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Use of Lethal Control to Reduce Habituation to Blank Rounds by Scavenging BirdsAuthor(s): Andrew T. Baxter and John R. AllanSource: The Journal of Wildlife Management, Vol. 72, No. 7 (Sep., 2008), pp. 1653-1657Published by: Wiley on behalf of the Wildlife SocietyStable URL: http://www.jstor.org/stable/25097748 .

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Tools and Technology Note

Use of Lethal Control to Reduce Habituation to BlankRounds by Scavenging Birds

ANDREW T. BAXTER, Bird Management Unit, Central Science Laboratory, Department for Environment, Food and Rural Affairs, Sand Hutton,York Y041 1LZ, United KingdomJOHN R. ALLAN, Bird Management Unit, Central Science Laboratory, Department for Environment, Food and Rural Affairs, Sand Hutton, York Y0411LZ, United Kingdom

ABSTRACT Scavenging bird deterrence frequently fails due to habituation. We demonstrated such habituation by gulls and corvids toblank rounds used in a dawn-to-dusk regime at a landfill site in southern England. We then combined blank rounds with live rounds and shotbirds whenever they attempted to land. Gull numbers declined significantly despite only 1.9% of the population being shot. Corvid numbersreturned to precontrol levels despite 52.7% of the population being shot.We suggest that shooting reduces gull habituation to blank rounds butis ineffective at reducing habituation by corvids. (JOURNAL OF WILDLIFE MANAGEMENT 72(7):1653-1657; 2008)

DOI: 10.2193/2007-458KEY WORDS blanks, corvids, deterrence, gulls, habituation, lethal, shooting, waste management.

Gulls {Larus spp.) and corvids {Corvus spp.) are frequentvisitors to landfill sites (Coulson et al. 1987, Baxter 2001).The presence of large numbers of these birds often leads toconflict with humans. Gulls and corvids are known carriersof human pathogens such as Salmonella, Escherichia coli, andavian botulism, and they have the potential to transferdisease to water storage reservoirs (Benton et al. 1983,Butterfield et al. 1983, Ortiz and Smith 1984, Monaghan etal. 1985, Sutcliffe 1986). Concentrations of scavenging birdscause considerable nuisance, and their defecation isdetrimental to amenity areas and increases health risks tolocal residents (Fasham 2000). Scavenging birds congregating around a site also alter habitats through trampling andchanging site fertility and impact sensitive wildlife byincreasing pr?dation (Bergman 1982, Bazely et al. 1991,Smith 1995, Harris andWanless 1997). Gulls and corvidstravel upwards of 20 miles to feed at landfill sites each day(Horton et al. 1983,Wingfield-Gibbons et al. 1993, Baxter2003), and these regular movements of birds often conflictwith flight safety (Dolbeer et al. 1993, Civil Aviation

Authority 2007). Preventing birds from foraging at landfillsites can therefore have significant environmental, social,and economic benefits (Temby 2000).

An array of deterrence techniques is available to reduceproblems associated with birds foraging at landfill sites(Aylward 1995, Baxter 2000, McDonald 2001, Bomfordand Sinclair 2002, Baxter and Allan 2006). The effectivenessof some techniques varies, however, between locations andcan diminish with time (Baxter and Robinson 2007). Blankfiring pistols, gas cannons, and rope bangers, which all deterbirds with a loud acoustic report are frequently used butoften result in habituation (Bomford and O'Brien 1990,McDonald 2001). Lethal deterrence with live rounds also isused in many parts of the world but can be problematic.

Widespread shooting to control bird damage to agriculturewas considered ineffective inAustralia (Fleming 1990), andE-mail: [email protected]

social and security concerns resulted in it rarely beingregarded as

aprimary solution at airports and other publicplaces (Anning 1998). Lethal control is, however, widelyused as a bird control technique in the United Kingdom but

only after other systems have been shown to be ineffective(HerMajesty's Stationery Office 1985).Anecdotal evidence suggests a combination of shootingand nonlethal deterrence is both publicly more acceptable

and results in greater effectiveness than the use of nonlethaltechniques alone (Cleary and Dolbeer 1999). Occasionalshooting with live rounds, for example, increased the effectof gas cannons on waterfowl present in Danish wetlands(Meltofte et al. 1996). Changes in bird behavior also have

been reported where combinations of deterrence techniqueswere in use. Flying trained falcons to support a shootingregime at John F. Kennedy International Airport (JFK)resulted in fewer target birds being shot, but itwas not clearwhether those birds viewed falcons as an indicator of thepresence of shotguns or whether they responded to eachdeterrent separately (Dolbeer et al. 1993, Dolbeer 1998). Itis important, therefore, to document whether lethal control,applied to re-enforce a non-lethal regime, can be effective atreducing habituation, deterring birds, and reducing thenumbers of birds that might otherwise be shot if lethalcontrol alone were used. We aimed to evaluate whether gullsand corvids habituate to the use of blank rounds, whetherthat habituation can be reversed through the deployment oflive rounds, and if so,what level of lethal control is required.STUDY AREA

We selected a landfill site situated 11 km from the southcoast of England that consistently attracted many thousandsof gulls to forage on the 250,000 tons of domestic wastedeposited each year. The total area of the site wasapproximately 25 ha within which one (


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10,000

8,000-4

Blank roundson Blank rounds off

?GullsICorvids

Figure 1. Number of gulls and corvids at a landfill site in Devon, UnitedKingdom, between week 1 and week 12 of a study starting on 23 October2000 inwhich blanks were deployed from week 4 to 8.

for gulls and corvids. Two adjacent 20-ha arable fieldsprovided similar resting locations, whilst 2 mature woodlandareas (3 ha and 6 ha), also adjacent to the landfill site,provided day and night roosting areas for corvids. Gulls flewin from the coast each morning and returned to roost thereat dusk.

METHODSMonitoring took place over a 23-week period from 23October 2000 to 31 March 2001. We recorded number ofgulls and corvids present every hour between dawn andmidday or midday and dusk on one randomly selectedmorning and one randomly selected afternoon each week.Herring gulls {Larus argentatus) and black-headed gulls

{Larus ridibundus) accounted for 99.8% of all gulls present,and we amalgamated them into one gull category. Rook{Corvus jrugilegus), carrion crow {Corvus corone), andjackdaw {Corvus monedula) made up 100% of corvids and

we amalgamated them into one corvid category. Werecorded all gulls and corvids feeding on the active tippingarea at the beginning of each hour. We then walked astandard route to record birds elsewhere on site and in theadjacent fields and woods. We discarded counts of birds thatwe observed moving between areas to prevent duplicatingrecords. We spent 10 minutes counting birds on the activetipping area and 5 minutes each at 4 vantage points coveringthe site and surrounding area. We used standard flockestimation methods as described by Bibby et al. (2000). Wecounted birds using 10 X 42 binoculars (Swarovski, Absam,

Austria) or a 20-60X telescope (Kowa, Nagoya, Japan).No deterrence was implemented from week 1 to week 3.From week 4 toweek 8, bird control was implemented by anautonomous pest control company that stationed one staffmember at the edge of the active area from dawn to dusk 7days aweek. Blank rounds were fired from a starter pistol,and the total number fired per week was recorded to the

Table 1. Number of blank rounds fired during a 5-week period inNovember-December 2000 and the mean hourly counts of gulls and corvidsat a landfill site inDevon, United Kingdom.Approx. no.Week of blank rounds Mean no. Mean no.trial fired gulls seen/hr corvids seen/hr4 1,000 4,798.33 131.585 1,150 2,364.08 102.676 1,100 4,595.10 211.70

7 850 4,967.20 245.608 900 7,705.20 420.60

nearest 50 rounds. No deterrence was deployed from week 9to week 12. Dawn-to-dusk deterrence (combination control) using blank rounds supplemented with live rounds (no.5 shot; 32 g) was then deployed from week 13 to week 19.

Live rounds were only used when gulls or corvids attemptedto land on the site. The numbers of live rounds fired, blankrounds fired, and birds culled were recorded by the pestcontrol company during the combination control phase ofthe trial. No control was implemented from week 20 toweek 23. Lethal control was deployed under UnitedKingdom Department for Environment, Food and RuralAffairs Wildlife and Countryside Act 1981 licences(WLF100085).We analyzed bird count data using SPSS forWindows(SPSS Inc., Chicago, IL). We used Kruskal-Wallis and

Mann-Whitney U tests as we could not normalize the dataor homogenize the variances.

RESULTSThe mean hourly count of gulls recorded across the wholewaste

management site during the preblank deterrenceperiod was 5,714. 89 and corvids was 489.00 for corvids (no.of hr [n] = 44). Reductions in overall gull numbers {x ?5,714.89 to 4,535.81 gulls, n = 54, U= 2.000, P= 0.046)and corvid numbers {x= 489 to 198.43 corvids, n= 54, U=5.412, P < 0.001) were recorded when blank rounds werefired. Reductions through time were negligible (Fig. 1).We did not observe a difference in numbers betweenpreblank deterrence and the final week of blank rounds forgulls {U= 0.991, P= 0.323) or corvids {U = 0.529, P =0.620). A mean of 1,000 blanks were fired each week (Table1) during the deterrence period. Birds were observedforaging

within 10 m of pest control staff and ignoringuse of blanks. We considered that this signified totalhabituation had occurred. Gull numbers continued toincrease after blank deterrence was removed {x= 5,714.89to 7,117.84 gulls, n = 33, Mann-Whitney U= 2.041, P =0.042), whereas corvid numbers remained stable {x= 489 to420.6 corvids, n = 33, U= 1.452, P= 0.148).

Reductions in gull (and corvid) numbers were achieved bycombination control (Fig. 2). Gull reductions {x= 1,086.65,n= 78, U= 6.037, P < 0.001) were maintained and showeda 97.95% decline from precontrol levels by the final week ofthe trial {U = 3.436, P < 0.001). A reduction in corvidnumbers also occurred {x= 263.85, n = 78, U= 3.207, P =

1654 The Journal ofWildlife Management 72(7)

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10,000

8,000-H

6,000i

2 4.000H

2.000H

10 11 12 13 14 15 16 17 18 19 20 21 22 23Week

?Gulls)Corvids

Figure 2. Number of gulls and corvids at a landfill site in Devon, UnitedKingdom, between week 9 and week 23 of a study starting on 23 October2000 inwhich blanks were re-enforced with lethal rounds between week 13and week 19.

0.002), although corvid numbers returned to precontrollevels by the final week of the trial {U= 0.765, P= 0.445).The number of gulls shot per week (Table 2) during thecombination phase declined (Kruskal-Wallis % = 33.06, P< 0.001), whereas numbers of corvids shot each week did

not decline (Kruskal-Wallis %2= 10.93, P = 0.09). Themean total number of blank and lethal rounds fired per weekduring the combination trial (280) was less than the meantotal number of blank rounds fired during the blanks onlytrial (1,000).

Gull numbers did not return to precombination controllevels {x = 7,117.84) following cessation of combinedcontrol {x= 2,633.27, n = 15,Mann-Whitney U= 3.606,P < 0.001). Gulls were observed on-site but did not feed for2 weeks after the end of the combination deterrence. Corvidnumbers did not change between pre- and postcontrolperiods {x= 414.06 to 559.4 corvids, n= 15, U= 1.541, P=0.124).

DISCUSSIONBoth groups of birds habituated to the use of blank rounds

when used alone. Gull and corvid numbers returned to, orincreased above, precontrol levels by the end of the blanks

trial. Habituation to blanks continued when the combination trial began until live rounds were deployed. If birds hadresponded to the use of blank rounds in this phase, lethalcontrol would not have been needed. Our results aretherefore consistent with those of other studies where noisegenerating techniques were used (Bomford and O'Brien1990). Birds quickly became habituated to one continuouslyused technique and remained in the immediate area makingcontinual attempts to feed. This behavior matches exactlythe definition of habituation as "response decrement as aresult of repeated stimulation" (Harris 1943:385). Combination control was effective at reducing habituation by gullsbut was not effective at reducing habituation by corvids.The addition of lethal control had a substantial effect ongull numbers. Although numbers did not decline due toculling, fewer gulls visited or remained near the site as thetrial progressed, which contrasts markedly with the findingsof Dolbeer et al. (1993), where the use of lethal deterrenceagainst gulls at JFK resulted in large numbers being shotannually. At JFK if the only feeding resource available togulls was located on the opposite side of the airfield to theirbreeding colony, gulls were left with no choice but tocontinue crossing the airfield to provision-dependent young.Our study, however, focused on one feeding area in winter.Gulls are known to travel long distances between roostingand feeding sites in the United Kingdom (Horton et al.1983), and in our study they were not tied to a nearbybreeding colony. Gulls were therefore presented with achoice of remaining on the landfill site, and risking beingshot, or moving to other feeding sites.Over several years, the eventual death toll at JFK wassignificantly more than the total number of gulls present inthe area. In our study, however, most gulls killed (84.6%)were removed in the first 2 weeks of control. It ishypothesized that, in the landfill environment where birdsare focused on one area, gulls would have witnessed theoutcome of shooting activity. Gulls would therefore havebeen able to associate the sound of blank or live rounds witha potentially lethal outcome, which agrees with the findingsof Baxter and Allan (2006), where gulls responded equally tohunting falcons (that caught and killed target birds) and lurefalcons (that were used to simulate the flight of a huntingfalcon but never caught birds). Gulls at JFK were dispersedover the length of the airfield while in transit betweenbreeding and feeding

sites. Most birds may not, therefore,

Table 2. Number of live and blank rounds fired per week during a 7-week period in January-February 2001, and the mean hourly counts of gulls and corvidsat a landfill site inDevon, United Kingdom.Week of No. live No. blank Total no. of Mean no. No.Mean no.o.

trial rounds fired rounds fired rounds fired gulls seen/hr gulls shot corvids seen/hr corvids shot13 188 164 352,433.90 76 176.20714 218 245 463,597.64 39 175.36015 154 139 29387.67 4 271.00316 156 175 33184.77 10 236.00117 88 99 1876.08 2 307.46518 98 93 1917.00 0 306.92319 62 82 14446.14 5 395.43 9

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have witnessed the killing of other individuals. Without thisvisual confirmation of the effect of lethal control, birdswould not have been able to learn of the risk posed by a loudacoustic report.

The results of combination control on gulls contrastedmarkedly with results achieved against corvids. After aninitial decline, corvid numbers returned to precontrol levelsby the end of the combination period despite 52.7% beingshot. The effect against corvids was similar to that ofshooting and poisoning of galah {Cacat?a roseicapilla) andsulfur-crested cockatoo {C galerita) populations in NewSouth Wales, Australia. Shooting was revealed to havecaused significant local population declines (Ford 1990).Several factors could account for this higher proportion ofthe population being removed in comparison with gulls.Corvids in the United Kingdom are resident birds and donot migrate inwinter (Gregory andMarchant 1996). Thedecline in gull numbers in response to control would havepresented corvids with reduced competition at the landfillsite. Corvids may therefore have elected to spend longerperiods on the landfill site due to the increased availability ofresources. Unlike gulls, which regularly travel to an areapurely to feed on a landfill site (Horton et al. 1983), corvidsare likely to be present in the local environment despite thepresence of a landfill site. Corvids routinely use agriculturalland to forage (Waite 1984). Corvids did not, therefore,need to disperse significant distances from the landfill sitewhen control was implemented. Unlike gulls, little energeticoutput would then be required to return to the landfill site togain potentially significant foraging benefits.Corvids are classified as a pest species in the UnitedKingdom and can be shot under a general licence systemthat permits landowners to remove corvids if they arecausing agricultural damage. Corvids are likely to have hadprior experience of shooting and could, therefore, havelearned to avoid gunshot. Airports around the worldregularly experience groups of resident corvids that seemto be able to avoid bird controllers (Civil Aviation Authority2007). In a landfill environment, resident birds thatmaintain a safe distance from bird control staff could haveacted as lures to other less experienced birds that then moveon to the landfill and are shot.

Management ImplicationsLethal control provides a practicable mechanism forreducing habituation by gulls to nonlethal techniques atlandfill sites without needing to introduce high kill rates. Itdoes not seem to reduce habituation by corvids. Werecommend that lethal control be implemented in combination with blank rounds throughout daylight hours until

gulls demonstrate a response to blank rounds alone. If gullsbegin to ignore blank rounds, birds should then be shot untilthe response to blank rounds resumes. Implementing such asystem is likely to result in resource savings in terms of the

number of rounds used, reduced adverse publicity in relationto kill rates, and staff cost savings. Landfill site staffequipped with blank firing pistols, for example, couldundertake most deterrence activities, whereas staff with

specific shooting qualifications would only be required on alimited basis.AcknowledgmentsThis study was undertaken through ENTRUST funding viathe United Kingdom governments landfill tax credit schemeand administered by the North West Environment Trust.Viridor Waste Management, Biffawaste, and Caird Bardon

sponsored the trial at their landfill sites and Natural BirdControl staff implemented bird control. N. Dixon undertook fieldwork and A. Robinson provided useful commenton an earlier draft. We particularly thankM. Swiss and stafffrom Heathfield landfill site, without whom this work couldnot have been undertaken.

LITERATURE CITEDAnning, R. 1998. Knowledge of species biology is key to urban pest bird

management. International Pest Control 40:11-13.Aylward, M. F. 1995. Bird control at landfill sites. Pages 829-834 in

Proceedings of the 5th International landfill symposium. EnvironmentalSanitary Engineering Centre, Sardinia, Cagliari, Italy.

Baxter, A. T. 2000. Use of distress calls to deter birds from landfill sites nearairports. Pages 401-408 in Proceedings of the 25th InternationalBirdstrike Committee Conference, 17-21 April 2000, Amsterdam, TheNetherlands.

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