Bird Strikes and Aircraft Fuselage Color: a Correlational Study

Bird Strikes and Aircraft Fuselage Color: a Correlational Study

Human–Wildlife Interactions 5(2):224–234, Fall 2011 Bird strikes and aircraft fuselage color: a correlational study ESTEBAN FERNÁNDEZ-JURICIC, Department of Biological Sciences, Purdue University, Lilly Hall G-302, 915 W. State Street, West Lafayette, Indiana 47907, USA [email protected] JIMMY GAFFNEY, Department of Biological Sciences, Purdue University, Lilly Hall, 915 W. State Street, West Lafayette, Indiana 47907, USA BRADLEY F. B LACKWELL, USDA/APHIS/Wildlife Services’ National Wildlife Research Center, Ohio Field Station, 6100 Columbus Avenue, Sandusky, OH 44870, USA PATRICE BAUMHARDT, Department of Biological Sciences, Purdue University, Lilly Hall G-302, 915 W. State Street, West Lafayette, Indiana 47907, USA Abstract: Collisions between birds and aircraft (bird strikes) pose safety risks to the public, cost airports and airlines money, and result in liability issues. Recent research suggests that aircraft visibility could be enhanced to increase detection and avoidance by birds. We questioned whether aircraft color scheme might play a role in bird-strike frequency. We used public records of bird strikes along with information on fl ights that were gathered by federal agencies in the United States. We estimated the bird-strike rates and compared them among airline companies using different fuselage color schemes, while controlling for aircraft type. Using an avian vision modeling approach, we fi rst corroborated the hypothesis that brighter colors would contrast more against the sky than darker colors. We found differences in bird-strike rates among airline companies with different color schemes in 3 out of the 7 aircraft types investigated: Boeing 737, DC-9, and Embraer RJ145. With each of these aircraft, we found that brighter aircraft were associated with lower bird-strike rates. Brighter fuselages might increase the contrast between the aircraft and the sky and enhance detection and avoidance behavior by birds. Our fi ndings are not conclusive but suggest a specifi c hypothesis and prediction about bird responses to aircraft with different color schemes that deserves empirical testing in the future. Key words: aircraft color scheme, antipredator behavior, avian vision, bird strike, chromatic contrast, human–wildlife confl icts Since the late 1960s, various measures Additionally, Bernhardt et al. (2010) showed have been put forward to mitigate wildlife that the distribution of injuries on a sample of collisions with aircraft , particularly on airports birds known to have been struck by aircraft (e.g., Cleary and Dolbeer 2005, Blackwell et (bird strikes) indicates evidence of anti-predator al. 2009a). Seventy-two percent of wildlife– behavior, implying that birds responded to the aircraft collisions (primarily involving birds) approaching aircraft as a threat. that were reported to the U.S. Federal Aviation The possibility of enhancing aircraft visibility Administration (FAA) from 1990 to 2008 relative to ambient light conditions depends occurred at or below 152 m above ground upon certain att ributes of avian vision. Bird level (AGL; Dolbeer et al. 2009) and within the vision is diff erent from human vision. Birds have airspace above the air operations area of an eyes whose vitreous humor allows ultraviolet airport. light to reach the photoreceptors, which have Anecdotal information and recent research 4 diff erent types of visual pigments (compared suggests that enhancing avian detection and to the 3 types found in humans; Cuthill 2006). avoidance of aircraft is possible (e.g., see review As a result, birds can perceive a wider range of by Blackwell 2002). Specifi cally, research eff orts the visual spectrum than humans. Additionally, have concentrated on exploiting avian vision, as opposed to humans, birds have oil droplets the primary sensory path for birds (Walls 1942, within their photoreceptors that fi lter light Sillman 1973) via aircraft lighting (Blackwell before it gets into the visual pigment. Oil and Bernhardt 2004, Blackwell et al. 2009b) to droplets are believed to facilitate distinguishing enhance detection and avoidance behaviors. subtle diff erences between wavelengths (Martin Findings from Blackwell et al. (2009) also and Osorio 2008). The implication is that birds indicate that ambient light conditions play a key may perceive aircraft fuselages diff erently from role in how birds respond to vehicle lighting. the way humans do. Aircraft color • Fernández-Juricic et al. 225 There are theoretical models (Endler and could be an association between fuselage and Théry 1996, Vorobyev et al. 1998) that can bird-strike rates, using a correlational approach estimate the degree to which an object stands with public records of bird strikes (see below). out from the visual background from the visual Our approach was to compare fuselage perspective of a bird (i.e., chromatic contrast; color schemes within a given aircraft type to Endler 1990). Chromatic contrast varies in reduce confounding factors, such as design, relation to the spectrum of ambient light, maneuverability, and engine capabilities. the peak sensitivities of the photoreceptors and oil droplets in the avian retina, and the Test of the hypothesis degree to which the target object and the visual We tested whether a gradient from white to background refl ect ambient light (Endler 1990). blue coloration would be perceived diff erently For instance, the chromatic contrast of the by birds through the estimation of chromatic golden-headed manakin (Pipra erythrocephala) contrast, following Endler and Mielke’s (2005) male plumage varies at diff erent heights in approach. The species frequently struck by the forest due to the incidence of light that is aircraft (e.g., Passeriformes; Dolbeer et al. 2009) absorbed and refl ected to diff erent degrees have visual systems with diff erent sensitivity by vegetation. When males display to att ract in the short wavelengths (Hart and Hunt females, they choose perching heights that 2007); therefore, we used the 2 types of avian increase chromatic contrast; whereas, when visual systems (VS and UVS) in the chromatic they try to hide from predators, they perch contrast calculations. The violet-sensitive (VS- in branches that would reduce the chromatic type) avian visual system represents species in contrast in relation to the background (Heindl which 1 cone type has the peak sensitivity in the and Winkler 2003). violet regions of the spectrum. The ultraviolet In this study, we asked whether the aircraft sensitive (UVS-type) avian visual system is color scheme might play a role in bird-strike similar to the VS-type, but the peak sensitivity frequency (as per Philiben and Blackwell 2005). of 1 cone type is the ultraviolet region of the The assumption is that fuselages diff ering in spectrum. We used the sensitivities of the visual color would have diff erent spectral properties pigments and oil droplets as noted by Endler that would be perceived diff erently by birds. and Mielke (2005): (1) VS model: VS = 412 nm, Darker aircraft color schemes (i.e., color SWS = 452 nm (oil droplet = 459 nm), MWS= 505 schemes refl ecting litt le light) could potentially nm (oil droplet = 525 nm), and LWS = 565 nm reduce the contrast between aircraft and the (oil droplet = 588 nm); and (2) UVS model: UVS visual background (e.g., sky). Therefore, darker = 367 nm, SWS = 444 nm (oil droplet = 426 nm), aircraft may potentially reduce the ability of MWS = 501 nm (oil droplet = 529 nm), and LWS birds to detect aircraft in suffi cient time to avoid = 564 nm (oil droplet = 591 nm). a strike. We then predicted that the frequency We used the Tetrahedral Avian Colorspace of bird strikes would be higher in aircraft program (Stoddard and Prum 2008) to with darker color schemes and lower in those estimate chromatic contrast. We measured with brighter color schemes. We used public irradiance (i.e., the amount of photons at each records on bird strikes along with information wavelength) and refl ectance of the background on fl ights gathered by federal agencies in the (i.e., the percentage of light transmitt ed, rather United States. We estimated bird-strike rates than absorbed, by the sky at each wavelength) and compared them among airlines with at a golf course under both sunny and partly diff erent fuselage color schemes but with the cloudy light conditions, and entered them into same aircraft type to minimize confounding the model. We used 3 objects that provided a factors associated with airframe aerodynamics. gradient from dark to bright coloration: the white of a sheet of plastic, the light blue color Methods of a plastic container, and the blue cover of a We tested our hypothesis that darker coloration notebook. We took multiple readings (range would be more diffi cult for birds to detect from 5 to 10) of irradiance and refl ectance and the background fi rst by using a chromatic averaged them. We acknowledge that these contrast model. Next, we tested whether there objects are not representative of the actual 226 Human–Wildlife Interactions 5(2) aircraft fuselage materials or color, but were 700 used only to estimate if a brighter color would Qr (X) ³ Q(O, X)Cr stand out more from the background from the 300 perspective of the avian visual system. where Qr(X) is the total photon capture at We used a Stellarnet EPP2000 portable distance X of 1 cone type, Q(λ, X) is the total spectroradiometer (Tampa, Fla.) to measure radiance spectra reaching the eye, and Cr is the refl ectance and irradiance. We recorded photon capture probability spectrum of each refl ectance every 0.5 nm (range 300-700 nm). cone class. We used a micron fi ber optic probe with a We scaled the summed Q(X) for the 4 avian tungsten krypton light source housed in a black cones types to 1 (following Uy and Endler plastic block sheath. The probe was positioned 2004). The values were plott ed in a tetrahedral at a 45˚ angle to prevent glare.

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