U.S. airports as (undesirable) zoos

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U.S. airports as (undesirable) zoos

Master Essay, 19-08-2017

Marloes Kosse S1887335

Supervisors:

R.F. Storms, MSc Prof. dr. C.K. Hemelrijk

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Abstract

Aircraft collisions with wildlife are a serious economic and safety problem worldwide. According to the FAA National Wildlife Strike Database, the number of reported strikes with U.S. civil aviation has increased from 1,847 in 1990 to a record of 13,795 strikes in 2015. This increase in wildlife strikes is primarily associated with an increase in air traffic. To decrease the economic and safety problem of aircraft collisions with wildlife, it is important to explore how wildlife hazards in aviation can be reduced. The aim of this study is to examine (1) what attracts wildlife on and near U.S. airports, (2) which hazardous wildlife species live on and near U.S. airports, and (3) how wildlife strikes can be prevented. Airports are often attractive to wildlife because airport habitats provide them their three primary needs: food, water and shelter. According to the FAA National Wildlife Strike database, the most hazardous species of U.S. civil aircraft are the Canada goose (Branta Canadensis), white-tailed deer (Odocoileus virginianus), snow goose (Chen Caerulescens), red-tailed hawk (Buteo jamaicensis) and bald eagle (Haliaeetus leucocephalus). The hazardous of wildlife can be determined by the probability of strikes happening again in the future and the severity of the strikes. The severity depends on the body mass and flocking behaviour of the species. The probability depends on the location, season and time of day. Wildlife strikes can be reduced by habitat modification. For example modification of water resources, grassland, agriculture and the placement of fences. Mammals can be excluded by fences, but the ability of birds to fly makes excluding them very tough. Active repellents in terms of auditory repellents, visual repellents and chemical repellents are used to reduce bird strikes. In conclusion, to reduce wildlife strikes in aviation effectively it is recommended to locally determine the attractors of hazardous wildlife on and near specific airports, identify the hazardous species, and carefully consider the methods to control their presence. To obtain reliable and precise results a local reporting plan needs to be initiated in order to generate a comprehensive database.

1. Introduction

Aircraft collisions with wildlife are a serious economic and safety problem worldwide (FAA 2016;

Thorpe 2003). People, especially airport personnel, have been aware of the risk wildlife strikes pose for a long time. A bird strike on 15 January 2009 increased the public awareness to the issue. On this date an Airbus 320 ingested Canada geese in both engines on a height of approximately 880m AGL (Above Ground Level). Both engines failed, resulting in an emergency landing in the Hudson River.

Although all 155 people on board survived 15 people needed a hospital treatment. The Airbus was total loss; due to the severe damages it was too costly to repair (CNN 2016; FAA 2016b; Marra et al.

2009).

The Federal Aviation Administration (FAA), an operating mode of the U.S. Department of Transportation, conducts research to ensure that U.S. commercial and general aviation is safe (FAA 2017). The FAA maintains a database of wildlife-civil aircraft strikes. This National Wildlife Strike Database is developed from incidents that are voluntary reported by pilots and other airport personnel (Crain et al. 2015; FAA 2016b). Figure 1 shows the number of strikes reported yearly to the FAA. The number of reported strikes has increased from 1,847 in 1990 to a record of 13,795 strikes in 2015 (Figure 1; FAA 2016b). This increase in wildlife strikes is primarily associated with an increase in air traffic, but also with the fact that modern aircraft are larger, faster and quieter (Blackwell et al. 2009;

Cleary & Dolbeer 2005). Since 1988 wildlife strike have destroyed more than 247 aircraft and killed over 262 people worldwide (FAA 2016b).

It is estimated that wildlife strikes cost the civil aviation industry worldwide more than 1.4 billion dollars annually, with U.S. costs being estimated to be more than 600 million dollars per year. (Biondi et al. 2011; DeVeault et al. 2017; FAA 2016b; Swaddle et al. 2016). These costs arise from aircraft repair

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4 and aircraft downtime which result in costly delays and cancellations (Allan & Orosz 2001; Anderson et al. 2015).

Figure 1. Number of reported wildlife strikes with civil aircraft, USA, 1990-2015. The total 166,276 strikes involved birds (160,894), terrestrial mammals (3,561), bats (1,562), and reptiles (259). Source: FAA (2016).

To decrease the economic and safety problem of aircraft collisions with wildlife, it is important to explore how wildlife hazards in aviation can be reduced. This study examines (1) what attracts wildlife on and near U.S. airports, (2) which hazardous wildlife species live on and near U.S. airports, and (3) how wildlife strikes can be prevented.

2. Attractants to wildlife on and around U.S. airports

Airports are often attractive to wildlife because airport habitats provide them their three primary needs: food, water and shelter (Harris & Davis 1998; DeVault et al. 2008; DeVault et al. 2017; Dolbeer et al. 2000; FAA 2007). It also provides space for loafing, perching, resting, reproduction, assembly and escape (ACI 2005; FAA 2007).

Water

Water resources attract wildlife because they provide breeding, loafing, roosting, drinking and foraging sites to birds and mammals (DeVault et al. 2008; DeVault et al. 2009; Transport Canada 2004). Multiple studies show that hazardous bird species to aviation are associated with water. Based on FAA National Wildlife Strike Database 1990-2009, 10 of the 15 most hazardous bird species groups are strongly associated with water (DeVault at al. 2011). In addition, based on the FFA National Wildlife Strike Database 1990-2007, 6 of the 8 bird species that are categorized in the extremely high hazard category, are associated with water (Dolbeer & Wright 2009). An inevitable water resource at airports is storm water; after a rain shower, airports move water away from runways, taxiways and aprons to ensure the safety of aircraft operations (Blackwell et al. 2008). The water runoff will go to detention ponds, which hold water for a maximum of 48 hours, or to retention ponds, which are permanent pools of water (Blackwell et al. 2008; Blackwell et al. 2009). Although these ponds are needed for a safe runway, safety is reduced due to the attraction to wildlife (Blackwell et al. 2013). Besides the controlled ponds,

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5 little ponds can occur in un-managed poorly drained places (DeVault et al. 2017; FAA 2007). Other water resources like natural waterbodies (e.g. lakes) and wetlands (e.g. marshes) near airports are also attractors to wildlife (DeVault et al. 2009; DeVault et al. 2017; Servoss et al. 2000).

Food

Wildlife can also be attracted to environments due to their food availability. The suitability of a site for foraging depends on the species of wildlife: natural vegetation and agricultures might be preferred by animals with an herbivorous diet (DeVault et al. 2008; Washburn & Seamans 2012). The availability of prey can be an attractor for predatory species (Crain et al. 2015). It has been common for airports to be surrounded by grasslands, which are attractive for birds and mammals to graze on (DeVault et al.

2008; Washburn & Seamans 2012). The availability of rodents such as mice and squirrels in these mature grasslands can in turn support the presence of predators like coyotes and raptors (Blackwell &

wright 2006; Crain et al. 2015; Transport Canada 2004). There has been a trend of replacing these grasslands with agriculture, which also provides numerous food sources to wildlife in terms of agricultural crops like corn, soybeans, grains and cereal (Baxter & Robinson 2007; Biondi et al. 2011;

Jorde et al. 1983). Garbage is also a major (urban) food source on and around airports. Wildlife, for example gulls and foxes, enjoy having an easy obtained meal from trash cans or disposal sites (Servoss et al. 2000; Solman 1978; Transport Canada 2004).

Shelter

Having shelter is important to many behaviours of wildlife. Animals need cover for roosting, perching, nesting, escaping and foraging (DeVault et al. 2017). Airports and their environment provide both natural and urban cover for wildlife. Examples of urban shelters are airport buildings, parking garages and hangars (DeVault et al. 2017; Khalafallah & El-Rayes 2006; Wright et al. 1998). Examples of natural shelters are trees, shrubs, wildflowers and land covers like agriculture, forest patches and grassland (Blackwell et al. 2013; DeVault et al. 2017).

3. Hazardous wildlife on and near U.S. airports

A variety of animals live on and around airports. More than 680 wildlife species are reported in the National Wildlife Strike Database of 1990-2015 (FAA 2016b). However, these species are not equally hazardous to aviation.

The relative hazard of wildlife species to aircraft depends on the probability of strikes happening again in the future and the severity of the strikes (ACI 2005; Allan et al. 2003). Figure 2A and 2B show how the probability and the severity of the strikes are defined. The probability depends on the average number of strikes per year, based on airport data. The severity depends on the percentage of strikes causing damage, based on national data. Figure 2C shows how the severity and the probability determine the potential hazard of wildlife.

The FAA National Wildlife Strike Database of 1990-2015 contains information about strikes per species (FAA 2016b). Note that this is done for all U.S. civil airports resulting that the probability can not be determined by the method of Figure 2. As shown in Table 1 these species can be ranked by total number of strikes, number of strikes with damage and costs resulting from strikes. Appendix I contains more extensive versions of this table presenting information about percentage of strikes with damage, bodyweight, number of strikes with multiple animals and severity category.

There is a positive correlation between body mass and severity (Dolbeer et al. 2000; DeVault et al.

2011). This may explain why smaller species like the European starling, mourning dove, killdeer, American kestrel, and barn swallow have the most total strikes, but a (very) low severity. Visa versa, species like the snow goose, turkey vulture, and bald eagle have relatively fewer strikes, but are

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6 classified in the extremely/very high category of severity (Appendix I). The species shown in Appendix I that classified in the (very) low severity category have a weight of 0.181 kg and less, while species that belong to the (extremely/very) high severity category weigh 1.043 kg and more (Appendix I;

Transport Canada 2004). DeVault et al. (2011) found that body mass and hazard level only held for wildlife with a body mass less than about 1 kg, while in this study the American kestrel (109-118 g) is classified in the very low severity category and the lighter European starling (77-95 g) is classified in the low severity category (Appendix I; Transport Canada). These outliers can be explained by the flocking behaviour of the European starling. In total 34% of European starling strikes were with multiple animals, while only 4% of American kestrel strikes were with more than one animal (Appendix I). This corresponds with the finding that aside from body mass, flocking behaviour is also a determinant of severity level to aircraft (DeVault et al. 2011; Dolbeer et al. 2000; Dolbeer &

Eschenfelder 2003).

Figure 2. Potential hazard of wildlife. The probability of strikes happening again in de future (A) and the severity of the strikes (B) determine the potential hazard of wildlife (C). The red colour indicates that the current residual risk requires further action to reduce it. The orange colour indicates that the current residual risk requires a review of available options and possible action. The green colour indicates that no further action is required beyond measures currently in place. Sources: ACI (2005);

Allan et al. (2003); Dolbeer & Wright (2009).

In this part of the paper, the focus will be on the species from the top 5 ranking of wildlife by costs resulting from strikes, because these costs cover both probability and severity. The higher the probability and/or severity the higher the costs; cost represent the relative hazard of species to aviation.

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7 Total strikes Total strikes with damage Reported costs ($)

Rank Species strikes Species

Strikes with damage

Species Reported

costs ($) 1 Mourning dove 7.566 White-tailed

deer 851 Canada goose 127.119.594

2 American kestrel 4.550 Canada goose 781 White-tailed deer 45.749.554 3 Killdeer 4.509 Turkey vulture 325 Snow goose 32.518.339 4 Barn swallow 4.105 Red-tailed hawk 323 Red-tailed hawk 27.637.113 5 European starling 3.930 Rock pigeon 247 Bald eagle 26.352.206

Table 1. Top 5 ranking of wildlife species by total number of strikes, total number of strikes with damage and costs resulting from strikes reported to the National Wildlife Strike Database of 1990-2015. Source: FAA (2016).

Canada goose (Branta Canadensis)

The National Wildlife Strike Database of 1990-2015 contains 1584 strikes with Canada goose. In total 49.3% of these strikes caused damage with total costs of $127,119,594 (Appendix I). One of these strikes caused 2 human deaths and another 15 strikes caused in total 117 injured humans (FAA 2016b).

With their weight of 3.6-4.5 kg they can cause severe damage to an aircraft, especially when they fly in a flock (Rudlege et al. 2015; Transport Canada 2004). Strikes between Aircraft and Canada geese possibly occur due to the overlap in fly altitude during landing and take-off (Rutledge et al. 2015;

Transport Canada 2004). For example, Canada geese around Piedmont Triad International Airport (Greensboro, North Carolina, USA) flew at a mean altitude of 17.2m Above Ground Level (AGL) with a maximum recorded altitude of 63m AGL (Rutledge et al. 2015).

Striking risk of Canada Geese varies per season. During the moult period (1 June - 15 July) the chance of geese being involved in aircraft strikes is very small. After the moult period Canada geese move considerably more, which increases the strike risk (Rutledge et al. 2015). During autumn geese migrate from the cold north to warmer southern areas (Bruggink et al. 1994; Tacha et al. 1991). Throughout their journey Canada geese concentrate in farm fields, wetlands, and nature reserves for foraging and resting (Bruggink et al. 1994; Tacha et al. 1991; Transport Canada 2004). Airports can provide these foraging and resting areas which increases the chance of aircraft collisions during migration (Baxter &

Robinson 2007; Biondi et al. 2011; Blackwell et al. 2008; Bruggink et al. 1994; DeVault et al. 2008; Jorde et al. 1983; Kaminski & Prince 1977). This shift of Canada geese populations to southern areas may result in a smaller strike risk around northern airports during winter. During spring migration when Canada geese return to their habitats, there is again an increased risk for strikes with aviation (Bruggink et al. 1994; Tacha et al. 1991). After spring migration the nesting and breeding season starts (Rutledge et al. 2015). Canada geese nest and breed on islands in open water habitats which can be available near airports (Kaminski & Prince 1977; Naugle et al. 1997; Ness & Klaver 2016, Owen & Black 1990;

Rutledge et al. 2015).

Most movements of Canada geese occur within the first 2 hours after sunrise and around sunset. This means that aircraft experience the highest risk on collisions with Canada geese during dusk and dawn (Figure 2). At daytime Canada geese fly in flocks to nearby feeding grounds (Owen & Black 1990). Most

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8 Canada geese fly to grasslands and fields with agricultures like cereal and rice to forage which are common on and near airports (Baxter & Robinson 2007; Naugle et al. 1997; Nichols 2014; Rutledge et al. 2015; Washburn & Seamans 2012). At night Canada geese rest at mudflats, for example in wetlands (DeVault et al. 2017; Owen & Black 1990).

White-tailed deer (Odocoileus virginianus)

The National Wildlife Strike Database of 1990-2015 contains 1016 strikes with white-tailed deer. One of these strikes caused a human death and 19 other strikes caused in total 27 human injuries. Due to their heavy weight (40-180 kg) 83.7% of these strikes caused damage with total costs of $45,749,554 (Appendix I; FAA 2016b). White-tailed deer live in herds so strikes with more than one deer occur (Appendix I; Lagory 1986). Most strikes occur during landing roll, followed by take-off run.

White-tailed deer are most active during autumn, when deer are on the move because of the rut (Beier

& McCullough 1990; Biondi et al. 2011; Hawkins et al. 1971; Wright et al 1998). This increases the strike risk with white-tailed deer (Figure 3B). White-tailed deer are cautious animals, yet are known as the most frequently struck mammals at airports. When they get surprised by noise and caught in landing lights of an oncoming aircraft their natural behaviour is to freeze which can result in strikes (Transport Canada 2004). White-tailed deer are most active during dusk and night. Accordingly aircraft collisions with white-tailed deer mostly occur at night and most incidents per hour occur during dusk (Figure 3D;

Biondi et al. 2011). White-tailed deer prefer to use areas with open vegetation during the night and areas with closed vegetation during daytime (Beier & McCullough 1990). Most of their time is spent to foraging (Beier & McCullough 1990; Lagory 1986). White-tailed deer are herbivourus and eat grass, forbs, and leaves, which makes the airport including its vegetated environment an attractive foraging habitat (Biondi et al. 2011; Chamrad & Box 1968; Waller & Alverson 1997; Wright et al. 1998). Figure 3C indicates that white-tailed deer prefer to live in grassland during winter, followed by closed forest.

During spring, summer, and autumn white-tailed deer live mostly in closed forest, followed by grassland. These land covers are well known on and near airports (Biondi et al. 2011; Wright et al.

1998).

Figure 2. Frequency of Canada goose movements categorized by hours after sunrise, Greensboro, North Carolina, 2008-2009. The number -1 represents the hour prior to sunrise.

From Rutledge et al. (2015)

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Figure 3. Incidents and habitats of white-tailed deer. A. Percentages of white-tailed deer incidents with U.S. civil aircraft by aircraft movement, U.S.A., 1990-2009. B. White tailed deer incidents with U.S. civil aircraft by month, U.S.A., 1991-2009. C. Use of habitat types by white-tailed deer on the George Reserve, Michigan, 1982-1983. The Dashed line indicates the availability of each vegetation type on the Reserve. D. Number of white-tailed deer incidents with U.S. civil aircraft by time of day, U.S.A., 1990-2009. Source: Biondi et al. (2011); Beier & McCullough (1990).

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Snow goose (Chen Caerulescens)

The National Wildlife Strike Database of 1990-2015 contains 130 strikes with snow geese. In total 77.7% of the snow goose collisions caused damage. Three of these strikes caused in total 3 human injuries. Even through there were relatively few strikes, snow geese still caused damage for

$32,518,339 (Appendix I; FAA 2016b). With their weight (2.3-3 kg) and flocking behaviour they can cause serious damage to an aircraft (FAA 2016b; Transport Canada 2004). Most strikes with snow goose occur higher than 152 Above Ground Level (AGL), during the climbing and descending phases of the flight (FAA 2016b). This is remarkably higher than Canada geese (Rutledge et al. 2015). The fact that snow geese, unlike Canada geese, do not forage in grassland at airports but rather in agricultural fields and marshes outside airport fences may contribute to this difference (FAA 2016b). This means that collisions with snow geese occur when they travel between foraging and loafing areas.

Snow geese start with nesting and breeding in June (Kerbes et al. 1990; Reed et al. 2003). They nest preferably in wetland habitats like wet meadows and ponds which can be available near airports (Blackwell et al. 2008; Hughes et al. 1994). In August the moulting season starts. During this period, moulting adults and their young goslings are especially vulnerable to terrestrial predators. Therefore they prefer habitats with some form of refuge such as open water during their moult (Hughes et al.

1994; Reed et al. 2003). Natural waterbodies near airports provide these geese a suitable moult habitat (DeVault et al. 2009; DeVault et al. 2017). Most non-breeders and failed nesters (nest destroyed or abandoned before hatch) in cold winter areas migrate to warmer areas before moulting. Other snow geese in cold winter areas migrate during autumn to warmer areas and return during spring (Reed et al. 2003). This indicates that the risk of airstrikes with snow geese is higher in July and during the migration periods in spring and autumn. During winter the risk of airstrikes with snow geese will be lower in northern America, because the shift of snow geese populations to southern areas (Reed et al.

2003)

Wetlands near airports provide suitable habitats for snow geese, especially when they are closely located to cropland and grassland where they can forage (Biondi et al. 2011; Blackwell et al. 2008;

DeVault et al. 2009). Snow geese fly in flocks to forage areas at dawn daily were they eat food like seed, grain, soybeans, grasses and forbs. At noon they return to water where they loaf, rest, bathe and drink. At dusk they fly again to foraging areas and after dusk they return to water to roost on water (Alisauskas & Ankney 1992; Gauthier et al. 2005). In total 75% of the strikes with snow geese occur at night (FAA 2016b). This means that the risk of aircraft collisions is highest after dawn before roosting.

Red-tailed hawk (Buteo jamaicensis)

The National Wildlife Strike Database of 1990-2015 contains 2243 strikes with red-tailed hawks.

Despite their average body mass of 1.1 kg only 14,4% of the strikes caused damage (Appendix I; FAA 2016b). However, one of these strikes caused 8 human deaths and another 7 strikes caused in total 9 injured people (FAA 2016b). In total the strikes have cost $27,637,113 (Appendix I; FAA 2016b). Most strikes (82%) with red-tailed hawks in the National Wildlife Strike Database of 1990-2004 occurred below 30.5m Above Ground Level (AGL). In total 63% of the red-tailed hawk strikes occurred on the ground at 0 AGL (Blackwell & Wright 2006).

As shown in Figure 4, most aircraft collisions with red tailed hawks occur during summer, when young hawks start with flying (Blackwell & Wright 2006; Dolbeer 2006). Blackwell et al. (2007) found population growth as a factor contributing to an increase in bird strikes with raptors. Most red-tailed hawks live in pairs and nest in habitats with upland forest, hunting areas and cropland. Their nests are isolated high in trees in forests with a lower tree density or in trees on a slope (Bernardz & Dinsmore 1984). Accessibility of water in nesting habitats results in a higher prey availability (Luttich et al. 1970).

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11 Habitats near airports contain diverse land covers like forest, grassland, and wetland which make it suitable nesting and hunting areas for red-tailed hawks.

Most strikes with red-tailed hawks occur during daytime, when they are active and mainly perch in their hunting habitat (Blackwell & Wright 2006; Leyhe & Ritchison 2004). Red-tailed hawks generally eat mammals (mainly rodents), birds and reptiles (Fitch et al. 1946). They are highly adaptable hunters.

Their food habits are adjusted to the presence and visibility of their prey (Errington 1933; Fitch et al.

1946; Luttich et al. 1970). For example, the nesting time of red-tailed hawks overlaps with the time young squirrels are abundantly available as food for the young hawks (Fitch et al. 1946). Common hunting habitats of red-tailed hawks are open areas like, non-forested marsh, pasture and farmland with enough perch sites (Bernard & Dinsmore 1984; Howell et al. 1978; Leyhe & Ritchison 2004).

Airports provide suitable perching places with high prey availability (Blackwell & Wright 2006).

Bald eagle (Haliaeetus leucocephalus)

The National Wildlife Strike Database of 1990-2015 contains 226 strikes with bald eagles (Appendix I;

FAA 2016b). In total 28.9% of the strikes with bald eagles caused damage. Four of these strikes caused in total 7 human with injuries (FAA 2016b). The strikes had costs of $26,352,206 (Appendix I; FAA 2016b). Because of their body mass (3.6-6.4 kg) bald eagles are capable of causing serious damage to aircraft (Transport Canada 2004). Information from 3 databases (1990-2013) show that about 17% of damage strikes with eagles cause damage on multiple areas of aircraft. Aircraft engines and wings were most often damaged. Furthermore Washburn et al. (2015) reported that most eagle-aircraft strikes occur at or below 30m Above Ground Level (AGL), when aircraft are landing or taking off (Washburn et al. 2015).

Bald eagles start nesting in March and breed in April and May. During the summer most young eagles remain near the nest site (Isaacs et al. 1983). Bald eagles have their breeding territories in their hunting habitat. These territories are located near water where nests are built in the top of dominant trees and

Figure 4. Strike reports of red-tailed hawks by the U.S. civil aviation industry, U.S.A., 1990- 2004. Strikes with turkey vultures, black vultures and unknown vultures are also provided in this figure as ‘Vulture’. Source: Blackwell & Wright (2006).

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12 have a high prey availability; territories that can be provided by airports (Andrew & Mosher 1982;

Garrett et al. 1993; Thompson et al. 2005; Watson et al. 1991). Some bald eagle populations migrate during winter depending on the availability of food like fish, birds and mammals (Frenzel 1984; Garrett et al. 1993; Jackman et al. 2007). These populations travel thousands of kilometres to find a habitat with food in abundance, for example garbage landfills (Elliott et al. 2006; Knight & Knight 1983).

Despite the extra activity during migrations, the frequency of aircraft strikes with bald eagles does not vary among months (Washburn et al. 2015).

When food is in abundance all day long, eagles primarily forage during early morning and late afternoon. When food is restricted eagles forage longer and during periods when most food is available (Elliott et al. 2006). Figure 5 shows that most aircraft collisions with bald eagles occur during day (Washburn et al. 2015). This is likely due to airports being included in their hunting area. Old-growth forests and wetlands near coastal and estuarine areas, like the environment of coastal airports, are common habitats for bald eagles (Garrett et al. 1993).

4. Strike prevention

To decrease the number of strikes, wildlife on airports has to be managed (DeVault et al. 2013; FAA 2016b; Thorpe 2003). Important managing techniques are habitat modification and the use of active repellents (Davis & Harris 1998).

Habitat modification

With habitat modifications the environment is made less attractive to hazardous species for aviation by mitigating and eliminating their attractants (FAA 2007). Figure 6 shows the recommended distances for habitat modification.

Figure 5. Distribution of the time of day for eagle-aircraft collisions with U.S.

civilian and military aircraft, U.S.A., 1990-2013. Source: Washburn et al. (2015).

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Figure 6. Separation distances within which hazardous wildlife attractants should be eliminated or mitigated. Perimeter A:

For airports serving piston-powered aircraft, hazardous wildlife attractants must be 1.5 km from the nearest air operations area. Perimeter B: For airports serving turbine-powered aircraft, hazardous wildlife attractants must be 3 km from the nearest air operations area. Perimeter C: 8 km range to protect approach, departure and circling airspace. Source: FAA (2007)

Some small modifications can reduce large attractants. For example the attraction to garbage can be reduced by sealed trash cans (Cleary & Dolbeer 2005). Other modifications are more complicated like the modification of water resources, grassland, agriculture and the placement of fences.

Modification of water resources

By modification of water resources the attraction and access of wildlife can be reduced. Coverage of waterbodies is the most effective way to manage relative small waterbodies such as detention and retention ponds (DeVault et al 2017). With physical barriers like wire-grid systems, nets, bird balls and floating covers the attraction of wildlife to these ponds can strongly decrease (FAA 2007; Barras &

Seamans 2002; Blackwell et al. 2013; Martin et al 1998). Each of these systems has its advantages and disadvantages. Wire-grid systems are relatively inexpensive in purchase and maintenance, but are not effective to all bird species. Although, it excludes extremely hazardous species like Canada geese and Snow geese (Barras & Seamans 2002; Lowney 1993). Nets, bird balls (7.5 cm diameter, complete coverage) and floating covers completely exclude birds, but are expensive in purchase and maintenance (Barras & Seamans 2002; Harris & Davis 1998; Martin et al. 1998; Clearly & Dolbeer 2005). Additionally, the bird balls can constitute a new hazard for aviation when they blow away on the airport runway (Barras & Seamans 2002; Martin et al. 1998). New ponds should be built following the separation distances of Figure 6 (FAA 2007). Larger waterbodies can be modified to less attractive

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14 areas for specific chosen animals. For example, open waters can be made less attractive for geese by planting emergent vegetation (DeVault et al. 2017; Owen & black 1990).

Modification of grassland

Grassland, the main land cover near airports, has multiple functions to wildlife. Tall grass is attractive to ground nesting birds and rodents as cover to predators like raptors (Barras & Seamans 2002; Hesse et al. 2010; Washburn & Seamans 2004). Short grass is attractive to wildlife like gulls as loafing and/or foraging area (Barras & Seamans 2002; DeVault et al. 2011). Therefore it is important to examine site- specific which bird species are most hazardous to determine the most effective habitat modifications.

According to wildlife discussed in this paper grass should be short on and near airports. Long grass attracts the hazardous red-tailed hawk and bald eagle because of the prey availability while Canada geese and white-tailed deer have no specific preferences (Barras & Seamans 2002; Seamans et al.

1999; Seamans et al. 2007; Washburn & Seamans 2004). Another factor that contributes to the attraction of grassland is the plant species composition. Grasses with low nutritional quality and palatability can reduce the attraction to grassland. For example, zoysiagrass is not preferred as food by Canada geese (Washburn & Seamans 2012). Grasslands with more wildflowers than grasses are less attractive to heavy bird species, which decreases the risk of bird strikes with damage (Harris & Davis 1998).

Modification of agriculture

Many U.S. airports lease parts of their land for agriculture (Blackwell et al. 2009). Agriculture fields are attractants of the three most hazardous species for aviation (Canada goose, white-tailed deer and snow goose). However, quitting with leasing has an economic effect. Especially on small airports which operate on limited budgets the extra income of leasing in needed (DeVault et al. 2009). Attractive crops like corn can be replaced by less attractive crop species. Martin et al. (2013) suggested to cultivate crops with low palatability or abundant seed resources. A suitable alternative would be biofuel crops.

Fencing

Mammalian hazards can be reduced by perimeter fencing (DeVault et al. 2008). The fences are physical barriers which prevent hazardous mammals and potential prey from entering aircraft bases (Biondi et al. 2011; Crain et al. 2015). The FAA recommends chain link fences higher than 3m with 3-strand barbed outriggers. Some species burrow, so fences should be buried 1.2 m deep in a 45° angle (FAA 2014) (FAA 2016a; FAA 2014). Due to the costs of fencing, many airports fence just essential parts of the perimeter (DeVault et al. 2008). The biggest challenge of fencing is to exclude deer, However, it seems that electric fences are the solution; excluding 99% of the deer effectively (FAA 2016a).

Active repellents

Habitat modification decreases the strike risk at airports, but is not effective to all species. Mammals can be excluded by fences, but the ability of birds to fly makes excluding them very tough (FAA 2016a;

FAA 2016b). Therefore, repellents are needed in terms of auditory repellents, visual repellents and chemical repellents.

Auditory repellents

Many wildlife species are attuned to sounds in their environment. Strange loud noises, such as caused by pyrotechnics, scare them away. Pyrotechnics are noise-making shells which are fired from shotguns and other pistols. The banging noise of the explosion scare birds away (Davis & Harris 1998; Mott et al. 1980). Despite an extra visual effect of the explosion, birds will habituate to the noise (Gilsdorf et al. 2002; Seamans et al. 2013). Using repellents with natural sounds, habituation takes more time

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15 (Davis & Harris 1998). For example bird calls; many bird species make distress and alarm calls to warn their conspecifics when they are in danger (Davis & Harris 1998). By recording and broadcasting these species-specific calls, birds will fly off (Gorenzel & Salmon 1993). A disadvantage of this technique is that not all species use alarm and distress calls. Additionally, since all species have their own specific call many different sounds should be broadcasted (Davis & Harris 1998). Other examples of auditory repellents are gas cannons and AV-alarm, but these are very susceptible for habituation (Davis & Harris 1998).

Visual repellents

Birds associate visual stimuli like scarecrows and guard animals with danger. Scarecrows are one of the oldest devices that have been used as local visual repellent which can scare birds away from agriculture. This relatively cheap repellent is due to habituation only useful for short-term deterrence (Davis & Harris 1998). Guard animals as (relatively cheap) repellents are not susceptible to habituation.

For example, border collies control nuisance of Canada geese. Leaving the dogs 24 hours a day for a longer period of time on the property, will effectively scare the birds away (Castelli & Sleggs 2000).

Other examples of visual repellents are reflecting tape, predator models and balloons. However, these are susceptible for habituation (Davis & Harris 1998).

Chemical repellents

Some chemical repellents are also part of habitat modification, because they make attractants less attractive. Thee agent Methyl Anthranilate is a non-toxic substance with an aversive taste for birds (Davis & Harris 1998). It can be spread in water and over food sources like grassland and crops (Belant et al. 1996; Davis & Harris 1998; Mason & Clark 1996). Disadvantages of this repellent are the high costs, it can be washed away or diluted by rainfall and it does not repel hungry birds (Davis & Harris 1998). Other chemical repellents are frightening agents like Avitrol. This toxic substance is usually added to bait which attracts birds initially. Ingestion causes disorientation and erratic behaviour followed by distress calls. This behaviour alarms other birds which will fly off. An overdose of Avitrol causes death.

5. Discussion

The aim of this study was to examine (1) what attracts wildlife on and near U.S. airports, (2) which hazardous wildlife species live on and near U.S. airports, and (3) how wildlife strikes can be prevented.

The presence of water, food and shelter attracts wildlife to airports (Harris & Davis 1998; DeVault et al. 2008; DeVault et al. 2017; Dolbeer et al. 2000; FAA 2007). These attractants include natural resources like natural waterbodies, wetlands, prey, and natural vegetation and out of men made recourses like detention ponds, retention ponds, agriculture, garbage, and buildings (Blackwell et al.

2013; DeVault et al. 2008; DeVault et al. 2009; DeVault et al. 2017; Khalafallah & El-Rayes 2006; Servoss et al. 2000; Solman 1978; Transport Canada 2004; Washburn & Seamans 2012; Wright et al. 1998).

According to the FAA National Wildlife Strike Database 1990-2015, most hazardous species on U.S.

airports are the Canada goose (Branta Canadensis), white-tailed deer (Odocoileus virginianus), snow goose (Chen Caerulescens), red-tailed hawk (Buteo jamaicensis) and bald eagle (Haliaeetus leucocephalus). The hazardous of a species can be determined by the severity of the strikes and the probability of strikes happening again (ACI 2005; Allan et al. 2003). The severity depends on the body mass and flocking behaviour of the species. The probability of strikes depends on the location, season and the time of the day (DeVault et al. 2011; Dolbeer et al. 2000; Dolbeer & Eschenfelder 2003). In this study we found that animals with a body mass higher than 1 kg (Canada goose, white-tailed deer, snow goose, red-tailed hawk and bald eagle) have a high severity. The flocking behaviour of the Canada

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16 goose and snow goose also contributes to the severity of the strikes. The five most hazardous species have the greatest risk of aircraft collisions under the following circumstances:

- Canada geese: during morning and evening hours in the migration seasons throughout spring and autumn on airports near wetland, open water, grassland and/or agriculture.

- white-tailed deer: during dusk and night throughout autumn on airports near closed forest and grassland.

- snow geese: after dawn (before roosting) during de migration periods in June, spring and autumn on airports near wetland, grassland and/or agriculture.

- red-tailed hawks: during daytime all year long on airports near upland forest, agriculture and open areas with prey.

- bald eagles: at daytime during migration periods in winter and spring on coastal airports near old-growth forests, wetlands.

Wildlife management can decrease the number of strikes (DeVault et al. 2013; FAA 2016; Thorpe 2003). Habitat modification such as modifications in water resources, grassland, agriculture and the placement of fences makes the environment less attractive to hazardous species (FAA 2007). (Barras

& Seamans 2002; Blackwell et al. 2013; DeVault et al. 2008; Martin et al. 1998; Martin et al. 2013; FAA 2007; FAA 2016; DeVault et al 2017; Washburn & Seamans 2004). Attractors are species specific and therefore it is impossible to introduce habitat modifications that make the environment less attractive to all wildlife; wildlife management should focus on the attractants of most hazardous species. Habitat modifications are not enough to reduce bird strikes; active repellents like auditory repellents, visual repellents, and chemical repellents are needed (FAA 2016b). It appears that artificial repellents such as reflecting tape and AV-alarms are more susceptible for habituation than natural repellents like alarm calls and guard animals. To prevent habituation, repellents should alternate (Davis & Harris 1998).

This study has certain limitations. Firstly, the strikes in the FAA Wildlife Strike Database are reported on a voluntary basis. As a result, the total number of incidents is most likely an underestimation.

Secondly, this study focused on all civil U.S. airports, which causes the data to be generalized and no airport specific recommendation could be presented. Nevertheless, the results of this study give an overview of the hazardous wildlife on and near U.S. airports and the possible prevention methods, providing a basis for individual assessments of airports.

In conclusion, to reduce wildlife strikes in aviation effectively it is recommended to locally determine the attractors of hazardous wildlife on and near specific airports, identify the hazardous species, and carefully consider the methods to control their presence. To obtain reliable and precise results a local reporting plan needs to be initiated in order to generate a comprehensive database.

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Appendix I

Table 2. Top 5 ranking of wildlife species by total number of strikes reported to the FAA National Wildlife Strike Database of 1990-2015. Number of strikes with damage, percentage of strikes with damage, bodyweight, number of strikes with multiple animals, costs resulting from strikes, and severity category are also provided. Source: FAA (2016); Transport Canada (2004).

Rank Species Total

strikes

Total strikes

with damage

Percentage of strikes with damage (%)

Bodyweight (kg)

With multiple

animals

Reported costs ($)

Severity category

1 Mourning dove 7,566 182 2.4% 0.091-0.181 1,165 9,113,292 Low

2 American kestrel 4,550 28 0.6% 0.109-0.118 190 2,070,127 Very low

3 Killdeer 4,509 51 1.1% 0.086-0.108 437 4,099,230 Very low

4 Barn swallow 4,105 15 0.3% 0.009-0.027 754 86,159 Very low

5 European starling 3,930 129 3.0% 0.077-0.095 1,348 7,127,433 Low

Table 3. Top 5 ranking of wildlife species by number of strikes with damage reported to the National Wildlife Strike Database of 1990-2015. Number of total strikes, percentage of strikes with damage, bodyweight, number of strikes with multiple animals, costs resulting from strikes, and severity category are also provided. Source: FAA (2016); Transport Canada (2004).

strikes

with damage

Percentage of strikes

with damage (%)

With multiple animals

Severity category 1 White-tailed deer 1,016 851 83.7% 40-180 79 45,749,554 Extremely

high

2 Canada goose 1,584 781 49.3% 3.6-4.5 662 127,119,594 Extremely

high

3 Turkey vulture 639 325 50.9% 1.5 36 12,824,279 Extremely

high

4 Red-tailed hawk 2,243 323 14.4% 1-1.2 49 27,637,113 High

5 Rock pigeon 2,899 247 8.5% 0.318-0.408 900 12,036,517 Moderate

Table 4. Top 5 ranking of wildlife species by costs resulting from strikes reported to the National Wildlife Strike Database of 1990-2015. Number of total strikes, number of strikes with damage, percentage of strikes with damage, bodyweight, number of strikes with multiple animals, costs resulting from strikes, and severity category are also provided. Source: FAA (2016); Transport Canada (2004).

strikes

with damage

Percentage of strikes

with damage (%)

With multiple animals

Severity category

1 Canada goose 1,584 781 49.3% 3.6-4.5 662 127,119,594 Extremely

high 2 White-tailed deer 1,016 851 83.7% 40-180 79 45,749,554 Extremely

high

3 Snow goose 130 101 77.7% 2.3-3 77 32,518,339 Extremely

high

4 Red-tailed hawk 2,243 323 14.4% 1-1.2 49 27,637,113 High

5 Bald eagle 226 88 28.9% 3.6-6.4 14 26,352,206 Very high

U.S. airports as (undesirable) zoos