, and the risks (and public/stakeholder perceptions of risk) to other potential target species, notably racing pigeons. Knowledge of local pigeon race routes and training areas might be important in this context. Some organisations with extensive experience of flying birds of prey (such as the National Birds of Prey Centre in the UK) clearly view the use of birds of prey to control problem bird species in urban areas as inappropriate, for reasons relating both to the safety of the birds of prey themselves and to their likely effectiveness (e.g. see www.nbpc.co.uk/control.htm for further information on such concerns).

6.1.3 Human disturbance

Following the trialling of various intensive methods (and combinations of methods) to disturb nesting gulls from the Abbeystead gull colony on moorland in Lancashire, it was felt that disturbance by humans was the most successful of the non-lethal techniques tested and was subject to the habituation that occurred rapidly with other methods (J. Coulson & M. O'Connell pers. comm.). The technique involved the presence of humans in the colony for virtually all the day-light hours from early March to the end of May and over three years has totally cleared (in combination with some localised culling of more persistent individual gulls) about 90% of a target area containing around 15,000 large gulls (J. Coulson & M. O'Connell pers. comm.). In 2004, these human disturbance methods were used on gulls in parts of the centres of South Shields and Sunderland, using hydraulic platforms to reach some sites (and birds of prey as additional disturbance in some areas) in South Shields. Control areas that were free from disturbance were also monitored. Counts of gulls in the treatment and control areas suggested that in South Shields these techniques reduced the gull population by 70% in one year (J. Coulson pers. comm.). None of this work has been published to date to allow us to evaluate the potential of the techniques for more general application however.

6.2 Manipulation of nesting areas

A range of methods and devices have been designed specifically for excluding gulls from nesting areas, such as buildings, but there are few rigorous studies to assess the effectiveness of the different techniques in limiting access or breeding, or that address the areas to which any gulls that are excluded are displaced.

6.2.1 Methods for preventing access, landing or nesting

Suspended monofilament lines have successfully prevented gulls from occupying established nest sites (Blokpoel & Tessier 1983, Morris et al. 1992, Blokpoel et al. 1997), and along with suspended wires, also from landing on small areas of open water (Ostergaard 1981, Blokpoel & Tessier 1984). Experience with excluding Ring-billed Gulls from 'natural' nest sites in the Great Lakes region suggests that the effectiveness of such approaches can be influenced by the colour and visibility of the nylon lines used, and also by the size of colony and how long it has been established (Maxson et al. 1996): bright coloured nylon string was slightly more effective than monofilament at deterring gulls, and birds in larger, established colonies with a history of successful breeding were less likely to be deterred.

Even where effective, a general problem with erecting lines or wires is the potential for gulls, and also non-target species, to become entangled within them. Outside of urban areas, judicious spacing of lines has been shown to permit the entry of smaller non-target species, for example waders and terns that gull exclusions have aimed to encourage (Morris et al. 1992) but some individuals can still become entangled (Maxson et al. 1996). It is established that collision risks for birds on overhead wires or fencing can be reduced through marking to make the obstructions more visible ( e.g. Janss & Ferrer 1998, Baines & Andrew 2003). Marking lines that have been placed to exclude gulls could reduce collision risks in a similar manner and potentially make the devices more effective at deterring gulls, but such marking (such as the use of 'flags') can add to weight and wind resistance, reducing the resilience of the barriers in exposed positions.

When used specifically in urban areas, the practicality of physical barriers will depend on whether effective arrays of wires or lines can be attached securely in areas where gulls nest or might potentially nest. On flat roofs, any array of lines will need to be extensive to effectively deter nesting gulls: for ground nesting gulls, parallel lines spaced at 16 metre intervals were shown to effectively excluded Herring Gulls but the smaller Ring-billed Gulls (comparable in size to Common Gulls) were thought to require a spacing of less than 6 metres (Belant 1997). The height of lines above the surface appear to be of lesser importance, although any structures present above the lines might be used for landing so that gulls can subsequently walk under lines to nest sites. The use of netting as an alternative to lines or wires can exclude gulls from nesting from appropriately treated sites ( e.g. Coulson & Coulson 1999-2003, Rock in prep), but problems with collisions and entanglement might remain if these are poorly installed or maintained (Rock in prep).

The physical blocking of nest sites in urban areas may prove successful locally in that gulls are excluded from a single building or part of a building. Several Local Authorities in Scotland reported success in the use of such techniques to exclude gulls from nesting on buildings ( e.g. Dundee City, Aberdeen City, Dumfries & Galloway) and that the techniques were almost 100% successful if the design and placement of the devices were correct for each specific building. Bendy plastic spikes have also been used with success to deter Lesser Black-backed Gulls from nesting in a housing estate in Kilmarnock, by placing these at precise favoured nesting locations on approximately one-fifth of the houses and bungalows on the estate (Wellpark Action Group, pers. comm.; see section 3.8). Wires placed on the roofs of warehouses in Dumbarton have also been reported as successfully excluding almost 500 pairs of breeding gulls (I. Gibson, pers. comm.).

The high levels of site tenacity shown by many gulls to their breeding sites (see Chapter 5) mean that exclusion of individual pairs from a building, or group of buildings, may simply lead them to settle on neighbouring buildings as long as suitable nesting sites are available there. For such techniques to be effective across the scale of even a small town, gull excluders may need to be erected, and importantly maintained, to cover all potential gull nest sites over a wide area. There are many practical problems to overcome if Local Authorities wish to achieve such coverage, including the major ones of gaining access to private properties, health and safety considerations and cost (including who pays for the work; see Chapter 7 also for legal issues). The physical blocking of nesting sites with netting or spikes in Dumfries was not extensive enough to restrict either the numbers or the distribution of the (mostly Lesser Black-backed) gulls breeding there (Coulson & Coulson 2000).

6.2.2 Manipulation of the nesting substrates

The manipulation of the substrates on which gulls chose to nest has been tried with varying success. Mowing grass deterred Silver Gulls in Australia (Smith & Carlile 1993) and may be successful for local control of Lesser Black-backed Gulls within some colonies in Britain ( e.g. Inchcolm in the Firth of Forth , J. Calladine, pers. obs.). Although this technique may be an option for gulls nesting on 'natural' substrates within urban areas, it is clearly not an option for deterring roof-nesting gulls. In Tasmania, covering causeways where Silver Gulls nested with netting and bitumen to make them smooth was unsuccessful at deterring breeding gulls (Skira & Wapstra 1990): that study may have parallels for roof nesting gulls, especially those on flat roofs. Some Local Authority representatives in Scotland reported anecdotal evidence that changing the colour of roofs or the material of which they were made (which could also influence their thermal properties) deterred breeding gulls but this had not been tested rigorously and the effect not consistent between sites and species. For example, the replacement of corrugated asbestos roofs on warehouses in Glasgow with flat roofs of stainless steel was believed to have prevented Lesser Black-backed Gulls from nesting successfully because their nests were washed away (I. Gibson, pers. comm.) but such flat steel roofs appear not to deter Herring Gulls from nesting in Aberdeenshire (C. Campbell, pers. comm.). Clearly further details of the context of such changes are essential in interpreting any suggested effects or differences between locations and species.

6.2.3 Creation of alternative nesting areas

Although rarely tested for gulls, it may be possible to encourage urban colonies to switch breeding areas so that colonies are concentrated at sites where there is less perceived conflict with human interests. Such a process would probably need to involve: (i) creation of suitable nesting habitat (suitable substrate in a setting that rendered nesting areas free from ground predators; see 6.2.2 above) in a location away from human interests; (ii) pro-active attraction of the gulls to the area, perhaps with the use of decoys/sounds; and (iii) use of suitable methods (see other sections of Chapter 6 and Table in section 8.2) to disturb gulls from current breeding locations that are perceived to be problematic.

6.3 Manipulation of food sources

The overall availability of all food sources (both natural and human-generated) within or close to urban areas must be a contributory factor influencing the distribution and abundance of urban gulls, as this is an essential requirement for survival and reproduction (see also Chapter 5). Equally, reductions in the availability of food have been implicated in the observed declines of Herring Gulls and Lesser Black-backed Gulls at several 'natural colonies' that have been studied ( e.g. Bergman 1982, Pons 1992, Perrins & Smith 2000) and are speculated to have had a wider impact at the population level in Britain and Ireland (Mitchell et al. 2004). Although no studies rigorously quantify the diet of urban gulls in Britain, it is widely perceived but not substantiated that garbage, both at tips and as litter in streets (e.g. remains of take-away food), is important (see Chapter 3).

The importance of refuse tips for providing food for gulls appears to vary geographically (or with a complex interaction of factors that may be area-, colony- and species-specific), with some studies showing them to be a major food source ( e.g. Davis 1974, Mudge 1978, Mudge & Ferns 1982, Sibley & McCleery 1983a), while others suggest that they are a less preferred alternative to 'natural' food and of relatively minor importance ( e.g. Kihlman & Larsson 1974, Coulson et al. 1987). The importance of refuse in the diet can also differ between individuals, with some specialising in garbage ( e.g. Sibley & McCleery 1983a,b), and the use of such discards can also vary seasonally (Belant et al. 1993).

The elimination of garbage as a food source for gulls in urban areas is likely to be effective only in the absence of suitable alternative food supplies within a suitable ranging distance and, clearly, this is likely to vary between specific locations. In one study, Herring Gulls showing specialised feeding preferences, such as for garbage, were found to select other similar sites for feeding when one foraging site became unproductive (McCleery & Sibley 1986), so that 'garbage specialists' may be expected to look for other such food sources if one site is removed. Along an urban stretch of the River Tyne in northeast England, the combined number of six species of gulls declined concurrent with a reduction in food available in the form of untreated sewage (Raven & Coulson 2001), although responses by the different species varied: numbers of Lesser Black-backed Gulls and Kittiwakes increased, while numbers of Herring Gulls did not change significantly. The differences in response to the reduction in sewage waste were attributed to differences in the ecology and breeding status between the species in the area, implying that the simple removal of one food source (albeit an apparently important one) need not necessarily lead to declines in the numbers of all gulls where alternative food resources are available.

During a two-week programme of deterring gulls (predominantly Herring Gulls and Lesser Black-backed Gulls) from a land-fill site near Gloucester in March 2004 using trained falcons and pyrotechnics, based on counts of birds and resighting of colour-ringed individuals, about two-thirds of the displaced gulls were found at other nearby landfill sites, while the remainder found different food sources (such as on agricultural land or in intertidal areas; Rock 2004). Although, only a short-term study and conducted before egg laying, when the birds were less likely to have strong ties to the feeding areas closest to their nesting areas, this study again demonstrates the apparent ease with which gulls may switch to alternative feeding sites and food types.

The control of urban gull colonies through limiting food availability does appear to be an option worthy of fuller investigation. In order to predict the likely effects of action to limit the food availability of birds at any given colony, it would be necessary to have knowledge of (i) the current food sources used by the gulls, (ii) the likely limits to the foraging range of the species (on which some information already exists, although not for urban-dwelling gulls; Chapter 5), and (iii) all other potential food sources within the likely foraging range, even if these appear not to be of current importance (there is data available on the diets of gulls from some studies but few for urban-dwelling birds; Chapter 5). Even with the above information assembled, well-designed field studies that monitor the effects of experimental manipulation of the food sources of urban gulls will be required to test the assumptions made and refine the prediction process.

6.4 Restriction of breeding success

Gulls are relatively long-lived bird species, with estimated annual survival rates for Herring Gulls (from natural colonies) ranging from 88% to 93.5% (Chabrzyk & Coulson 1976, Coulson & Butterfield 1986, Migot 1982, Wanless et al. 1996) and for Lesser Black-backed Gull and Black-headed Gull, 91% and 82-89% respectively (Wanless et al. 1996, Lebreton 1994). Thus aside from any potential deterrence effect caused by disturbance at breeding colonies through these control measures (Section 6.1), the longevity of adults and also the 2-4 year period of immaturity before breeding (Chapter 5) will tend to make control via the restriction of breeding success a lengthy process. Some studies, for example one on the Isle of May, suggest that the colony of Herring Gulls there was limited effectively by restricting reproductive output, although this was not as successful for Lesser Black-backed Gulls (Wanless et al. 1996). The difference in response between species may have been due to the generally greater mobility of Lesser Black-backed Gulls (Chapter 5), although Wanless et al. (1996) found no direct evidence to support such an hypothesis. On a wider scale, it is also strongly suspected that traditional egg collecting by humans limited some gull colonies in Britain until at least the 1950s ( e.g. Davis 1974, Lloyd et al. 1991). Therefore persistent and long-term control measures to limit breeding success could potentially be effective for some sites and species, but effectiveness is likely to depend on several factors, notably choice of an effective treatment method, thorough control (treatment of a high proportion of nests) and low levels of immigration from other productive colonies: the presence of adult birds at a site, even with a small number only rearing young, might still attract some potential recruits from other sites (see Section 5.4).

The largest documented control programme that used restriction of breeding output as a principal control measure was carried out in Maine, USA from 1940-1952, during which time around 800,000 Herring Gull eggs were sprayed with oil (Kadlec & Drury 1968). Breeding numbers began to decline 4-5 years after the programme started, coincident with the expected recruitment year of the first cohort to which control was applied. Although rates of decline were at times greater than that expected if adult mortality alone was responsible (25-30% per year, suggesting additional disturbance effects leading to redistribution of both breeding adults and a reduction in recruitment), the programme ended because it was not felt to be achieving its aims over a wide enough area and also for financial reasons (Kadlec & Drury 1968, Thomas 1969).

6.4.1 Treatment or removal of eggs or nests

Methods of limiting or preventing successful breeding that have been documented include the removal or destruction of eggs and chicks, puncturing, shaking eggs or injecting eggs to kill the embryo, and coating eggs with oil (such as paraffin) to suffocate the embryo. Gulls can lay replacement clutches following the destruction or removal of eggs ( e.g. Harris 1964, Vermeer et al. 1991), necessitating repeat visits to destroy or remove eggs, although the frequency of relaying is reduced if eggs are destroyed late during incubation (Nisbet & Drury 1972), and the same applies if chicks are destroyed.

The treatment of eggs to prevent them from hatching (whilst leaving the nest and eggs intact) will generally prevent gulls from relaying and thus reduce the need for repeated treatments per nest, as long as the birds continue to incubate the treated eggs. Repeated visits to a colony will normally still be necessary if the aim is to treat all eggs, however, due to the variation in egg laying dates that is likely to occur. Eggs that have been treated by puncturing can sometimes be recognised by incubating gulls, leading to their rejection and subsequent relaying (Corkhill 1970). Methods that result in the death of embryos within eggs, although generally more time consuming per treatment, largely eliminate the problems associated with gulls relaying fresh clutches. Such treatments include vigorous shaking, injection with an embryonicide ( e.g. formalin), or coating eggs with oil to prevent gases moving through the eggshell and thereby suffocating the embryo (Kadlec & Drury 1968, Thomas 1972, Blokpoel & Hamilton 1989, Christens & Blokpoel 1991).

In the Scottish context, a number of Local Authorities have used the treatment or removal of eggs or nests to limit numbers of breeding gulls in a manner that they have perceived to be successful in achieving their individual aims. For example, nest removal followed by proofing has been perceived as successful in limiting breeding numbers on a housing estate in Kilmarnock (see section 3.8), in limiting breeding numbers in some areas of East Lothian (F. Mackay, pers. comm.) and in reducing the breeding colony size in Eyemouth (D. Watney, pers. comm.), although detailed counts of the changes in breeding numbers are not available in these cases to allow us to provide a fully quantitative scientific appraisal of the effects.

6.4.2 Introduction of predators

The introduction of nest predators, as a potential alternative method for controlling breeding productivity, has been used successfully on some island gull colonies in North America (using foxes, racoons or pigs; Kadlec 1971, Belant 1997). Such an approach often has limitations in that non-target species can also be taken by the predators and the predators will require alternative food if they are to remain in the long-term and prevent gulls from recolonising. In urban areas, where predators of eggs and chicks, such as rats and cats, are generally abundant in any case, it is likely that gulls will be nesting in areas that are largely inaccessible to them in the first place and hence this is unlikely to be a suitable option for the control of urban gull colonies.

6.4.3 Contraception

In principal, contraceptive techniques could be used to restrict the breeding success of gulls and may have applications in future. Immuno-contraception, via baits and vectors (which are likely to be the only practical options), is undeveloped as a technology currently, however, and its ecological application has not been tested appropriately (Barlow 2000). One Local Authority in Scotland provided us with information on an attempt to control urban pigeons in Venice using contraceptive techniques but it was concluded that this is not an effective control option (for further information see: http://www.aberdeencity.gov.uk/acc_data/committee%20reports/cs_env_r4_4_040907.pdf). Even if suitable contraceptive drugs become available, it may prove difficult to control the level of drug intake by gulls, however, because they 'rush feed' , which would be likely to lead to overdose and potentially adverse or even lethal effects (J. Coulson pers. comm.).

6.5 Removal of adult birds

The live capture and transportation of fully grown gulls away from problem areas is potentially practical, although relocated gulls would probably (or even certainly) return to the area of capture (Belant 1997); hence this is unlikely to be a useful control method except perhaps for the removal (to a large distance from the colony) of isolated 'problem' birds or pairs. The effective removal of adult birds from a colony is likely to necessitate killing them.

Due to the generally high survival rates of adult gulls and their delayed maturity (age of first breeding; Chapter 5), the culling of breeding adults will invariably have a more immediate effect on numbers than control methods aimed solely at restricting breeding output. A number of gull culls have been well documented and have proven to be an effective means of controlling some local breeding gull colonies ( e.g. Thomas 1972, Wanless & Langslow 1983, Coulson et al. 1982, Coulson 1991). Amongst the documented culls, narcotic bait (typically alpha-chlorolose) placed by nests has been used widely. The territorial nature of nesting gulls permits the targeting of specific nests, species or areas using this approach.

One of the best documented large-scale culls of breeding gulls was carried out on the Isle of May in the Firth of Forth, where over 40,200 Herring Gulls and 5,300 Lesser Black-backed Gulls were removed: a combined colony of over 17,000 pairs of gulls in 1972 was reduced to between 2,500 and 3,000 pairs (Wanless & Kinnear 1988, Coulson 1991). Associated with the reduced colony size, some changes in the demography of these Herring Gulls were recorded: the age of first breeding declined and natal philopatry increased during the following period when culling was carried out at a lower level (Duncan 1978, Coulson et al. 1982, Coulson 1991). These changes were interpreted as density-dependent responses that tended to compensate for the increased mortality due to culling and to some degree mitigated against the effectiveness of the cull. However, in addition to the removal of gulls killed during the cull, there appeared to an additional influence on recruitment with up to 6,000 individuals (young reared on nearby undisturbed colonies) potentially deterred from settling to breed on the Isle of May (Coulson 1991). During the culls, breeding success was obviously low, in that breeding adults were systematically killed. It follows that if recruitment can be influenced by performance-based conspecific attraction (Section 5.4), then a colony undergoing an effective cull of breeding adults is likely to be less attractive to potential recruits.

An alternative to killing gulls at breeding colonies is to target them at feeding sites, such as refuse dumps. Here there may be more potential to trap and kill, shoot or poison concentrations of birds ( e.g. Hakkinmen & Nummelin 1980). The local breeding birds that survive might also become deterred by the disturbance caused by either trapping or shooting, however, and learn to avoid the feeding areas where such activities are undertaken, such that only a proportion of the colony is removed. For this reason, poisoning or narcotising at feeding sites may be a more effective alternative, although it need not necessarily target a high proportion of gulls breeding in urban areas (see section 5.5). Botulism poisoning, whereby gulls inadvertently ingest a toxin produced by the bacterium Clostridium botulinum that is associated with some rubbish tips (particularly during warm weather), has been implied as a causal factor for some observed declines of Herring Gulls and Lesser Black-backed Gulls ( e.g. Ferns 1984, Sutcliffe 1986, Madden & Newton 2004). It follows that deliberate poisoning at feeding sites may potentially be an effective method for control. Such approaches will not be selective in terms of the breeding origins of the individual gulls (could target urban and non-urban breeders in the absence of existing information on the origins of birds using any given feeding site) however. The likelihood of affecting non-target species will also generally be greater at feeding sites than at nest sites.

6.5.1 Use of narcotics

Narcotic baits, such as alpha-chlorolose, do not generally kill birds immediately on ingestion but, rather, result in 'drowsiness' and death over a period of hours. In practice, where baits are administered at colonies, access needs to be restricted to prevent disturbance and to maximise the proportion of birds that die whilst sitting on or close to nests. Carcasses can then be removed and appropriately disposed off. If disturbance to the colony occurs, gulls that have ingested baited food may fly away and die elsewhere. The control of gulls in urban areas using narcotics baits at nests is only likely to be practical in situations where (i) access to nest sites is possible (for placing the baits), (ii) baited areas can be sealed from human disturbance temporarily and (iii) carcasses can be cleared up for disposal. Such criteria are unlikely to be met in city centres or residential areas but such techniques might be practical on some industrial sites with restricted public access.

The use of alpha-chlorolose to cull gulls in a urban setting has been documented by Scarborough Council (see presentation by T. Fenter at the Gloucester Urban Gulls Conference: http://www.gloucester.gov.uk/libraries/templates/page.asp?URN=2162, which gives further details on the summary that follows here). Roof nesting was first recorded in Whitby in 1942 and Scarborough in 1967. The Council carried out a programme of control, initially using egg pricking and substitution, but from 1976-1990 using narcotic bait. Between 1978 and the late 1980s, the combined population of gulls in Scarborough, Whitby and Staines decreased from 1400 to 500. Some property owners refused the Council access to cull gulls, so that complete clearance was not possible, and the culling also led to protests from the 'animal rights' movement (J. Coulson, pers. comm.). In 1991, the culling was abandoned for a number of reasons (including the lack of cost effectiveness) and control is now primarily by nest clearance. After the abandonment of culling, the gull population increased again (peaking at c.3000 in 2001 and now standing at c.2750). The Council was refused permission for a licence to resume use of narcotic baits and now clears nests and eggs from buildings on request, provides proofing materials at cost, gives advice and guidance on gull control, and is advocating the need for a national strategy to control gulls.

6.5.2 Shooting

The shooting of gulls at breeding colonies, as well as targeting breeding birds, may also eliminate potential recruits (young birds attending colonies) and potentially also scare birds away through disturbance (Section 6.1). The shooting of gulls has been proven effective as a control measure on some small islands ( e.g. Casey et al. 1995) and is also reported to have been effective at reducing or eliminating some colonies of Lesser Black-backed Gulls on moorland (Calladine 2004). Issues of access and safety are obviously likely to limit the use of shooting within urban areas, however, and, as with all control measures, it is essential that appropriately licensing is in place (Chapter 7), Police advice is sought and guidance followed. The perceptions of the public to the use of shooting in urban areas must also be a major factor when considering the use of this control technique.

Contact

Email: Central Enquiries Unit ceu@gov.scot

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