Testing the efficacy of deterring systems in two gull species

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Abstract

During recent decades, populations of some gull species have dramatically increased causing management problems; as a result of this, a number of deterring systems have been implemented. In this study, three commonly used scaring methods (visual, acoustic, falconry) were tested at a refuse dump. In order to evaluate the efficacy of the methods and, in particular, habituation occurrence, an index was developed that can be used for comparing such methods and for evaluating their employment in an integrated management protocol. Although the tests lasted for a few weeks, the analytical methodology revealed that the employed methods were effective only for short periods of time.

Introduction

The environmental problems caused by superabundant animal species, particularly “bird pests” have become increasingly acute during the past decades (Feare, 1991). Owing to their high ecological adaptability, their competitive behaviour and their abundance, gulls are often considered pests (Vidal et al., 1998). The feeding and roosting habits of gulls drawn by the new trophic resource available in refuse dumps are often considered a nuisance or responsible for damage to personal property, or else they affect human activities (Blockpoel, 1976, Benton et al., 1983, Belant, 1997, Raven and Coulson, 1997, Dolbeer and Eschenfelder, 2002, Rock, 2005).

A management policy to reduce access to anthropogenic resources, at least during the pre-laying period, could have an effect on the presence of gulls and on their reproductive success (Pons, 1991, Bosch et al., 1994, Kilpi and Ost, 1998, Duhem et al., 2002, Duhem et al., 2003). In the long-term, preventing access to a constant and highly predictable food source could result in a reduction in the presence of gulls in the area, favouring the reduction of economic impacts on human activities affected by gulls.

Dissuasive devices are used in integrated wildlife damage management to reduce the impacts of animals, but the effectiveness of such devices is often variable (Belant and Ickes, 1997, Blackwell et al., 2002, Baxter, 2000, Stevens et al., 2000). Non-lethal and non-chemical techniques are to be preferred in creating an environment unattractive to birds (Haag-Wackernagel, 2000). Although physical barriers can permanently exclude birds from a site, they may not be economical or compatible with the purpose or design of the facility. Visual and acoustical devices to scare and repel birds are often the methods for managing bird problems. However, the efficacy of most of these devices has not been objectively tested in the field (Stewart, 1984, Griffiths, 1988). Devices with ultrasonic (i.e., >18 kHz, the upper frequency level heard by humans) output are most appealing because they are not conspicuous or distracting and do not produce annoying sounds. The utility of ultrasonic sounds for repelling birds has no apparent biological foundation because most birds do not hear higher frequencies than humans can hear (Frings and Frings, 1967, Dooling et al., 2000). Pest birds such as starlings, Sturnus vulgaris, and house sparrow, Passer domesticus, have hearing ranges from 0.2 to 18 kHz (Brand and Kellogg, 1939, Summers-Smith, 1963), while those of pigeons, Columba livia, go from 0.1 to 10 kHz (Necker, 1983). In addition, even if birds could detect ultrasonic sounds, they generally become quickly used to sounds, and this would make the devices ineffective for long-term control (Murton and Wright, 1968, Bourdeau, 1975, Blockpoel, 1976, Murton and Westwood, 1976). Many other methods have been tested, such as eye devices (Woronecki, 1988), mylar flags (Belant and Ickes, 1997), bio-acoustic (Bridgman, 1969, Brough, 1969, Iljichev, 1986), pyrotechnics, chemical repellents (Belant et al., 1996, Belant et al., 1997a, Belant et al., 1997b, Blackwell et al., 1999), laser beams (Blackwell et al., 2002) and integrated bird hazing systems (Stevens et al., 2000).

Due to the peculiar areas where dissuasive devices can be used (such as airports, mills, landfills, crops, etc.) and to the response of different species, the efficacy of the methods should be tested in advance and calibrated in order to obtain better results. We assumed that the test of a method, or the comparison of more than one, if carried out in a highly attractive place, and conducted with standardized methodology would give important information on their efficacy and applicability. Based on data obtained from experimental work, we propose the use of two new indexes that may be combined in evaluating the efficacy of deterring systems. As refuse dumps are highly attractive foraging sites for gulls, they were selected as testing sites.

Section snippets

Animals

We studied the response of the two most abundant species of Laridae in refuse dumps in Italy: the yellow-legged gull, Larus michahellis (Naumann, 1840), and the black-headed gull, Larus ridibundus (Linnaeus, 1766). These species were chosen firstly because of their abundance in Italy and then because they are easy to test in the wild.

The test arena

Commonly used deterring methods were compared during a seven-week test period (six days per week) at a landfill in San Donà di Piave (45.63N 12.57E, Venice, Italy)

Quantitative aspects

No correlation was found between bird occurrence, temperature or input of refuse. During the whole period of 37 days, we recorded a daily average number of 556 ± 394.05 yellow-legged gulls (max 1973.75–min 125.75) and 536 ± 228.81 black-headed gulls (max 981.25–min 41.25) (Table 1). The proposed treatments affected the average daily occurrence of both species inside the refuse dump (Table 1).

Data collected were randomly ordered; movements of the two species of gulls proved to be significantly

Discussion

Our analyses indicated a slight difference between the two species throughout the testing period. The yellow-legged gull exhibited a negative trend in numbers and no correlation between inside and outside numbers, meaning that birds once scared disperse and move to quieter roosting and feeding areas. On the other hand, no trend was found in numbers of black-headed gull and no correlation between inside and outside numbers, meaning that birds move while waiting for the disturbance to stop, but

Conclusion

We suggest scaring methods should be tested in a close-to-limit-condition test field, determining the timing of habituation occurrence. This information may then be employed when preparing a dissuasive protocol including different scaring methods. Knowledge about scaring methods efficacy and bird habituation to them is essential for a more effective timetable in dissuasive protocols.

Acknowledgements

We are grateful to LIPU (the Italian BirdLife International partner), Aviotek, I. Busso, L. Panzarin and A. Ballan for participating in this research project. The comments of N. Baccetti, G. Cherubini, L. Serra, P. Bennet, J. Burge and two anonymous referees greatly improved earlier drafts of the manuscript. We would also like to thank the Hunting and Fishing Office of the Province of Venice and the Environmental Office of San Donà di Piave City Council for allowing this project to take place.

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