Raptor Electrocution: A Case Study on Ecological Traps, Sinks, and Additive Mortality James F. Dwyer*

ABSTRACT The recovery from human persecution of some upper wildlife species coupled with ongoing expansion of human-dominated landscapes is leading to increased human–wildlife interactions in urban environments. Raptors in particular are drawn to high concentrations of potential nest sites and prey, and are colonizing cities across North America. These birds are encountering novel and sometimes dangerous situations such as exposed overhead electric systems, which can operate as ecological traps, create population sinks, and lead to additive mortality. Ecological traps occur when the cues animals use to evaluate quality are decoupled from the true quality of the environment. Sinks occur in areas where mortality exceeds . Additive mortality can occur when mortality factors that are

not density dependent continue to operate proportionally as numbers decrease. Herein, I use the situation of Harris’s undergraduate education hawks (Parabuteo unicinctus) colonizing Tucson, AZ, to illustrate these concepts to students in an introductory college course. The case is deliberately loaded with the terminology of ecology, and is intended to be delivered early in the course so that as students explore the case and the vocabulary, their questions probe related material. Student inter- est and questions thus dictate the order of content delivery as students realize and seek to expand the boundaries of their knowledge. Ecology is rarely one-sided and multiple competing hypotheses should be encouraged to facilitate student transitions to higher level modes of thinking. Together, these approaches will increase student interest in detailed cover- age of the typical curriculum of an introductory ecology course.

he electrocution of raptors was first identified as a Tsignificant source of mortality in the United States Impact Statement in the early 1970s (Olendorff, 1972; Smith and Murphy, 1972; Boeker and Nickerson, 1975). Since then, biologists Birds of prey are colonizing urban around the world. and industry personnel have come to understand how and In doing so they encounter novel risks for which their evo- why raptors are killed routinely on some pole configura- lutionary history has not prepared them. This case study on tions but not on others, and have identified techniques to ecological traps uses the expansion of the Harris’s hawk pop- make those poles raptor-safe (Liguori and Burruss, 2003; ulation in Tucson, AZ, to create a compelling student-lead APLIC, 2006; Lehman et al., 2007). This work has been exploration of many topics typically covered in an introduc- conducted primarily in rural areas, but habitat selection by tory college ecology course. raptors is increasingly leading birds to settle in urban areas. For example, in North America, natal dispersing Peregrine como and Guerrieri, 2008), and booted eagles (Hieraaetus falcons (Falco peregrinus) are increasing in Los Angeles, penntus) are associated with urban areas in Central Spain San Francisco, and San Diego, CA (Kauffman et al., 2004), (Palomino and Carrascal, 2007). red-tailed hawks (Buteo jamaicensis) are breeding in urban This is by no means a complete list of raptor species and suburban areas of Milwaukee, WI (Stout et al., 2006), breeding in close proximity to humans, nor are all spe- and Cooper’s hawks (Accipiter cooperii) are wintering cies or individuals so tolerant. One species that can be successfully in Terre Haute, IN (Roth and Lima, 2003). In particularly tolerant of humans is Harris’s hawks (Parabu- Europe, goshawks (Accipiter gentilis), generally considered teo unicinctus). This case focuses on Harris’s hawks in a shy forest species, have colonized the city of Hamburg, a densely populated urban area, Tucson, AZ. In Tucson, Germany (Rutz, 2008), black kites (Milvus migrans) have Harris’s hawks have expanded from about 10 nesting pairs established breeding colonies in Rome, Italy (De Gian- in 1975 (W. Mader, personal communication, 2003), to 46 pairs in 1993 (Dawson and Mannan, 1994), to 62 pairs in 2004 (Dwyer and Mannan, 2007). With this increase, Department of Fisheries and Wildlife Sciences, 106 Cheatham Hall, came increased interactions between Harris’s hawks and Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. Received 8 Jan. 2009. *Corresponding author (biojimmi@ overhead electric utility structures, resulting in an eventual yahoo.com). electrocution rate of at least 1.3 birds per territory within 300 meters of nests within 8 weeks of fledging (Dwyer and J. Nat. Resour. Life Sci. Educ. 38:93–98 (2009). Mannan, 2007), and documentation of wild raptors sur- doi:10.4195/jnrlse.2009.0002u • http://www.JNRLSE.org viving with electric injuries (Dwyer, 2006). This case will © American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA explore the role of overhead electric systems as ecological

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JOURNAL OF NATURAL RESOURCES & LIFE SCIENCES EDUCATION VOLUME 38 2009 93 traps for Harris’s hawks, the potential role of urban Tucson as a population sink, and the potential consequences of additive mortality. After completing this case, students will understand these concepts, and will have participated in discussions that prime them to engage in discussions of related topics such as dispersal and habitat selection. Students will also have expanded their understanding of the unique situations faced by urban wildlife, and will gain increased awareness of the ecology operating around them daily, even in urban environments. The Case An adult female Harris’s hawk (Fig. 1) perches at the edge of her nest and surveys her territory. She is 13 years old, and has raised young in this pine tree nest each year since she was 4 years old. This season has been good; she has three young birds in her nest, and they are growing Fig. 1. An adult female Harris’s hawk (Parabuteo unicin- fast. Her mate is hunting to provide food for her and their tus) (photo by James F. Dwyer). nestlings, and to defend their territory against non-breeding floaters and other trespassing birds of prey. The territory Harris’s hawks feed mainly on passerines and small includes a dry riverbed, a grassy city park, a convenience mammals. Urban Tucson supports large numbers of both, store, a gas station, an apartment complex, a neighborhood primarily as a consequence of human activities of cultivat- of 200 single family homes, and about 400 power poles. A ing plants and maintaining standing water in what would new house has just been completed across the street from otherwise be an expanse of dry desert. Exotic avian spe- her nest, and a new power pole has been installed. Living cies are especially plentiful, and exotic pine trees provide in an urban area requires habituation to these kinds of perfect nest sites. Thus, Harris’s hawks sampling Tucson changes and the Harris’s hawks accept them readily. The during dispersal find an environment resembling their female hawk’s mate appears in flight from behind a nearby natural habitat where essential components are replaced pine tree. He is carrying a freshly caught rabbit for nest- with acceptable and sometimes more plentiful substitutes. undergraduate education lings who will soon be flying to catch their own meals. Pine tress replace natural scrubby nest sites, power poles Harris’s hawks are non-migratory birds of prey and live replace saguaro cacti for demonstrating hierarchies, and in social groups of up to seven birds. These groups always advertising territories, and introduced birds supplement contain a dominant mated pair, and can also include young the richness and diversity of the native prey . of the year, young of previous years, and unrelated birds Individual raptors with the temperament and behavioral (Bednarz, 1995). In raptors, females tend to be larger than plasticity necessary to adapt to the din of urban life can males and more dominant. Only the mated pair breeds, and settle in urban areas, eat especially well, grow quickly, and all other group members older than about 9 months con- reproduce abundantly. tribute prey to raising the mated pair’s young. To enforce After a month watching her nestlings grow, the female this system, Harris’s hawks maintain a strict Harris’s hawk knows it is time for them to fly. With a gentle hierarchy within groups wherein individuals demonstrate rattle she entices them to leave the nest and move among their status by perching together on a single object. The the branches of their nest tree. She leads them hop-flying most dominant individuals perch highest on the object, branch to branch, and then jumps to the roof of the new and the subordinate individuals perch below them (Dawson house. Begging loudly for food, the young follow her and and Mannan, 1991). In Tucson this behavior historically are rewarded with scraps of pigeon. The male calls out now. took place on saguaro cacti. Today many of the cacti have He has returned with more prey to the power pole where been removed, and power poles dot the landscape. Har- he habitually transfers his catch to his mate. She sees him, ris’s hawks now routinely demonstrate their social structure and with another gentle rattle, she joins him. There is just within and between groups by advertising their territory enough space between the uninsulated wires for the two and their rank while perched together on power poles. of them. Then, one of the young flies to the pole to be fed Harris’s hawks have found a substitute in their new habitat and lands between the two adults. As the young bird lands, for an essential, but now minimized component of their old each of his wings brushes against each of his parents. Both habitat. parents take a step back to make room, and as they do, Only the dominant female is allowed to provision prey each touches one of the wires they’ve been standing beside to the nestlings. She watches as her mate brings the food since the pole was installed. Instantly, 10 thousand volts as close as he dares, and as he swoops gracefully with it to of electricity flow from one of the wires, into and through the cross arm of the newly installed power pole. She then the adult female, into and through the fledgling, then the makes a short flight, and alights beside him. As they stand adult male, and then into the other wire. In a split second between two wires, she takes the rabbit, and returns to the all three birds drop to the ground dead. The remaining two nest. As soon as he has delivered the prey, he turns away young birds cry for food, but their cries go unanswered. to hunt again.

94 JOURNAL OF NATURAL RESOURCES & LIFE SCIENCES EDUCATION VOLUME 38 2009 dead. The floater doesn’t know this, and enters the terri- tory as stealthily as he can. Slowly, he moves through the area searching for rivals. Finding none, he begins adver- tising near the nest site, and soon attracts a female. The power pole where the previous breeding pair died is in a perfect place to simultaneously observe the nest site and the surrounding territory, and the new pair soon establishes a habit of perching there just as the previous pair did. Then one rainy afternoon, electricity arcing through puddles on the cross-arm electrocutes these two birds as well. Raptors of various species routinely compete for the nest sites, and it isn’t long until a great-horned owl discovers the area and then meets the same fate. In the latter two cases, the new birds did not even contact energized wires;

rather, the birds were standing across puddles that were in undergraduate education contact with the wires. Electricity flowed from one wire into a puddle, through the two birds, and into another puddle that was in contact with the other wire. These birds never saw what happened to the first pair, so they couldn’t have learned any better, and they ended up dying on the same dangerous pole. They were drawn to the pole for exactly the same reasons the first pair was, and for exactly the Fig. 2. An adult Harris’s hawk (Parabuteo unicintus) same reasons that the next pair is likely to be. This terri- (solid dark breast, left cross arm), and two fledgling hawks tory has become a place where mortality regularly exceeds (streaked dark and light breasts, center and right cross productivity, and where the mortality factor is as likely to arm) perched together on a power pole. Just visible in the strike any large raptor using the nest site regardless of the photo are the folded wings of an electrocuted third fledgling quality of the individual. lying dead on top of the transformer (photo by James F. Dwyer). Harris’s hawks routinely congregate around fallen group members for days or weeks. Learning Objectives After completing this case study, students will be They will stay near the bodies of their parents and brother able to: for as long as they can (Fig. 2), but will soon grow hungry. 1. Identify three types of ecological traps, define popu- With no adults to teach them to hunt, they will starve. lation sources and sinks, and describe additive and To a human, electricity can be simple: it flows when a compensatory mortality. circuit is closed, it does not flow when a circuit is open. 2. Infer 4 stakeholder groups in this issue and interpret When you flip a light switch, you close a circuit and the their motivations. resistance generated by electrical energy flowing through 3. Practice critical thinking and the development of mul- your light bulb’s filament is converted to heat and light tiple competing hypotheses (Chamberlain, 1897). energy. To a bird electricity is incomprehensible. Birds don’t understand that electricity can be dangerous when flow- Teaching Note ing, but harmless when not flowing. When a bird perches on a wire, or perches on a power pole and touches only one Case Objectives wire, the electricity has no place to flow. Thus, electricity This case study is designed to engage students in an does not pass through the bird, and no harm is done. When introductory college ecology course in higher levels of the bird touches two wires at the same time, or touches learning through structured discussions in an active learn- one wire and another bird while that bird touches another ing environment. Students should work together to synthe- wire, the electricity can flow. The bird then becomes just size communal understandings of ecological traps, sinks, like the filament in a light bulb. Electrical energy flowing and additive mortality. Doing so will facilitate the students through the bird is converted to heat and light energy when overcoming incorrect preconceived notions, and advancing the bird’s body acts as a resistor, and the animal is usually discussion and critical thinking skills. This case study should killed. In this case, the individual birds were drawn into a be presented early in the course as it contains a number of situation where sudden environmental change (the addi- key words intended to trigger questions in related content tion of the power pole) decoupled the cue the birds used to typically delivered in an introductory ecology course. Thus, evaluate habitat quality (abundant advertising and hunt- while the apparent focus from the student’s perspective is ing perches) from the true quality of the environment (low ecological traps, population sinks, and additive mortality, because mortality risk was high). the actual consequence is that students will (perhaps with- A floater male has been visiting this territory every out realizing it) request additional information on population month for 2 years. His goal has been to evict and replace sources, compensatory mortality, habitat selection, disper- the breeding male. Until now, the breeding male defended sal, wildlife population management, and human–wildlife his mate and territory, but now both territorial adults are

JOURNAL OF NATURAL RESOURCES & LIFE SCIENCES EDUCATION VOLUME 38 2009 95 interactions. The instructor should be prepared to deliver population, and levels. The latter raptors mini lectures on these or other required topics as student did not learn from the consequences suffered by the early interest focuses on them. This case study offers a construct raptors because (1) animals are probably unable to convey upon which additional concepts not typically taught in an the complex information required to fully understand the introductory ecology class can also be assembled. Specifi- risks of the electric system, and (2) even if they could com- cally, the arenas of environmental ethics and environmental municate, each bird died the first time a negative conse- law could be briefly explored. quence occurred so there was no opportunity to learn. [Present mini-lecture on source-sink dynamics, metapo- Teaching the Case pulations, , and offer discussion ques- I recommend that instructors review introductory con- tion 4 as a follow up to reinforce learning.] cepts and develop 20-minute mini-lectures in each of the 4. Not all (or even many) population sinks are nec- areas outlined in “Discussion Questions” as student discus- essarily the result of ecological traps. How else can sions should enter these areas. If the case is taught early sinks arise? (analyze, create) Sinks can arise wherever in the semester, it can be mentally referenced by students territorial animals are distributed across habitats of unequal through the remainder of the course, thus providing a quality. If all of the highest quality habitat are occupied, framework for reconstruction of subsequent modules. I also animals may chose to attempt to breed in lower quality provide several discussion questions that may be useful habitats. Such decisions can result in relative decreases in in keeping conversations focused on learning. As in Villa- productivity, up to and including situations where mortality magna and Murphy (2008), I have identified in parentheses exceeds productivity. the level of thinking that each question targets. 5. How is the number of raptors present in the area likely to influence electrocutions near this nest Discussion Questions site? (understand) Electrocutions are likely to occur if 1. Why did these hawks die? Why didn’t they learn there are any raptors using this nest site regardless of the not to approach the wires? (understand) In two words: density of raptor nests in the surrounding area. Thus, the ecological trap. An ecological trap is a situation where mortality factor is a density-independent factor that strikes sudden environmental change decouples the cue an animal birds indiscriminately regardless of the number, status, uses to evaluate habitat quality from the true quality of or quality of the individuals, and is likely to cause addi- the habitat. Ecological traps are encountered by individual tive mortality. Electrocution in particular is likely to strike animals as a result of specific behaviors, and can occur females more because female raptors are bigger and thus more likely to bridge two differentially energized conduc- undergraduate education where: (1) cues indicating good habitat have increased but actual habitat quality has not, (2) cues haven’t changed but tors. Female raptors of many species also routinely exclude habitat quality has decreased, and (3) cues have increased their mates from provisioning young. Thus, prey captured and habitat quality has decreased. by the adult male must be transferred from the male to [Present mini-lecture on three types of ecological traps, the adult female, and then from the adult female to the and offer discussion question 2 as a follow up to reinforce young. Because the female make twice as many transfers learning.] as the others, and these transfers often happen on power 2. What are some other examples of ecological poles, the female is placed in potentially dangerous situa- traps? (apply) Sea turtle hatchlings hatch at night and tions twice as often. Young birds can also be at greater risk orient to light cues to find the ocean. Light pollution from because they tend to bump into objects, including ener- beachfront structures can misdirect hatchlings inland where gized electric equipment, as they are learning to balance survival is unlikely. and orient to polar- and fly. ized light to lay eggs. For thousands of years the primary [Present mini-lecture on additive vs. compensatory mor- source of polarized light was sunlight reflected from water, tality and growth curves, and offer discussion question 6 as where the eggs of these species develop normally. Asphalt a follow-up to reinforce learning.] roads also reflect polarized light. These insects respond 6. Is deer hunting as it is practiced in the United to the polarized light from asphalt and lay their eggs on States today likely to be additive or compensatory roads where the eggs desiccate and die. Oil slicks and mortality? (apply) Limits to deer hunting in the United other reflective surfaces can function identically. Indigo States are typically set after careful calculation of expected Buntings (Passerina cyanea) are attracted to forest edges. . Theory suggests that the environment Historically, forest edges were widely separated enough to where a given deer population lives can support a certain prevent edge-specialist predators from developing sub- number of individuals through some expected survival stantial populations. Edges are now common due to human bottleneck, typically over-winter survival. Any animals fragmentation of landscapes, and bunting productivity is exceeding the carrying capacity are expected to die via reduced in territories with high proportions of edge habitat. , starvation, disease, accident, and so forth. See Robertson and Hutto (2006) for an in-depth review of Harvesting these individuals is probably compensatory these cases. because they are not expected to survive anyway; spe- 3. Why did the next pair of hawks die? Why didn’t cifically, the mortality is compensated for in other ways. they learn from the previous pair? (understand) In Harvesting more individuals than the difference from the two words: ecological sink. A sink is an area where mortal- total minus the carrying capacity is typically viewed as ity exceeds productivity. Sinks can be viewed at territory, additive mortality. Harvesting only a particular sex, age,

96 JOURNAL OF NATURAL RESOURCES & LIFE SCIENCES EDUCATION VOLUME 38 2009 or morphology class can result in additive mortality for the more closely to energized equipment. Burying power lines affected segment of the population. effectively eliminates raptor electrocutions, but is extremely 7. Why are wildlife in general, and raptors in expensive when it is possible, burying is not possible for particular, moving into urban areas? (remember, very high voltages or in some substrates, and burying can apply, create) Human-dominated landscapes are expand- be very environmentally damaging. Insulating energized ing continuously into previously undeveloped areas (think parts prevents raptors from being able to contact danger- urban sprawl). Many animals are either incapable of move- ous portions of equipment, is relatively inexpensive, rela- ment on the scales required to outpace development, or tively ecologically friendly, and allows companies to target are unable to establish new territories in remaining habitat specific pole types and locations. where other individuals already exist. Many raptor species [Present mini-lecture on multiple competing hypotheses, in North America in particular are recovering from popu- and offer discussion question 10 as a follow up to reinforce lation crashes that resulted from eggshell thinning, and learning.] populations are now expanding. Direct persecution of rap- 10. Create a raptor protection plan that takes into tors (through shooting) also once was more prevalent than account the ecology of raptors, the costs and ben-

it is today, and consequently some raptors that previously efits of various electrocution prevention techniques, undergraduate education avoided living near humans may now be more tolerant. and the values of as many stakeholders as possible Higher trophic level species like raptors and coyotes (Canis (create). Such plans, when effective, have tended to latrans) also enter urban areas to capitalize on increased target poles with specific dangerous configurations (trans- prey density resulting from human provisioning of pas- formers, switchboxes, etc.) in specific areas (open spaces serines via bird feeders and small mammals via accessible used for hunting, poles around nests used for advertis- food scraps and cover. These factors probably operate ing) for retrofitting. This minimizes the costs to financially synergistically to press wildlife into human-dominated motivated stakeholders while maximizing the satisfaction landscapes. of ethically motivated stakeholders by preventing most [Present mini-lecture on human–wildlife interactions, electrocutions. and offer discussion question 8 as a follow-up to reinforce [Mini-lectures on animal territoriality, dispersal, and learning.] habitat selection are also likely to be appropriate to course 8. Who are the human stakeholders in raptor goals and relevant to student-generated questions.] electrocution issue? (analyze) A compete answer to this question should include: (1) The electric company that Evaluation owns and powers the equipment where birds are killed, Student learning can be assessed in variety of tradi- and who would have to pay the equipment and manpower tional methods. Multiple choice or fill in the blank tests costs to make poles raptor-safe. (2) Electric company could effectively assess capture and assimilation of topical customers who can lose power when electrocutions disrupt materials. Testing at this level assures the instructor that the delivery system, who ultimately shoulder the finan- the materials were sufficiently grounded such that higher cial burden borne by the electric company when electric learning could occur. The objectives of this module are that charges increase, and who may feel a measure of respon- students will construct knowledge of ecological concepts at sibility when the equipment they rely on kills birds. (3) higher levels of learning. Short-answer questions will better Wildlife enthusiasts, urban residents, and non-governmen- facilitate evaluation at these levels. Students should be tal agencies (Audubon Society, Defenders of Wildlife, etc.) able to defend their positions using the concepts discussed who object on a variety of ecological and ethical grounds to in class. These positions should be cogently argued, and indiscriminate killing of wildlife. (4) Government agencies do not need necessarily to mirror the instructor’s opinions. (U.S. Fish and Wildlife Service, state wildlife management The creation of multiple competing hypotheses should be offices) responsible for enforcing laws. The electrocution of encouraged throughout these discussions. raptors is illegal, and has been federally prosecuted (see United States vs. Moon Lake Electric Association, Inc.; http://www.animallaw.info/cases/caus45fsupp2d1070.htm; Conclusion verified 27 Apr. 2009). The electrocution of raptors is an ongoing global concern 9. Why are raptors sometimes electrocuted when (Lehman et al., 2007). Efforts to prevent the electrocution they routinely perch on power lines without negative of raptors in developed countries have had mixed results. effects? (understand) And how can raptor electrocu- Scientists and industry personnel are learning when and tion be prevented? (apply, create) Electrocution only how raptor behavior can be manipulated, and when and occurs when current flows through an organism. If an how power line construction standards must be amended, organism does not simultaneously contact two differen- but birds continue to die in the interum. Efforts to prevent tially energized conductors, electricity can not flow, and the electrocution of raptors in developing countries often electrocution does not occur. Prevention: Wildlife biologists falls a distant second to bringing electric systems on line as and industry personnel have tried installing various types quickly as possible, and little to no raptor-safety consider- of perch deterrents on power poles so birds will choose ations are included in structure designs. Thus, the ecologi- to land elsewhere. These deterrents tend to cause the cal traps described here continue to close on individual birds to avoid only the deterrent device specifically, but raptors on a daily basis around the world. Ecological traps not the whole pole, and can cause raptors to perch even can be very difficult to demonstrate because we must

JOURNAL OF NATURAL RESOURCES & LIFE SCIENCES EDUCATION VOLUME 38 2009 97 understand settlement cues and habitat quality, relative to Robertson, B.A., and R.L. Hutto. 2006. A framework for under- expected productivity in the absence of the potential trap. standing ecological traps and an evaluation of existing Thus, very few ecological traps have been well documented evidence. Ecology 87:1075–1085. doi:10.1890/0012- in scientific literature (Robertson and Hutto, 2006). Despite 9658(2006)87[1075:AFFUET]2.0.CO;2. the challenges, research in this area should enable scien- Roth, T.C., and S.L. Lima. 2003. Hunting behavior and diet of tists to identify, correct, and even prevent the ecological Cooper’s hawks: An urban view of the small-bird-in-winter paradigm. Condor 105:474–483. doi:10.1650/7219. traps in the future. Rutz, C. 2008. The establishment of an urban bird popula- tion. J. Anim. Ecol. 77:1008–1019. doi:10.1111/j.1365- Acknowledgments 2656.2008.01420.x. Brian Murphy’s graduate course at Virginia Tech on Smith, D.G., and J.R. Murphy. 1972. Unusual causes of raptor mor- Pedagogy for the Natural Resources Sciences prompted this tality. J. Raptor Res. 6:4–5. article, and Dr. Murphy’s comments on early drafts greatly Stout, W.E., S.A. Temple, and J.R. Cary. 2006. Landscape features improved it. The Tucson Electric Power Company provided of red-tailed hawk nesting habitat in an urban/suburban access to poles and other hardware, and worked tirelessly environment. J. Raptor Res. 40:193–199. doi:10.3356/0892- to prevent the electrocution of raptors in the Tucson area. 1016(2006)40[181:LFORHN]2.0.CO;2. An education and community outreach grant from the Villamagna, A.M., and B.R. Murphy. 2008. Water resources manage- Tucson Electric Power Company provided funding. ment in the Lerma–Chapala basin, Mexico: A case study. J. Nat. Resour. Life Sci. Educ. 37:102–110. References APLIC. 2006. Suggested practices for avian protection on power lines: The state of the art in 2006. Edison Electric Institute, Avian Power Line Interaction Committee, and the California Energy Commision, Washington DC, and Sacramento, CA. Bednarz, J.C. 1995. Harris’s hawk (Parabuteo unicintus). The birds of North America. In A. Poole (ed.) Cornell Lab of Ornithology, Ithaca, NY. Boeker, E.L., and P.R. Nickerson. 1975. Raptor electrocutions. Wildl. Soc. Bull. 3:79–81. Chamberlain, T.C. 1897. The method of multiple working hypoth- undergraduate education eses. J. Geol. 39:155–165. Dawson, J.W., and R.W. Mannan. 1991. Dominance hierarchies and helper contributions in Harris’ hawks. Auk 108:649–660. Dawson, J.W., and R.W. Mannan. 1994. The ecology of Harris’s hawks in urban environments. Final report submitted to Ari- zona Game and Fish Department. Heritage Grant G20058-A. Arizona Game and Fish Department, Tucson, AZ. De Giancomo, U., and G. Guerrieri. 2008. The feeding behavior of the black kite (Milvus migrans) in the rubbish dump of Rome. J. Raptor Res. 42:110–118. doi:10.3356/JRR-07-09.1. Dwyer, J.F. 2006. Electric shock injuries in a Harris’s hawk population. J. Raptor Res. 40:193–199. doi:10.3356/0892- 1016(2006)40[193:ESIIAH]2.0.CO;2. Dwyer, J.F., and R.W. Mannan. 2007. Preventing raptor electrocu- tions in an urban environment. J. Raptor Res. 41:259–267. doi:10.3356/0892-1016(2007)41[259:PREIAU]2.0.CO;2. Kauffman, M.J., J.F. Pollock, and B. Walton. 2004. Spatial structure, dispersal, and management of a recovering raptor population. Am. Nat. 164:582–597. doi:10.1086/424763. Lehman, R.N., P.L. Kennedy, and J.A. Savidge. 2007. The state of the art in raptor electrocution research: A global review. Biol. Conserv. 136:159–174. doi:10.1016/j.biocon.2006.09.015. Liguori, S., and J. Burruss. 2003. Raptor electrocution reduction pro- gram, 2001–2002 Rep. Prepared by HawkWatch International for Pacificorp, Salt Lake City, UT. Olendorff, R.R. 1972. Eagles, sheep, and power lines. Colorado Outdoors 21:3–11. Palomino, D., and L.M. Carrascal. 2007. Habitat associations of a raptor community in a mosaic landscape of Central Spain under urban development. Landscape and Urban Planning 83:268– 274. doi:10.1016/j.landurbplan.2007.04.011.

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