COMPETITION IN THE AIR: BIRDS VERSUS AIRCRAFT

The first known aircraft fatality that was directly attributable to a bird occurred in 1912, when a gull (Larus sp.) was caught in the control cables of an aircraft, causing it to crash. Since that time, aircraft have generally increased in size to carry more passengers. Bird–aircraft conflicts are becoming more common recently, which is possibly due to increased numbers of both aircraft (e.g. an estimated 28 million jets now take off in the United States as compared to 18 million in 1980) and some kinds of bird species (e.g. Canada Geese [Branta canadensis], in the United States have quadrupled to 2 million since 1985). Between 1990 and 1998, there were an estimated 22,000 bird–aircraft collisions in the United States, which cost an annual $400 million in aircraft repairs. This bird–aircraft conflict takes place around the world, although the species, situations, and severity differ. It is estimated that at least 350 people have been killed in bird–aircraft collisions worldwide. Understanding bird–aircraft conflict is critical due to monetary reasons and the potential threat to human life. Despite the severity of the situation, bird–aircraft conflict has largely remained on the fringes of rigorous ornithological investigations, and sound ornithological understanding is still required to find long-term management solutions for that conflict. I hope that this review will stimulate ornithologists to show more interest in this crucial issue. On average, the aircraft of the U.S. Air Force incur 2,500 bird strikes annually (Lovell 1997). Out of those, one human death occurs per 2,000 strikes (Neubauer 1990). Most air crashes occur when a bird hits the windshield or is inducted into the engine. In terms of civilian aircraft, over 5,000 bird strikes were reported in the United States during 1999 alone. Between 1950 and 1999, 286 serious bird-related accidents of military aircraft (in which the aircraft were destroyed or there were fatalities) occurred in 32 countries. Of those accidents, 63 were fatal, which resulted in 141 deaths (Richardson and West 2000). These bird-strike incidents, at least in some cases, are minimum estimates because pilots only report 20 to 30% of actual strikes (Burger 1985). Pilots are thought to underreport bird strikes either because they are unaware of the strikes or because of the inconvenience of filing reports (Solman 1978, Linnell et al. 1999, Brown and Hickling 2000). Sometimes, strikes by large bird species (>350 g) such as Brahminy Kites (Haliastur indus) and Cattle Egrets (Bubulcus ibis) go unreported by pilots (N. Sodhi pers. obs.). Therefore, runway carcass searches must supplement pilot reports to correctly evaluate the bird threat at airports. The cost of repairing an aircraft damaged by a bird strike can vary from very little to millions of dollars when an aircraft is lost. The aircraft component that is most frequently damaged by bird strikes is the engine. International Civil Aviation Organization's (ICAO) analysis shows that bird strikes damaged 200 engines on or near airports around the world in 1996. The cost of repair due to bird ingestion can range from $250,000 to $1 million, depending on the type of engine. However, there have been cases in which the cost of aircraft repair has been as high as $6 million, as was the case for an Air France Concord that was struck by a number of Canada Geese in 1995 on approach to the John F. Kennedy International Airport in New York City. It is predicted that bird–aircraft conflict will become costlier due to the plans for increased numbers of wide-bodied jets in the air (Robinson 2000). The cost of the bird management program at the Christchurch International Airport in New Zealand is about twice that of repairs to aircraft that are damaged by bird strikes. However, that does not include the costs of lost flight time, passenger disruption, and passenger safety (Chilvers et al. 1997). Annually, aircraft spend 461,000 h on ground in the United States due to bird strikes (Cleary et al. 1999). The cost of bird strikes in terms of human morbidity and mortality has not been rigorously investigated (Neubauer 1990). One human fatality can cost up to $2.5 million. Other studies show that bird management actions have halved the cost of repairs to aircraft that are damaged by birds (e.g. Solman 1973). Military aircraft are usually more vulnerable to bird strikes than civil aircraft because they typically travel at high speeds at low altitudes (30 to 300 m), where most birds fly. Approximately 54% of the bird strikes on military aircraft and 90% of those to civil aircraft around the world occur in or near to airfields (e.g. during take off) (Smith 1986, Neubauer 1990, Cleary et al. 1999). However, those figures should be viewed with caution because bird strikes en route can go unreported. Military aircraft are also vulnerable at bombing ranges where pilots do not always adequately detect approaching birds (Neubauer 1990). The number of reported bird strikes on military aircraft in the United States increased steadily between 1974 and 1987. However, that could have been due to heightened pilot awareness of the need to report collisions. Thus, bird–aircraft collisions are not uncommon and can result in loss of life and high costs. Airfields can provide good resources (e.g. foraging and nesting sites) for some bird species (e.g. Kershner and Bollinger 1998). However, they can be hazardous habitats due to the danger of getting hit by an aircraft. The ability to avoid an aircraft may involve learning to judge the threat and flying in a manner to evade it successfully. As bird strikes typically occur four to six times per 10,000 aircraft movements, it is possible that most individual birds succeed in evading an aircraft. However, it is critical to understand why evasive behavior does not always work. Birds should typically be good at sound and color signal detection. Those abilities, however, can vary with species and individuals. How nutritional stress, parental duties, disease, and ecotoxins (e.g. neurotoxins) affect a bird's ability to evade an aircraft remains poorly understood (Kelly et al. 2000). For example, carcasses versus live individuals in airports can be compared to determine whether dead individuals have disproportionately more parasites. Therefore, exciting research avenues remain open to understand which characteristics may make individuals more likely to collide with aircraft. It is also possible that due to a lack of previous near-fatal encounters, most birds do not perceive an aircraft as a threat or potential predator. Limited evidence suggests that the amount of air traffic affects birds' evading abilities. The chance of bird strikes increases with the reduction of air traffic on a runway (Burger 1985). Birds probably get acclimatized to the lack of traffic and become less vigilant. Therefore, airport mangers must take specific action (e.g. disperse birds before resuming aircraft activity) when a runway has been inactive for several hours. Recent design improvements might have made aircraft more vulnerable to bird collisions. Due to public and economic pressure, quieter, larger, and faster aircraft have been developed. Faster and wider-bodied aircraft are struck more often by birds than are the older, narrower-bodied jets (Burger 1983). For example, birds strike 737 passenger jets less frequently than the larger 767 jets (Chilvers et al. 1997). With the wider bodied aircraft, birds have to fly twice as far to escape than they do for the older small-bodied aircraft. Perhaps birds are also unable to hear the newer, larger- bodied quieter aircraft. Engine recording playbacks have shown that the escape distance from third generation quieter jet engines is much less than older, noisier engines (Solman 1981). At least for some species, aircraft noise may have little affect on daily activities (Conomy et al. 1998a). Furthermore, it may be hard for birds to distinguish aircraft noise from background noise at airports. Species respond differently to aircraft characteristics (e.g. visual and auditory cues; Conomy et al. 1998b), suggesting that some bird species might be better at learning to avoid aircraft, but the evidence remains anecdotal. For example, American Crows (Corvus brachyrhynchos), Northern Harriers (Circus cyaneus), and American Kestrels (Falco sparverius) were not reported to strike aircraft, despite being common at the John F. Kennedy International Airport in New York City (Burger 1985). Numerous questions remain unanswered as to why some birds do not or cannot perceive the aircraft as threat. Modifications to the newer aircraft might have made them less detectable and difficult to evade. Around the world, gulls (Larus spp.) account for a majority of strikes on civilian as well as military aircraft (e.g. Van Tets 1969, de Jong 1970, Solman 1978, Burger 1985, Smith 1986, Dolbeer et al. 2000). At the Lihue Airport in Kauai, Hawaii, the body mass of birds that hit the aircraft ranges from 13 to 1,300 g (Linnell et al. 1996). Individuals of heavier bird species are more hazardous to aircraft (Dolbeer et al. 2000). The average body mass of the bird species that caused fatalities or injuries to aircraft occupants is 5.1 kg (Neubauer 1990). Several authors have suggested that disproportionately more immature individuals may be involved in aircraft strikes. Significantly more young than adult individuals of Herring (L. argentatus), Ring-billed (L. delawarensis), and Laughing (L. atricilla) gulls strike aircraft at the John F. Kennedy International Airport (Burger 1985). However, such is not the case for the Great Black-backed Gull (L. marinus). The reason for these species differences is not clear, but young individuals are probably either less capable of perceiving an approaching aircraft as a threat or less successful at evading it. All things being equal, a solitary individual will cause less damage to an aircraft than will a flock. The number of birds that strike aircraft varies with species. Usually, ducks, geese, herons, owls, and doves collide with aircraft as individuals. However, shorebirds and starlings usually hit aircraft in flocks. Numerous factors can affect bird strikes on aircraft. Below, I discuss some of the more important factors. At the Christchurch International Airport in New Zealand, bird strikes peak at midmorning (0900), and there is another, smaller peak at night (2000) (Chilvers et al. 1997). However, strikes by sparrows peak at about 0800, whereas those by gulls peak at midday. At the John F. Kennedy International Airport, most gull strikes occur between 0500 and 0900 (Burger 1985), but non-gull strikes do not show any diurnal peak time. Although approximately 10 to 17% of bird strikes can occur during night (Neubauer 1990, Satheesan and Grubh 1992), nocturnal birds are generally ignored in bird strike monitoring and control. That may be partly due to difficulty in sampling nocturnal birds and, in some cases, difficulty in accurately assigning the timing of strikes to bird carcasses that are found at airports. For the U.S. Air Force aircraft, 61% of bird strikes occur during clear weather, when both birds and aircraft are more active (Neubauer 1990). To save energy, migratory birds usually use tail wind to fly. However, wind speed does not significantly affect bird strikes (Manktelow 2000). There is a positive correlation between bird strikes and mean monthly rainfall at Lihue Airport (Linnell et al. 1996). That correlation is probably because of increased seed production along the runways during the rainy months, which attracts granivorous birds. Similar results have been found in the United Kingdom (Manktelow 2000). The chance of a bird strike is 5× higher during the migratory season than at other times (Jerome 1976). Bird strikes with the U.S. Air Force aircraft usually peak coinciding with the spring and fall migration (Neubauer 1990). Other authors have reported similar results (e.g. Blokpoel 1976). A large number of fatigued birds probably results in more bird strikes during migration. Heightened pilot awareness during the migratory season may also be at least partly responsible for more reporting. More bird strikes occur in April than at any other time of year at the Christchurch International Airport. That is the time when fledglings are abundant, and they are possibly less successful at evading aircraft (Chilvers et al. 1997). Approximately 90% of bird strikes occur <1,500 m above ground, but there are records of bird strikes at altitudes >2,000 m (Satheesan 1990). For military aircraft, 56% of bird strikes occur at <300 m above the ground (Neubauer 1990). Jerome (1976) makes a number of recommendations for pilots to minimize bird strikes. They include scanning the skies before take off, avoiding taking off into the sun, switching the aircraft lights on in areas of high bird concentration, keeping the windshield heat on to withstand a greater impact force, and maintaining lower safe airspeeds. Above all, Jerome recommends that pilots should report bird sightings and suspected and actual bird strikes to control towers. Hence, pilot vigilance can prevent some, but not all, bird strikes. Aircraft speed is a major factor in crashes due to bird strikes (Niering 1990). That is because the kinetic energy that is dissipated during a bird strike increases with the aircraft speed. There is probably no jet engine in the world that can ingest as large a bird as a Canada Goose and still fly (Eschenfelder 1990). Based on bird-strike data, efforts are underway to improve aircraft so that they can withstand a greater impact (Niering 1990). Those efforts include new material designs for aircraft engine compressor blades, stronger windshield design, and more damage-resistant wings. For military aircraft, windshields need further strengthening modifications, and some of the older aircraft are probably still vulnerable during bird strikes (Neubauer 1990). Previous lessons are sometimes taken into account when making recommendations to improve aircraft design. When a DC10 remained in the air for 10 min after two of its three engines were hit by birds in 1973, the Bird Strike Committee of Europe recommended that European airbuses should have three engines instead of two (Solman 1978). The current engine certification standards remain vague and are primarily based on the amount of bird flesh ingestion rates. Artificial birds should be used for such tests because it is a humane course of action, and will probably assist in standardization across different aircraft manufacturers. However, existing bird use only body mass with little such as and is now underway to a better bird for aircraft engine and 2000). There still is a need to further the so that more aircraft are bird strikes were not However, in a aircraft at the International Airport in European into three of its four The aircraft and of the people on A review of bird strike at that time and a lack of in its In the to bird-strike reports from With the of the bird strike in bird strike 1998, on bird strikes were from and In the United airport is required to report bird strike that result in damage or However, to bird strike in the United States is have been used to the pilots and flight of potential bird has been used to bird and the (Solman et al. 2000). In the the Bird Strike of the U.S. Air Force a bird of bird pilots with on the specific and times of high bird the United has been used by the since its in A shows that can be in bird on (Lovell and Dolbeer 1999). for the United States is now on the with in for different times of the (e.g. and and similar monitoring are being used or in other areas of the world et al. et al. 2000). However, the are primarily based on bird migration need to be the migratory behavior of as has been predicted 2000). species are hazardous to aircraft. Dolbeer et al. (2000) the species to potential to aircraft in the United This was so that not and in the species. As heavier bird species such as and were more hazardous to aircraft than species such as sparrows and However, that should be viewed with caution because it not take behavior such as into A of birds can cause greater damage to an aircraft than can a In to the bird the of the is the to in the bird at airports. A course is for military and civilian airport in the United The course bird and recording and analysis 2000). The bird with similar There has been a that resources are for and bird–aircraft conflict et al. 2000). and avoiding the bird threat to aircraft are critical and more research is to the existing and the is important so that management is The recommends that airports should take to both and the bird to aircraft. airport has its specific bird that on the bird species and the and the airports. Hence, a management that across airports is not Bird management can be into or long-term action (e.g. the of and management so that airfields and become less for birds. One of the with control action has been Birds usually become to bird four to six and numbers in 2000). The other is that birds usually in an before and after the use of The of birds and has been at airports 1976). However, bird can also because more hazardous individuals that are about aircraft might That may be but the of can still be have sometimes been used to the bird For example, are being used to birds from airfields (e.g. International Airport at 1999). an at the International Airport in Canada resulted in a reduction of bird numbers over a year 2000). in (Falco and have been used to birds from airports during (Solman 1973). However, that has for airports that have during the night and during In the United airports are recommended to at 20 high (Smith That is based on the that most birds are to in areas because of difficulty in scanning for However, such can result in a high of which can birds of Airport in the United Kingdom has been due to numbers of Canada Pilots have been of these and around this and other airports are being to make less habitats for that species. Canada Geese have also been causing at and there is now a program underway to and the of have been used to the bird The International Airport in the United Kingdom in the on runway to prevent from on to the runways and birds (Smith However, the use of is not recommended due to possible such as and and 1990). For increased a of both and long-term management actions is used at some airports. At the John F. Kennedy International Airport, as as bird strikes occur of which over are caused by gulls (Dolbeer et al. Bird strikes between and 1990 increased more than The possible reason for that was that in the during the Laughing Gull numbers increased from to The of gulls in and resulted in a to reduction in gull strikes. A that to understand the factors high number of Laughing was also at that airport and The that Laughing were primarily to the in the areas of the action was recommended to such as areas and and to Similar have been (e.g. is to its to bird species and 1990, et al. 1998). At the John F. Kennedy International Airport, two the runways between and The on those are with such as bird to disperse birds (Burger 1985). active management action is being taken by that provide for and the of that management has also been at Airport in the United and at Airport in The (Smith 1986, Van action bird strikes in at Airport However, that was not the case at the Christchurch International Airport, where of the airport successful management (Chilvers et al. 1997). Due to large human high species and the bird at airports is usually and its management is For example, a that was at the International Airport in found that the of as well as and the airport and areas and birds of and management difficult due to the of into airport for activities such as and The management of in shows that management can still be possible in countries. Annually, million is in to repair aircraft that are damaged by numbers can be significantly by carcasses and 200 of civil and military (Satheesan and Satheesan 2000). and can use some of the recommendations at of airport to birds. For example, the of can be to nesting birds such as can be or can be so that they do not become for and It is much better to evaluate the bird before a for an as has been in et al. 2000). and near airports should during the of are involved in the bird threat at those airports. As shown management can the bird at airports. The that has made in understanding bird and can be for some bird species by them with resources such as and nesting Those birds can be hazardous to and aircraft. To or that airports around the world should have rigorous bird monitoring and management should also because not hazardous birds are to airports. airports may have different depending species and Hence, although a management may not be there is a need to a rigorous for bird to aircraft. There is also a need for better in to those in countries. The can be an for such that en route bird may have to be should there be in bird and migratory The to ornithologists more in bird its aircraft. ornithological may be required to or bird to aircraft. Great for ornithological research in the of bird–aircraft conflict and more will be into that conflict. With this I in no to that should have air over birds. Birds the for to aircraft, and now a understanding of could be the most in bird–aircraft collisions. I Smith for to this I a at the was I for during the and for and For more on the bird Bird for the United States is at More on bird strikes can be found at the International Bird Strike Committee Air Force The International Civil Aviation Canada Civil Aviation Canada and de Canada Civil Aviation United Kingdom Bird Strike Committee Bird Strike Committee Bird Strike Committee The Aviation

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