I wrote this report for a university course, "Topics in Avian Conservation". The text summarizes anthropogenic mortality of birds.
ANTHROPOGENIC MORTALITY OF BIRDS
OVERVIEW
Global bird populations are declining, for some species precipitously, as a result of multiple anthropogenic stressors (Sekercioglu et al. 2004, IUCN 2014). Adding to stressors that indirectly cause mortality (i.e. habitat loss, climate change), direct anthropogenic mortality affects hundreds of bird species and billions of birds each year (Loss et al. 2015). In rank order as they affect birds in North America, direct mortality sources include depredation by free-ranging cats, collisions with human-made structures, and poisoning. Ranking last, we might include a “catch-all” category including hazards such as entrapment, entanglement, burns, etc.. The only continent-wide estimates of these mortality sources are for North America. Furthermore, most research focuses on regional or smaller scales and assesses impacts on either a single or a small suite of species.
CATS
Depredation by the domestic cat (Felis catus) is the leading cause of anthropogenic mortality to birds. Cats have been implicated in at least 33 bird species' extinctions, making them one of the most important causes of bird extinctions world-wide (Nogales et al. 2004). The threat is greatest in closed systems, such as islands, whereas bird populations in continental-scale systems are less at risk. That said, population impacts of cat depredation and species-specific mortality vulnerabilities are not well understood and constitute priorities for research. Annual mortality due to cat depredation is estimated to be 100–350 million birds per year in Canada (Blancher 2013) and 1.4–4 billion birds per year in the U.S. (Loss et al. 2013).
Mitigation efforts include influencing cat owner behaviors to limit outdoor access and the control of feral cats. Interventions such as bells or “cat bibs” are not effective solutions. Trap-Neuter-Release (TNR) programs are generally opposed by the bird conservation community. TNR programs fail because they cannot spay and neuter a sufficient number of cats to affect feral cat numbers at the population level. Lethal control is effective but does not generally garner broad social support, although more research is needed as to the social acceptance of these control methods (Peterson et al. 2012, Lohr and Lepczyk 2013).
COLLISIONS
Birds collide with a variety of human-made structures including buildings, communication towers, wind turbines, vehicles, power lines, offshore oil rigs, and aircraft. Building collisions rank highest among these, killing an estimated 365 to 988 million birds annually in the United States (Loss et al. 2014) and 16–42 million per year in Canada (Machtans et al. 2013). High-rise buildings (≥12 stories) account for only a small portion (<1%) of overall bird–building collision mortality (Loss et al. 2014). Nonetheless, high-rise buildings represent opportunities to conduct research, to raise public awareness, and to implement mitigation measures. Downtown skyscrapers are good research sites because bird carcasses are easily detected, spatially concentrated (as a function of dense development), and freely accessible. Furthermore, studying collisions broadly across taxa is feasible since migrating birds have a 70% chance of flying through at least one major metropolitan area per season (Brown and Caputo 2007).
It is important not only to look at collision mortality in terms of magnitude, but also to put such quantities in context of bird abundance. For any given species, comparing annual mortality to abundance yields a relative “vulnerability index”. Findings suggest that not all species are at equal risk for colliding with structures. In fact, vulnerability to building collision varies widely, ranging from 1,066 times more likely to collide than average to 273 times less likely to collide than average (Loss et al. 2014). Among species which are highly vulnerable to building collisions, some are Birds of Conservation Concern, including Golden-winged Warbler (Vermivora chrysoptera), Painted Bunting (Passerina ciris), Canada Warbler (Cardellina canadensis), Wood Thrush (Hylocichla mustelina), Kentucky Warbler (Geothlypis formosa), and Worm-eating Warbler (Helmitheros vermivorum).
Research findings are mixed regarding population level effects of collisions. Arnold and Zink (2011) found that collisions with buildings and communication towers had no discernible population impacts in North America. However, some studies conducted elsewhere in the world and using finer geographic scales have found cases in which mortality from collisions did contribute to significant population declines. Some examples include the following: vehicle collisions for owls in Portugal (Borda-de-Agua et al. 2014); wind turbine collisions for vultures in Spain (Carrete et al. 2009); and wind turbine collisions for eagles in Norway (Dahl et al. 2012). Priorities for collision research include: assessing population impacts, annual collision patterns, data on various structure types in diverse geographic and ecological settings; assessing biases (e.g., detection and carcass removal); and mitigation options.
Although the problem of collisions is not confined to urban areas, cities have been leading the charge for public awareness and intervention. Popular media draws attention to bird kills at large urban buildings. Galvanized by organizations such as The Audubon Society, concerned citizens become citizen scientists who monitor bird fatalities and injuries. On the solutions side of things, cities are where we pioneer, demonstrate, and even boast of bird-safe innovations and policies. Lights-out programs, for example, have been reported to reduce per-building mortality by as much as 80% (Zink and Eckles 2010). Fritted glass, though difficult to incentivize in the private sector, can be prescribed or required in publicly-funded developments. For example, Minnesota’s “Buildings, Benchmarks, and Beyond” (b3mn.org) contains bird-safe guidelines that limit high-risk surfaces, trap-like recessed areas, and see-through structures.
POISONS
Poisons is the final major category of direct anthropogenic mortality of birds. At least 113 thirteen pesticides directly cause bird mortality, (Loss et al. 2015). Between 1 and 4.4 million birds are killed annually by pesticides in Canada (Calvert et al. 2013). One estimate suggests that during its peak use, Carbofuran resulted in the death of 17-91 million birds across US cornfields (Mineau 2005). Because Carbofuran resulted in quick mortality, it was easier to detect mortality than with some of its competitors. EPA began its effort to remove the pesticide completely from the market in 2006, including liquid forms. Carbofuran is still used around the world, legally and illegally. Studies show that pesticide use correlates with declining bird populations in the Canadian prairies (Mineau 2005b) and US agricultural lands (Mineau and Whiteside 2013).
Besides agricultural pesticides, other toxin sources and human behavior patterns are a significant threat to birds. The vulnerability of vultures to poisoning makes for an interesting case study. In North America, the California condor (Gymnogyps californianus) is one of many raptors threatened by lead poisoning in the form of lead shot. Vulture populations in India, Bangladesh, and Pakistan underwent precipitous declines in the 1990s. It wasn't until 2003 that researchers from the Peregrine Fund definitively linked the birds' deaths to the carcasses of cattle that had been treated with the anti-inflammatory medication diclofenac. One cattle carcass with diclofenac in its system can poison hundreds of carrion-feeding birds. By 2006, when the drug was officially banned, vulture populations had declined by 97 percent. In an 11-year period, the feral dog population increased by 7 million, to 29 million animals. This is attributed to the decline of the vultures, a key competitor for food resources. Deaths from rabies increased by nearly 50,000, which cost Indian society roughly $34 billion in treatment expenses, lost wages, and human lives lost. Current evidence suggests the declines have slowed, and in some regions, vulture numbers have even begun to increase. Yet populations of the three hardest-hit species remain a small fraction of it former millions.
In contrast unintentional mortality in India, Africa’s vulture crisis is possibly more complicated in its human dimensions. 61% vulture deaths reported since 1970 were caused by eating pesticide-laced carcasses (Ogada et al. 2015). Poisoning is happening through two paths linked to human behavior: livestock owners who perpetrate retaliatory poisoning to target large predators; and poachers who kill vultures to remove signal to enforcement. Just as in India, the problem is significant in scale because of how the poison moves through the food web. A single elephant carcass, for example, can kill six hundred vultures. In July 2012, 191 vultures died after feeding at a single poached elephant in a Zimbabwean park. Furthermore, poisoning is only one of several anthropogenic threats to vultures in Africa, threats which include market demand for traditional healing products (muti), increasing wind development, etc.. Over the next half century vulture numbers on the continent are projected to decline by 70 to 97 percent.
DISCUSSION
Across the entire landscape of anthropogenic mortality, there is momentum growing to gather data that informs full life-cycle population models and conservation. Specifically, Scott Loss, a leader in this arena, has put forward a suggestion to employ Integrated Population Models and Potential Biological Removal Models. Looked at individually, it might be argued that none of these big threats will ever be clearly additive. Finding incontrovertible evidence linking these threats to population declines may be an unreasonable goal. That said, it’s important that direct anthropogenic threats are taken in context, a context that includes documented population declines in many species and indirect stressors like climate change and habitat loss.
Finally, anthropogenic mortality may represent an opportunity for the conservation community to gain traction with the public. Perhaps the visibility of the problem, along with the fact that often the causes of death are not obscured by complex biological processes and ecological dynamics, may prompt response by the public. The public’s capacity for concern and action merits attention from the conservation community.
It is critical that we situate avian mortality within complex human contexts. Furthermore, I think it is useful to consider these issues with two parallel tracks of thinking: first, how do we identify conservation priorities so limited resources can be allocated?; and second, what can we learn from these cases in terms of our ethical commitments and obligations? Preventable mortality is part of a greater conversation about the appropriateness of how people relate with nature and others (Chan et al. 2016). Reducing harm represents ethical as well as biodiversity benefits. As our world becomes increasingly urbanized, the protection of biodiversity must meaningfully engage city-dwellers. Birds, aside from being biodiversity indicators for urban green-spaces (Strohbach et al. 2013), offer accessible opportunities for urbanites to observe natural organisms interacting with the environment and consider human–nature relationships.
REFERENCES
Arnold TW, Zink RM. 2011. Collision mortality has no discernible effect on population trends of North American birds. PLOS ONE 6:e24708
Blancher PJ. 2013. Estimated number of birds killed by house cats (Felis catus) in Canada. Avian Conserv. Ecol. 8:3
Borda-de- Agua L, Grilo C, Pereira HM. 2014. Modeling the impact of road mortality on barn owl (Tyto alba) populations using age-structured models. Ecol. Model. 276:29–37
Calvert AM, Bishop CA, Elliot RD, Krebs EA, Kydd TM, et al. 2013. A synthesis of human-related avian mortality in Canada. Avian Conserv. Ecol. 8:11
Carrete M, Sanchez-Zapata JA, Benitez JR, Lobon M, Donazar JA. 2009. Large scale risk-assessment of wind-farms on population viability of a globally endangered long-lived raptor. Biol. Conserv. 142:2954–61
Chan, K.M.A, P. Balvanera, K. Benessaiah, M. Chapman, S. Díaz, E. Gómez-Baggethun, R. Gould, N. Hannahs, K. Jax, S. Klain, G.W. Luck, B. Martín-López, B. Muraca, B. Norton, K. Ott, U. Pascual, T. Satterfield, M. Tadaki, J. Taggart, N. Turner. 2016. Why protect nature? Rethinking values and the environment. PNAS: Vol. 113, No. 6: 1462–1465.
Dahl EL, Bevanger K, Nyga ̊rd T, Røskaft E, Stokke BG. 2012. Reduced breeding success in white-tailed eagles at Smøla windfarm, western Norway, is caused by mortality and displacement. Biol. Conserv. 145:79–85
Hager SB, Cosentino BJ, McKay KJ, Monson C, ZuurdeegW, Blevins B. 2013.Window area and
development drive spatial variation in bird-window collisions in an urban landscape. PLOS ONE 8:e53371
Horikoshi K., Okochi I (eds.). Restoring the Oceanic Island Ecosystem. Springer 2010, page 205
Lohr, C.A., C. Lepczyk. 2014 Desires and Management Preferences of Stakeholders Regarding Feral Cats in the Hawaiian Islands. Conservation Biology Vol 28 Issue 2: 392–403.
Loss SR, Will T, Marra PP. 2015. Direct Mortality of Birds from Anthropogenic Causes. Annual Rev. Ecol. Evol. Syst. 46:99-120
Loss SR, Will T, Marra PP. 2013. The impact of free-ranging domestic cats on wildlife of the United States. Nat. Comm. 4:1396
Loss SR, Will T, Loss SS, Marra PP. 2014. Bird-building collisions in the United States: estimates of annual mortality and species vulnerability. Condor 16:8–23
Machtans CS, Wedeles CHR, Bayne EM. 2013. A first estimate for Canada of the number of birds killed by colliding with buildings. Avian Conserv. Ecol. 8:6
Mineau P. 2005. Direct losses of birds to pesticides—beginnings of a quantification. US For. Serv. Gen. Tech. Rep. PSW-GTR- 191.2005, US Dep. Agric., Washington, DC
Mineau P, Whiteside M. 2013. Pesticide acute toxicity is a better correlate of U.S. grassland bird declines than agricultural intensification. PLOS ONE 8:e57457
Nogales M, Vidal E, Medina FM, Bonnaud E, Tershy BR, et al. 2013. Feral cats and biodiversity conservation: the urgent prioritization of island management. BioScience 63:804–10
Peterson, M.N., B. Hartis, S. Rodriguez, M. Green, C. Lepczyk. 2012. Opinions from the Front Lines of Cat Colony Management Conflict. PLoS One. http://dx.doi.org/10.1371/journal.pone.0044616
Sekercioglu CH, Daily GC, Ehrlich PR. 2004. Ecosystem consequences of bird declines. PNAS 101:18042–47.
Ogada, D., Shaw, P., Beyers, R. L., Buij, R., Murn, C., Thiollay, J. M., Beale, C. M., Holdo, R. M., Pomeroy, D., Baker, N., Krüger, S. C., Botha, A., Virani, M. Z., Monadjem, A. and Sinclair, A. R. E. (2015), Another Continental Vulture Crisis: Africa's Vultures Collapsing toward Extinction. Conservation Letters. doi: 10.1111/conl.12182
IUCN (Int. Union Conserv. Nat.). 2014. The IUCN red list of threatened speciesTM. http://www.iucnredlist.org/.
Global bird populations are declining, for some species precipitously, as a result of multiple anthropogenic stressors (Sekercioglu et al. 2004, IUCN 2014). Adding to stressors that indirectly cause mortality (i.e. habitat loss, climate change), direct anthropogenic mortality affects hundreds of bird species and billions of birds each year (Loss et al. 2015). In rank order as they affect birds in North America, direct mortality sources include depredation by free-ranging cats, collisions with human-made structures, and poisoning. Ranking last, we might include a “catch-all” category including hazards such as entrapment, entanglement, burns, etc.. The only continent-wide estimates of these mortality sources are for North America. Furthermore, most research focuses on regional or smaller scales and assesses impacts on either a single or a small suite of species.
CATS
Depredation by the domestic cat (Felis catus) is the leading cause of anthropogenic mortality to birds. Cats have been implicated in at least 33 bird species' extinctions, making them one of the most important causes of bird extinctions world-wide (Nogales et al. 2004). The threat is greatest in closed systems, such as islands, whereas bird populations in continental-scale systems are less at risk. That said, population impacts of cat depredation and species-specific mortality vulnerabilities are not well understood and constitute priorities for research. Annual mortality due to cat depredation is estimated to be 100–350 million birds per year in Canada (Blancher 2013) and 1.4–4 billion birds per year in the U.S. (Loss et al. 2013).
Mitigation efforts include influencing cat owner behaviors to limit outdoor access and the control of feral cats. Interventions such as bells or “cat bibs” are not effective solutions. Trap-Neuter-Release (TNR) programs are generally opposed by the bird conservation community. TNR programs fail because they cannot spay and neuter a sufficient number of cats to affect feral cat numbers at the population level. Lethal control is effective but does not generally garner broad social support, although more research is needed as to the social acceptance of these control methods (Peterson et al. 2012, Lohr and Lepczyk 2013).
COLLISIONS
Birds collide with a variety of human-made structures including buildings, communication towers, wind turbines, vehicles, power lines, offshore oil rigs, and aircraft. Building collisions rank highest among these, killing an estimated 365 to 988 million birds annually in the United States (Loss et al. 2014) and 16–42 million per year in Canada (Machtans et al. 2013). High-rise buildings (≥12 stories) account for only a small portion (<1%) of overall bird–building collision mortality (Loss et al. 2014). Nonetheless, high-rise buildings represent opportunities to conduct research, to raise public awareness, and to implement mitigation measures. Downtown skyscrapers are good research sites because bird carcasses are easily detected, spatially concentrated (as a function of dense development), and freely accessible. Furthermore, studying collisions broadly across taxa is feasible since migrating birds have a 70% chance of flying through at least one major metropolitan area per season (Brown and Caputo 2007).
It is important not only to look at collision mortality in terms of magnitude, but also to put such quantities in context of bird abundance. For any given species, comparing annual mortality to abundance yields a relative “vulnerability index”. Findings suggest that not all species are at equal risk for colliding with structures. In fact, vulnerability to building collision varies widely, ranging from 1,066 times more likely to collide than average to 273 times less likely to collide than average (Loss et al. 2014). Among species which are highly vulnerable to building collisions, some are Birds of Conservation Concern, including Golden-winged Warbler (Vermivora chrysoptera), Painted Bunting (Passerina ciris), Canada Warbler (Cardellina canadensis), Wood Thrush (Hylocichla mustelina), Kentucky Warbler (Geothlypis formosa), and Worm-eating Warbler (Helmitheros vermivorum).
Research findings are mixed regarding population level effects of collisions. Arnold and Zink (2011) found that collisions with buildings and communication towers had no discernible population impacts in North America. However, some studies conducted elsewhere in the world and using finer geographic scales have found cases in which mortality from collisions did contribute to significant population declines. Some examples include the following: vehicle collisions for owls in Portugal (Borda-de-Agua et al. 2014); wind turbine collisions for vultures in Spain (Carrete et al. 2009); and wind turbine collisions for eagles in Norway (Dahl et al. 2012). Priorities for collision research include: assessing population impacts, annual collision patterns, data on various structure types in diverse geographic and ecological settings; assessing biases (e.g., detection and carcass removal); and mitigation options.
Although the problem of collisions is not confined to urban areas, cities have been leading the charge for public awareness and intervention. Popular media draws attention to bird kills at large urban buildings. Galvanized by organizations such as The Audubon Society, concerned citizens become citizen scientists who monitor bird fatalities and injuries. On the solutions side of things, cities are where we pioneer, demonstrate, and even boast of bird-safe innovations and policies. Lights-out programs, for example, have been reported to reduce per-building mortality by as much as 80% (Zink and Eckles 2010). Fritted glass, though difficult to incentivize in the private sector, can be prescribed or required in publicly-funded developments. For example, Minnesota’s “Buildings, Benchmarks, and Beyond” (b3mn.org) contains bird-safe guidelines that limit high-risk surfaces, trap-like recessed areas, and see-through structures.
POISONS
Poisons is the final major category of direct anthropogenic mortality of birds. At least 113 thirteen pesticides directly cause bird mortality, (Loss et al. 2015). Between 1 and 4.4 million birds are killed annually by pesticides in Canada (Calvert et al. 2013). One estimate suggests that during its peak use, Carbofuran resulted in the death of 17-91 million birds across US cornfields (Mineau 2005). Because Carbofuran resulted in quick mortality, it was easier to detect mortality than with some of its competitors. EPA began its effort to remove the pesticide completely from the market in 2006, including liquid forms. Carbofuran is still used around the world, legally and illegally. Studies show that pesticide use correlates with declining bird populations in the Canadian prairies (Mineau 2005b) and US agricultural lands (Mineau and Whiteside 2013).
Besides agricultural pesticides, other toxin sources and human behavior patterns are a significant threat to birds. The vulnerability of vultures to poisoning makes for an interesting case study. In North America, the California condor (Gymnogyps californianus) is one of many raptors threatened by lead poisoning in the form of lead shot. Vulture populations in India, Bangladesh, and Pakistan underwent precipitous declines in the 1990s. It wasn't until 2003 that researchers from the Peregrine Fund definitively linked the birds' deaths to the carcasses of cattle that had been treated with the anti-inflammatory medication diclofenac. One cattle carcass with diclofenac in its system can poison hundreds of carrion-feeding birds. By 2006, when the drug was officially banned, vulture populations had declined by 97 percent. In an 11-year period, the feral dog population increased by 7 million, to 29 million animals. This is attributed to the decline of the vultures, a key competitor for food resources. Deaths from rabies increased by nearly 50,000, which cost Indian society roughly $34 billion in treatment expenses, lost wages, and human lives lost. Current evidence suggests the declines have slowed, and in some regions, vulture numbers have even begun to increase. Yet populations of the three hardest-hit species remain a small fraction of it former millions.
In contrast unintentional mortality in India, Africa’s vulture crisis is possibly more complicated in its human dimensions. 61% vulture deaths reported since 1970 were caused by eating pesticide-laced carcasses (Ogada et al. 2015). Poisoning is happening through two paths linked to human behavior: livestock owners who perpetrate retaliatory poisoning to target large predators; and poachers who kill vultures to remove signal to enforcement. Just as in India, the problem is significant in scale because of how the poison moves through the food web. A single elephant carcass, for example, can kill six hundred vultures. In July 2012, 191 vultures died after feeding at a single poached elephant in a Zimbabwean park. Furthermore, poisoning is only one of several anthropogenic threats to vultures in Africa, threats which include market demand for traditional healing products (muti), increasing wind development, etc.. Over the next half century vulture numbers on the continent are projected to decline by 70 to 97 percent.
DISCUSSION
Across the entire landscape of anthropogenic mortality, there is momentum growing to gather data that informs full life-cycle population models and conservation. Specifically, Scott Loss, a leader in this arena, has put forward a suggestion to employ Integrated Population Models and Potential Biological Removal Models. Looked at individually, it might be argued that none of these big threats will ever be clearly additive. Finding incontrovertible evidence linking these threats to population declines may be an unreasonable goal. That said, it’s important that direct anthropogenic threats are taken in context, a context that includes documented population declines in many species and indirect stressors like climate change and habitat loss.
Finally, anthropogenic mortality may represent an opportunity for the conservation community to gain traction with the public. Perhaps the visibility of the problem, along with the fact that often the causes of death are not obscured by complex biological processes and ecological dynamics, may prompt response by the public. The public’s capacity for concern and action merits attention from the conservation community.
It is critical that we situate avian mortality within complex human contexts. Furthermore, I think it is useful to consider these issues with two parallel tracks of thinking: first, how do we identify conservation priorities so limited resources can be allocated?; and second, what can we learn from these cases in terms of our ethical commitments and obligations? Preventable mortality is part of a greater conversation about the appropriateness of how people relate with nature and others (Chan et al. 2016). Reducing harm represents ethical as well as biodiversity benefits. As our world becomes increasingly urbanized, the protection of biodiversity must meaningfully engage city-dwellers. Birds, aside from being biodiversity indicators for urban green-spaces (Strohbach et al. 2013), offer accessible opportunities for urbanites to observe natural organisms interacting with the environment and consider human–nature relationships.
REFERENCES
Arnold TW, Zink RM. 2011. Collision mortality has no discernible effect on population trends of North American birds. PLOS ONE 6:e24708
Blancher PJ. 2013. Estimated number of birds killed by house cats (Felis catus) in Canada. Avian Conserv. Ecol. 8:3
Borda-de- Agua L, Grilo C, Pereira HM. 2014. Modeling the impact of road mortality on barn owl (Tyto alba) populations using age-structured models. Ecol. Model. 276:29–37
Calvert AM, Bishop CA, Elliot RD, Krebs EA, Kydd TM, et al. 2013. A synthesis of human-related avian mortality in Canada. Avian Conserv. Ecol. 8:11
Carrete M, Sanchez-Zapata JA, Benitez JR, Lobon M, Donazar JA. 2009. Large scale risk-assessment of wind-farms on population viability of a globally endangered long-lived raptor. Biol. Conserv. 142:2954–61
Chan, K.M.A, P. Balvanera, K. Benessaiah, M. Chapman, S. Díaz, E. Gómez-Baggethun, R. Gould, N. Hannahs, K. Jax, S. Klain, G.W. Luck, B. Martín-López, B. Muraca, B. Norton, K. Ott, U. Pascual, T. Satterfield, M. Tadaki, J. Taggart, N. Turner. 2016. Why protect nature? Rethinking values and the environment. PNAS: Vol. 113, No. 6: 1462–1465.
Dahl EL, Bevanger K, Nyga ̊rd T, Røskaft E, Stokke BG. 2012. Reduced breeding success in white-tailed eagles at Smøla windfarm, western Norway, is caused by mortality and displacement. Biol. Conserv. 145:79–85
Hager SB, Cosentino BJ, McKay KJ, Monson C, ZuurdeegW, Blevins B. 2013.Window area and
development drive spatial variation in bird-window collisions in an urban landscape. PLOS ONE 8:e53371
Horikoshi K., Okochi I (eds.). Restoring the Oceanic Island Ecosystem. Springer 2010, page 205
Lohr, C.A., C. Lepczyk. 2014 Desires and Management Preferences of Stakeholders Regarding Feral Cats in the Hawaiian Islands. Conservation Biology Vol 28 Issue 2: 392–403.
Loss SR, Will T, Marra PP. 2015. Direct Mortality of Birds from Anthropogenic Causes. Annual Rev. Ecol. Evol. Syst. 46:99-120
Loss SR, Will T, Marra PP. 2013. The impact of free-ranging domestic cats on wildlife of the United States. Nat. Comm. 4:1396
Loss SR, Will T, Loss SS, Marra PP. 2014. Bird-building collisions in the United States: estimates of annual mortality and species vulnerability. Condor 16:8–23
Machtans CS, Wedeles CHR, Bayne EM. 2013. A first estimate for Canada of the number of birds killed by colliding with buildings. Avian Conserv. Ecol. 8:6
Mineau P. 2005. Direct losses of birds to pesticides—beginnings of a quantification. US For. Serv. Gen. Tech. Rep. PSW-GTR- 191.2005, US Dep. Agric., Washington, DC
Mineau P, Whiteside M. 2013. Pesticide acute toxicity is a better correlate of U.S. grassland bird declines than agricultural intensification. PLOS ONE 8:e57457
Nogales M, Vidal E, Medina FM, Bonnaud E, Tershy BR, et al. 2013. Feral cats and biodiversity conservation: the urgent prioritization of island management. BioScience 63:804–10
Peterson, M.N., B. Hartis, S. Rodriguez, M. Green, C. Lepczyk. 2012. Opinions from the Front Lines of Cat Colony Management Conflict. PLoS One. http://dx.doi.org/10.1371/journal.pone.0044616
Sekercioglu CH, Daily GC, Ehrlich PR. 2004. Ecosystem consequences of bird declines. PNAS 101:18042–47.
Ogada, D., Shaw, P., Beyers, R. L., Buij, R., Murn, C., Thiollay, J. M., Beale, C. M., Holdo, R. M., Pomeroy, D., Baker, N., Krüger, S. C., Botha, A., Virani, M. Z., Monadjem, A. and Sinclair, A. R. E. (2015), Another Continental Vulture Crisis: Africa's Vultures Collapsing toward Extinction. Conservation Letters. doi: 10.1111/conl.12182
IUCN (Int. Union Conserv. Nat.). 2014. The IUCN red list of threatened speciesTM. http://www.iucnredlist.org/.