Vultures of the World

Essential Ecology and Conservation

Keith L. Bildstein

• Publisher: Comstock Publishing Associates
• Year: 2022
• Print ISBN: 9781501761614, 9781501761614
• eBook ISBN: 9781501765032
• Topic: Ciencias biológicas
• Subtopic: Zoología

In Vultures of the World, Keith L. Bildstein provides an engaging look at vultures and condors, seeking to help us understand these widely recognized but underappreciated birds.

Bildstein's latest work is an inspirational and long overdue blend of all things vulture. Based on decades of personal experience, dozens of case studies, and numerous up-to-date examples of cutting-edge science, this book introduces readers to the essential nature of vultures and condors. Not only do these most proficient of all vertebrate scavengers clean up natural and man-made organic waste but they also recycle ecologically essential elements back into both wild and human landscapes, allowing our ecosystems to function successfully across generations of organisms. With distributions ranging over more than three-quarters of all land on five continents, the world's twenty-three species of scavenging birds of prey offer an outstanding example of biological diversity writ large.

Included in the world's species fold are its most abundant large raptors—several of its longest lived birds and the most massive of all soaring birds. With a fossil record dating back more than fifty million years, vultures and condors possess numerous adaptions that characteristically serve them well but at times also make them particularly vulnerable to human actions. Vultures of the World is a truly global treatment of vultures, offering a roadmap of how best to protect these birds and their important ecology.

1 Essential Ecology of Scavengers

In every community we should find herbivorous, carnivorous, and scavenging animals.

Charles Elton (1947)

The pioneering English ecologist Charles Elton was spot-on in his assessment of the importance of avian scavengers. But although carrion consumption is an essential aspect of animal ecology, the consumption of animal carcasses has been an academic backwater until recently. This scarcity of information has much to do with humanity’s long-standing aversion to the dead, a deep-seated adaptation to the possibility of human and livestock contagions.

Mounting evidence suggests that in most landscapes, decomposition and scavenging both remove and effectively recycle significant organic material, and as such are essential ecological processes worthy of study. Across the savannas of East Africa, an ecosystem in which these phenomena have been studied in considerable detail, scavengers and decomposers consume and recycle 70% of all of the large ungulate biomass, whereas lions and other predators remove only 30%. In the Arctic near Barrow, Alaska, scavengers, including grizzly bears, wolves, and Northern Ravens, removed half of all specifically placed large-mammal carcasses in summer; and foxes, least weasels, and lemmings scavenged 99% of all subnivean lemming carcasses in winter. In and around residential Oxford, England, where predators account for 60% of small-mammal deaths, scavengers and decomposers consume 40% of all carcasses annually. Examples such as these suggest that single-celled decomposers and multi-celled scavengers, including vultures, represent two of the most important groups of organisms in ecosystems. Appreciating their importance is key to our fully understanding how both natural and human-dominated landscapes function.

Ecological Functions of Scavenging and Decomposition

Understanding the ecological roles of decomposers and scavengers requires understanding differences in the functions and attributes of these organisms in ecosystems. A brief primer in ecosystem science and Newtonian thermodynamics is useful. As many learn in ecology 101, ecosystems are made up of both producers and consumers. Producers, or autotrophs, include plants and microorganisms that make their own food by transforming solar energy into chemical energy in the form of organic, carbon-based substances. Plants do so by means of the intricate biochemical process of carbon fixation known as photosynthesis, and several autotropic microorganisms do the same through various processes in chemosynthesis. Consumers, most of which are heterotrophs, then use this organic material to fuel their metabolic, behavioral, and reproductive activities. The transfer of food energy from producers through one or more animal consumers is called a predatory food chain. Ecological links in food chains are determined by how many types of animals are consumed or preyed upon by other typically larger predators. In many cases, animals—and some of their organic matter—are consumed by smaller organisms called parasites. In other cases—and this is where it gets interesting—the carcasses of dead animals that have been either preyed upon by larger animals or parasitized by smaller organisms, or have died by metabolic processes attributable to misfortune or accident, are consumed by decomposers, scavengers, or both.

Overall, decomposers and scavengers function similarly in ecosystems, the principal differences being their respective size and where and how their “feeding” occurs. Decomposers are mainly single-celled, saprophytic or saprozoic organisms, including bacteria, that feed extracellularly on dead material and subsequently absorb their food in place. Scavengers, most of which are multicellular organisms, including invertebrates and their young—think maggots—and vertebrates including vultures that feed on the fragments of carcasses and subsequently ingest them, do so either on-site or elsewhere. Energy transfer is key in all of this. The transfer of energy, or the “ability to do work,” from one form to another is governed by Newtonian Laws of Thermodynamics. Sir Isaac Newton’s First Law states that although energy can be transformed from one form to another, it can never be created or destroyed, whereas his Second Law states that energy transformations always involve the degradation of energy from more concentrated to more dispersed forms. All energy transformations, ecological and otherwise, are “leaky processes” that result in the dispersion of energy from a more valuable concentrated form to a less concentrated and, therefore, less valuable form that is less useful in doing work, ecologically and otherwise. Because of the laws of thermodynamics, “pyramids of energy” are said to occur in ecological food chains, with less useful energy being available at every subsequent link in the chain. This, in turn, is why decomposers and scavengers are essential in ecosystems. Although decomposers and scavengers, like other organisms, obey the laws of thermodynamics, their ecological actions as elemental recyclers serve to recapture and supply the materials or essential ingredients that other ecological actors, including both plants and consumers, need to maintain the energetic processes of primary and secondary productivity that enable the long-term continuity of ecological systems. Thus, whereas many of my colleagues in conservation education typically describe vultures, and for that matter other vertebrate scavengers, as nature’s “sanitation engineers,” these organisms in fact are much more than that. In addition to keeping ecosystems clean and reducing the likelihood of both livestock and human diseases, decomposers and scavengers perform essential work as elemental recyclers that serve to reconstitute the raw materials that propagate and perpetuate long-term ecosystem function.

Decomposers versus Scavengers

Decomposers and scavengers play critical roles in speeding material recycling in ecological systems. Strictly speaking, decomposers do their recycling mainly “out of body,” or extra-cellularly, and on-site. Scavengers, including vultures, remove organic material from decaying carcasses for subsequent digestion and disintegration elsewhere. The dependence of decomposers and scavengers upon similar, if not identical, resources—dead and dying organic matter—means that they compete with one another for their food. This ecological reality functions to hasten their feeding rates. Estimates suggest that without the gluttonous and frenzied feeding behavior of these organisms, decomposition would slow tenfold. Vultures compete not only with other avian scavengers but also with many additional types of organisms. Although the carcasses themselves are defenseless, the degree of both intra- and interspecies competition within these feeding communities can be brutal, both physically and chemically.

Hundreds, if not thousands, of species of decomposers settle on, within, and around both the carcasses and soon-to-be-carcasses of dead and dying animals, proceeding rapidly to mineralize organic waste while producing food for themselves and, in many cases, producing a suite of chemical agents that can affect the success of their competitors. As a result, decomposer metabolism at carcasses often proceeds at rates that heat the carcasses of disintegrating warm-blooded animals to temperatures that approach or exceed what they were prior to death.

Competition by decomposers at carcasses takes countless forms, many of which are largely unappreciated by avian biologists. Typically, the struggle begins as inter- and intra-species chemical warfare, as physical warfare is out of the question given the oftentimes enormous size differences among the combatants. In such situations, microbes quickly fabricate off-putting and often toxic metabolites to fend off other microbes and larger competitors. The creation of alcohol and other metabolites by many decomposers can inebriate, intoxicate, and sicken vertebrate scavengers, making them vulnerable to avian and mammalian predators. As a result, livestock, rodents, and herbaceous scavengers are known to avoid moldy grain and rotten fruit that can cause intestinal discomfort. Indeed, the well-known antibiotic fungal metabolite, penicillin, most likely evolved to forestall bacterial growth in “moldy” organic matter. The objectionable nature of over-ripe fruits and vegetables suggests the widespread nature of this type of chemical warfare among decomposers. That disease-causing Salmonella, Staphylococcus, and Clostridia produce toxins in the tissues they infest supports this line of reasoning.

The American evolutionary ecologist Daniel Janzen authored a paper intriguingly titled “Why fruit rots, seeds mold, and meat spoils” in 1977. In that paper, Janzen laid out a cogent argument that suggested that persistent interspecies competition between microbes and large organisms would create strong selective pressure on microbes to chemically render the seeds, fruits, and meats they were consuming inedible to vertebrates, leading scavengers to develop the ability to “ignore or mask objectionable flavors or odors that are also warnings and [thereafter] detoxify or otherwise avoid” them as much as possible.

In addition, many vertebrate scavengers host detoxifying symbiotic microbes to cope with rotting meat. Microbes in the lower gut of these individuals often create antibiotics that selectively clear this region of competitive microbes. A recent investigation of the microbiomes of 50 individuals of two species of New World vultures demonstrated the remarkable “conservancy” of a small number of types of gut flora dominated by the microbial genera Clostridia and Fusobacteria, which consist of species of common soil bacteria that typically are toxic to other vertebrates (for example, tetanus). Intriguingly, alligators, which also scavenge carrion, have similar microbial communities. The presence of Clostridia and Fusobacteria simply may be due to the organisms’ ability to outcompete other bacteria in the lower digestive tracts of New World vultures. Yet, it may be fostered by scavenging birds and reptiles for their own benefit in the breakdown and use of other carrion bacteria for their food value, while escaping the possible negative effects of related toxins. Several New World vultures possess antibodies for botulinum, leading researchers to speculate that they may be able to tolerate such toxins.

Another likely symbiotic relationship between vultures and their microbes involves urohidrosis, the socially unacceptable phenomenon of defecating on one’s feet and toes. Several New World vultures do this predictably and routinely. Although the behavior has long been attributed to evaporative cooling on particularly warm summer days, individual vultures do so in winter, suggesting the likelihood of an additional, antiseptic, function as individuals slog through putrefying carcasses. Whether this is true remains to be tested.

Finally, if carcasses become chemically less valuable to vultures over time, quickly locating and consuming carrion would be selected for.

Facultative versus Obligate Vertebrate Scavengers

Two types of vertebrate scavengers occur. Most vertebrate scavengers are facultative scavengers, species that sometimes but not always feed on carrion. A distinct minority are obligate scavengers, species that always, or almost always, feed on carrion. Because facultative scavengers are predatory as well as scavenging vertebrates, they are notably less specialized anatomically, behaviorally, and nutritionally for carrion finding and feeding, and they display fewer adaptations to the scavenging-only lifestyle than do obligate scavengers. Only several dozen avian scavengers are obligate, including vultures, several caracaras, and two species of giant petrels. No living mammalian scavengers are in the obligate group.

Facultative Scavengers. Facultative scavenging is phylogenetically widespread. Almost all predatory organisms, including the overwhelming majority of raptors and other carnivorous vertebrates, are facultative scavengers. The degree to which scavenging occurs varies opportunistically among species of facultative scavengers, as well as both seasonally and geographically within species, and, in some instances, individually. Jaguars, lions, and grizzly bears, for example, do not forego opportunities to feed on carcasses while searching for living prey, as do many eagles, hawks, and falcons. In fact, many predators routinely scavenge, including wolves, wild dogs, foxes, jackals, hyenas, bobcats, and lynx. Although fewer reptiles than mammals and birds routinely scavenge carrion, alligators and many lizards do, including Komodo dragons and other monitor lizards, all of which use their sense of smell to locate dead animals. Many predators scavenge by displacing less weaponized obligate scavengers and other predators from recently killed prey, whereas others feed on animals that have died from injury, disease, malnutrition, and other causes. Seasonality is important, with many carnivores switching from predation to scavenging during lean times of the year when live prey is less readily available. In some instances, predators undertake seasonal shifts in their feeding niches. In winter in North America, for example, wolverines typically search for and scavenge the carcasses of winter-storm-killed animals, whereas in summer they return to preying upon newborn reindeer and caribou. In East Africa, relationships among predator-scavengers are complex with predictable role reversals among species, for example, lions routinely scavenge prey remains left by hyenas and vice versa. Another complexity occurs when both species prey upon each other depending upon the circumstances, for example, the relative health of the individuals involved, the relative densities of their populations, and shifts in their group sizes and social structures. In more open habitats with limited cover, hyenas actually benefit lions when sustained pursuit predation is more profitable, whereas lions and leopards can benefit hyenas in brushier areas where successful lengthy pursuit can be more difficult for the latter.

Obligate Scavengers. Sixteen species of Old World vultures and seven species of New World vultures are generally recognized as the only truly obligate vertebrate scavengers, that is, the species that feed all but entirely on carrion and that require carrion for their livelihoods. (The California Condor, Andean Condor, and Eurasian Griffon are included here as “vultures” as well, as they are phylogenetically and ecologically part of this trophic assemblage.) All 23 species have large wingspans and light wing loadings that allow them to soar at low cost over large areas in search of episodically and ephemerally available dead animals (or carrion). All possess keen eyesight and several possess a keen sense of smell to help them locate carrion. All have reduced feathering around their heads that might otherwise cake their heads with putrefying material, at least occasionally. And all use social networks to help locate carrion more efficiently. This suite of somewhat specific adaptations allows them to outcompete other vertebrate scavengers that, in addition to feeding on carrion, depend on live prey to supplement their carrion diet.

Synthesis and Conclusions

1. Because of humanity’s long-standing aversion to the dead, microbial decomposition and carrion consumption by scavengers remain largely “neglected” aspects of healthy ecosystems despite their essential ecosystem function.
2. Decomposers are single-celled, saprophytic or saprozoic organisms, including bacteria, that feed extracellularly on dead material on-site. Scavengers are multicellular organisms, including both invertebrates and vertebrates, that feed on the fragments of carcasses, either on-site or elsewhere.
3. The ecological actions of decomposers and scavengers as elemental recyclers serve to recapture and supply the raw materials that other ecological entities, including plants and consumers, need to maintain the energetic processes of primary and secondary productivity necessary to sustain the long-term continuity of ecological systems.
4. Many microbial decomposers engage in chemical warfare by fabricating off-putting and, at times, toxic metabolites to ward off other microbes and larger competitors. This chemical process is why plants mold, fruits rot, and meats spoil.
5. Urohidrosis, the habit of defecating on one’s feet and toes practiced by several New World Vultures, which has long been attributed to evaporative cooling on hot days, also occurs in winter, suggesting the possibility of an antiseptic function against microbial infecti...

Table of contents

• Preface
• Introduction: Origins and Evolution of Vultures
• 1 Essential Ecology of Scavengers
• 2 Species Descriptions and Life Histories
• 3 Pair Formation and Reproduction
• 4 Food Finding and Feeding Behavior
• 5 Movement Behavior
• 6 Social Behavior
• 7 Vultures and People
• Appendix: Scientific Names of Vultures and Condors, and Other Birds, and Scientific Names of Other Animals Cited in the Text
• Glossary
• References and Recommended Readings by Chapter
• Index