Vultures of the World: Essential Ecology and Conservation

Vultures of the World

Essential Ecology and Conservation

Keith L. Bildstein

Comstock Publishing Associates

an imprint of

Cornell University Press

Ithaca and London

To Lindy, Corrine, Andre, Campbell,

Darcy, Munir, Peter, Sergio, and Todd

whose love for vultures

proved to be contagious

Contents

Preface

Introduction: Origins and Evolution of Vultures

1 Essential Ecology of Scavengers

2 Species Descriptions and Life Histories

New World Vultures

Black Vulture

Turkey Vulture

Lesser Yellow-headed Vulture

Greater Yellow-headed Vulture

King Vulture

California Condor

Andean Condor

Old World Vultures

Palm-nut Vulture

Bearded Vulture

Egyptian Vulture

Hooded Vulture

Indian Vulture

Slender-billed Vulture

White-rumped Vulture

Griffon Vulture

Rüppell’s Vulture

Himalayan Vulture

White-backed Vulture

Cape Vulture

Red-headed Vulture

White-headed Vulture

Cinereous Vulture

Lappet-faced Vulture

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

Preface

Vultures of the World owes its origins to a chance encounter at Hawk Mountain Sanctuary in the Appalachian Mountains of eastern Pennsylvania, USA, in the autumn of 2002. Two Princeton University scientists, David Wilcove and Martin Wikelski, were visiting the Sanctuary to take in the autumn raptor migration when I encountered them at the Sanctuary’s South Lookout while interpreting the flight for visitors. After discussing several of the big questions in raptor movement ecology, the three of us agreed that an in-depth study of the migrations of Turkey Vultures was long overdue. Doing so made sense for several reasons. First, Turkey Vultures were partial migrants, a migratory species in which some but not all individuals migrated, and although partial migrants were far more common than complete migrants, they remained less studied and the geographic complexities of their movements were less well understood than for the complete migrants. Second, Turkey Vulture populations were doing well at a time when most vultures were not, which offered the chance to study the ecology of a conservation outlier. Third, Turkey Vultures were both common and widespread in the Americas and could be observed in the backyards and, perhaps more important, the schoolyards of most children in the New World, making them accessible wildlife to both parents and educators throughout their range. Indeed, the species was common enough in New Jersey, USA, when I was growing up that I cannot recall when I saw my first Turkey Vulture, as they were as common in the sky as clouds.

As we concluded our conversation, Martin Wikelski promised to introduce me to one of his star students, Jamie Mendel, who had just committed to graduate school at Cornell University and was looking for a suitable study project. Less than a year later, Jamie and I were trapping Turkey Vultures; fitting them with satellite-tracking devices, heart-rate monitors, and core-body-temperature dataloggers; and following their movements and tracking metabolic costs. Since then my studies have expanded to include the movement behavior and ecology of migratory European vultures overwintering in Africa, as well the ecology of non-migratory Hooded Vultures in West, East, and southern Africa.

Like other raptor biologists, I have been fortunate in having patient mentors and, subsequently, colleagues who have been willing to share and retell their successful and unsuccessful experiences with me, not only while in the field but also later in the day over food and drink. I have carried on this tradition with my own students and colleagues, and much of what I have learned about vulture and condor biology results from these associations. One of my more memorable mentors was Frances Hamerstrom, a tough-as-nails, old-school role model who shepherded me, sometimes begrudgingly, through the whys and wherefores of what she would have called an introduction to conducting proper animal ecology. Hamerstrom had a series of rules for doing this successfully, three of which stand out. First and foremost was to understand that you were largely ignorant of your subject, and that there was always more to be learned. Second was that thoroughly understanding and appreciating the fundamental principles of ecology were essential. Third was that among the most important of these principles were the processes involved in population limitation. This book reflects those rules as it presents arguments for understanding the essential nature of vultures, the world’s scavenging birds of prey.

Vultures of the World begins with an introduction to the evolutionary processes that have led to the global diversity of scavenging birds of prey, along with a framework for discussing the ecological aspects necessary for sustaining that diversity. I then cover a range of topics that includes the poorly understood yet significant ecological processes of organic decay and decomposition; identification of the world’s 23 species of Old and New World vultures; the process of convergent evolution; the familial behavior of vultures along with their food-finding abilities, movements, and aspects of their social behavior; and a discussion of the factors that conspire to make vultures one of the most endangered groups of all birds—and how conservations work to protect them.

Although the book generally reflects my own views, it also summarizes the institutional knowledge of many of my colleagues and other professionals in the field who, over the years, have broadened my perspectives considerably, and without whom I would not have been able to describe vultures in this book. They include, but are not limited to, Dean Amadon, David Barber, Marc Bechard, Jim Bednarz, David Bird, Gill Bohrer, Andre Botha, David Brandes, Leslie Brown, Tom Cade, Bill Clark, Mike Collopy, Miguel Ferrer, Allen Fish, Laurie Goodrich, Maricel Grilli-Grana, Frances Hamerstrom, Katie Harrison, Todd Katzner, Roland Kays, Corrine Kendall, Robert Kenward, Lloyd Kiff, Sergio Lambertucci, Yossi Leshem, Mike McGrady, Bernd Meyburg, Ara Monadjem, Joan Morrison, Peter Mundy, Campbell Murn, Juan Jose Negro, Ian Newton, Rob Simmons, Jean-Francois Therrien, Jean-Marc Thiollay, Lindy Thompson, Simon Thomsett, Munir Virani, Rick Watson, Martin Wikelski, David Wilcove, and Reuven Yosef. I also thank my benefactors, Sarkis Acopian and the Acopian family, and my former employer, Hawk Mountain Sanctuary Association. Finally, I thank the fine people at Cornell University Press, including Kitty Liu, Susan Specter, Allegra Martschenko, and Candace Akins.

Also, note that the words raptor, vulture, and buzzard are often confused. Throughout this work I use the term raptor to describe all predatory and non-predatory birds of prey, or the hawks, eagles, falcons, vultures, and condors, and the term vulture to include all obligate scavenging and largely non-predatory birds of prey, including both vultures and condors. I use the term buzzard to describe predatory raptors in the genus Buteo.

I conclude with an important disclaimer. In addition to an impressive series of technical papers and books, two monumental monographs have made my work on vultures and condors far easier than otherwise possible. Leslie Brown and Dean Amadon’s Hawks, Eagles and Falcons of the World, published in 1968, provided me with an essential natural history of the vultures and condors of the world; and Peter Mundy, Duncan Butchart, John Ledger, and Steven Piper’s The Vultures of Africa, published in 1992, provided an effective model for describing the essential ecology of the world’s scavenging birds of prey.

Introduction

Origins and Evolution of Vultures

The bird student cannot help becoming envious on observing with what accuracy and amazing detail the student of mammals reconstructs the history of that class [with its] fossils … Bird bones, being small, brittle, and often pneumatic, are comparatively scarce in fossil collections.

Ernst Mayr (1946)

Before I describe the origins and evolution of the birds that ornithologists call vultures, I want to define what vultures are. The world’s living vultures represent 23 species of obligate scavenging birds of prey that, unlike raptors, feed principally or exclusively on carcasses they have not killed, but rather have died of other causes. Two of the largest western hemisphere or New World vultures are called condors and a few of the larger Old World vultures are called griffons. Nevertheless, ornithologists consider them all to be vultures. As do I. Although New World vultures are sometimes commonly referred to as buzzards—most likely because early English settlers in North America confused them with Old World Common Buzzards—New World vultures are not at all closely related to Old World buzzards, which are ancestrally distinct predatory raptors. With that nomenclature in mind, below I discuss the origins and evolution of this trophic assemblage known as vultures.

The world’s ecosystems are intrinsically efficient recycling machines. They have to be. Although certain biological taxa, most notably plants, capture and convert inorganic energy into forms that can sustain life, no biological entity, plant or animal, creates its own essential elements (technical terms and jargon are defined in the glossary), which, by definition, make up the indispensable building blocks of life. As a result, the constant recycling of life’s limiting essential elements is a critical ecological function of healthy ecosystems. A host of organic decomposers, microbial and invertebrate, serve this function admirably—as do vultures—the world’s only group of full-time vertebrate scavengers. Whereas many vertebrates are part-time scavengers, only the vultures do so full time.

But if recycling is essential in ecosystems, why are vultures the only full-time, obligate vertebrate scavengers? Why are there no obligate scavenging fish, amphibians, reptiles, or mammals? The answer lies in a unique suite of what evolutionary scientists call key innovations, specific characteristics that significantly increase an organism’s access to ecological resources that enable range expansion, as well as significant population growth, and, in most instances, adaptive radiation and speciation. Examples of key innovations include vertebrate lungs, the paired organs that evolved in a handful of aquatic and semi-aquatic vertebrates, which subsequently enabled the rapid diversification of land-based amphibians, reptiles, birds, and mammals. Innovations also include feathers, the integumentary structures that initially evolved as insulating elements in reptiles, and subsequently as airfoils in birds, enabling their rapid radiation. But what about key innovations in vultures? What innovations do they possess that allow them to be the world’s only obligate scavenging vertebrates? A suite of anatomical and behavioral vulturine traits associated with hyper-efficient soaring and gliding flight appears to fit the bill nicely.

The 20th-century American pioneer of animal ecophysiology Knut Schmidt-Nielsen once quipped that flight had the potential for being the most energetically costly form of animal locomotion. Although Schmidt-Nielsen is right theoretically, natural selection has been shaping avian flight for at least 100 million years, and low-cost soaring and gliding flight rank among its many successes. Today vultures use soaring and gliding flight to achieve full-time obligate scavenging in two ways. First, such flight elevates vultures above the landscape so they can see far enough to scan effectively both for carcasses and other vultures, and second, it allows them to travel long distances at minimal energetic cost while doing so. That vultures frequently rely on cues from one another to help them discover carcasses also increases the chances of successfully finding their next meal. Hence, social behavior serves as a key innovation in the development of obligate scavenging. As do their relatively large body mass and crop size, which allow them to feed sufficiently at a single meal to store food enough to sustain them for several days. Together, this suite of characteristics has enabled three distinct vulturine lineages to achieve something unique among vertebrates: an obligate scavenging lifestyle.

How did this unique avian trophic guild evolve? A somewhat far-fetched energetics model developed by two Scottish biologists helps explain why hyper-efficient soaring and gliding flight is critical. Graeme Ruxton and David Houston developed their energetic model to evaluate the potential for obligate scavenging in a ground-based vertebrate that few might have anticipated: the long-extinct, ferocious, meat-eating, and up to 15 metric-ton dinosaur Tyrannosaurus rex, a species the two concluded would have been capable of full-time, obligate scavenging in the Serengeti savannas of East Africa, at least without obligate avian scavengers. As far-fetched as it at first might seem, Ruxton and Houston’s case is convincing in that East Africa’s ungulate populations and the carcasses they provided would have been numerous enough to support the nutritional needs of a small population of these dinosaurs. However, only if Tyrannosaurus met a key requirement for successful obligate scavenging: hyper-efficient locomotion. Tyrannosaurus would have had to reduce the cost of movements to a bare minimum, something Ruxton and Houston suggest a lumbering, slow-moving version of the gigantic reptile could have accomplished. Although somewhat fanciful, their model demonstrates that low- to no-cost movement ecology is key to developing an obligate scavenging lifestyle, something vultures have managed to achieve by means of efficient soaring and gliding flight.

Although previous experts had proposed the Tyrannosaurus was a facultative scavenger more than a century ago, American paleontologist Jack Horner built a stronger case for their obligate scavenging lifestyle in 1994, when he posited that the species’ poor eyesight, puny forelimbs, grinding rather than piercing teeth, and ponderous gait combined to so compromise potential predatory behavior as to make obligatory scavenging a foregone conclusion. Though others have questioned Horner’s line of reasoning, the species’ towering upright adult stature of more than 5 meters certainly would have permitted it to scan larger areas of surrounding open habitats for large carcasses, which is another important scavenging trait. Equally important was its relatively high abundance (as compared to large co-occurring herbivorous dinosaurs). A multi-species census in fossil beds in the Hell Creek Formation of northeastern Montana ranked it as the second most abundant of all dinosaurs with population densities rivaling those of hyenas in the Serengeti of East Africa, which supports the likelihood of Tyrannosaurus rex being largely, if not entirely, a scavenging versus a predatory beast.

But enough about the why and how of vertebrate scavenger evolution; on to the where and when.

Ancient Avian Lineages

The fossil remains of vultures, like those of other large-bodied and large-boned birds, are reasonably well represented in the avian fossil record. Based on that record, avian paleontologists now recognize three genetic lineages of obligate scavenging raptors that fall into two avian families: Cathartidae, represented by seven living species of New World vultures, including the Turkey Vulture, Lesser Yellow-headed Vulture, Greater Yellow-headed Vulture, Black Vulture, King Vulture, California Condor, and Andean Condor; and Accipitridae, represented by 16 living species of Old World vultures in two distinct subfamilies, the Gypaetinae, including Palm-nut Vulture, Egyptian Vulture, and Bearded Vulture, and the Aegypiinae, including the Red-headed Vulture, Cinereous Vulture, Lappet-faced Vulture, Hooded Vulture, Indian Vulture (aka Long-billed Vulture), Slender-billed Vulture, Cape Vulture, Griffon Vulture, Rüppell’s Vulture, White-backed Vulture, Himalayan Vulture, and White-rumped Vulture. A fourth lineage, potentially obligate, but more likely facultatively scavenging, Teratornithidae, is detailed in Box 0.1.

Box 0.1 Teratorns: A Third Group of Obligate Avian Scavengers

Teratorns, an extinct New World lineage of large scavenging and predatory birds of prey, represent one of the most remarkable assemblages of avian fossils. Members of the extinct avian family Teratornithidae, these so-called wonder birds, included the largest of all flying birds.

The family Teratornithidae was introduced to science in 1909 when the University of California ornithologist and paleontologist Loye Miller who was in the process of retrieving and studying fossil remains from tar pits in Los Angeles, California, described one teratorn as follows:

Among many interesting forms of vertebrates taken from the Quaternary asphalt of the Rancho La Brea beds in Southern California have appeared several specimens of a very large bird which show marked divergence from recent forms [so] as to necessitate the establishment of a new genus.

Rancho La Brea had already yielded fossilized bones of scavenging raptors, including those of both California Condors and Turkey Vultures, but teratorns were distinct from these raptors. Bigger, more robust, and with anatomical hints of New World vulture affinities, the group now includes at least four extinct forms. The most widely recognized is Miller’s original find, Teratornis merriami, a bulked-up, Arnold Schwarzenegger–like version of the California Condor that stood about 30 in (75 cm) tall and weighed an estimated 33 lb (15 kg). As in other teratorns, merriami had an eagle-like bill, suggesting that it could guzzle smaller prey whole as well as hastily devour larger pieces of more massive prey in much the same way that hugely beaked bands of Steller’s Sea Eagles do today. It also possessed what Miller described as a high bridge of the nose and it lacked a nasal septum, features that link it to extant New World vultures. The La Brea tar pits would have been excellent habitat for scavengers like teratorns as its glossy surface probably attracted mammals and other vertebrates, numbers of which would have become mired in the shimmering puddles of liquid asphalt they had mistaken for watering holes in the dry California landscape.

Although Miller was quick to point out of the many differences between Teratornis merriami and extant New World Vultures, he was especially taken by the specimen’s head region, which displayed a striking similarity to that of the California Condor, and the evident preponderance of Cathartid affinities.

Subsequently discovered fossil teratorns include a smaller version of the La Brea specimen from southern Brazil, but the real subfamily show-stopper is Argentavis magnificens from more southern South America. Dating from the late Miocene 5 to 8 million years ago, this extinct teratorn stood 5 ft (1.5 m) tall, had a wingspan of more than 23 ft (7 m), and weighed more than 150 lb (70 kg). Specialists indicate that although it, too, was capable of soaring flight, strong Patagonian westerlies would have been necessary for takeoffs as individuals spread their enormous wings.

All fossil teratorns feature stout, fast-scampering legs, and both living prey and fresh carcasses, most likely, would have nourished all of them. The group’s disappearance at the end of the Pleistocene 11,000 years ago coincided with the large-scale collapse of New World ungulate megafauna, either directly, due to the disappearance of its food base, or because of climate change. The coincidental disappearance of teratorns and North American megafauna supports the idea that teratorns fed mainly on large carcasses and may have been episodically predatory as well.

German museum curator and ornithologist Hans Gadow was the first to note that so-called New World and Old World Vultures differed from each other, not only in their global distributions but also in several notable anatomical characteristics, including the perforated nares and double-notched sternum in New World but not in Old World vultures. Numerous analyses have confirmed these and other anatomical and behavioral differences, and the ancestral history of New World versus Old World vultures remains essentially settled. The once widely held belief that New World Vultures were more closely related to storks than to Old World Vultures and other birds of prey has largely been refuted. Assuming this to be realistic, the avian families Cathartidae and Accipitridae represent one of the best examples of what evolutionary biologists call convergent evolution, the independent evolution of similar characteristics in species or groups of species from different lineages resulting from ecological rather than evolutionary similarities (Box 0.2).

Box 0.2 Convergent Evolution

Convergent evolution, the independent evolution of structural or functional similarity in two distantly-related phylogenetically distinct lineages not based on common ancestry, provides evolutionary biologists with one of the most convincing arguments for the power of natural selection.

Widely recognized examples of convergent evolution in vertebrates include ecological and anatomical similarities between the phylogenetically distinct North American gray wolf and the southern hemisphere Tasmanian wolf; African sunbirds and American hummingbirds; sharks and dolphins; and New World and Old World vultures. Although the latter two lineages are not particularly closely related, the two exhibit many similarities, several of which include:

Anatomical similarities

Bare or minimally feathered heads. Most species in both groups feature completely bare or minimally feathered heads, most likely because it makes it easier for individuals to keep their heads clean when feeding on and inside of carcasses.

Oversized wings. Species in both groups have large, oversized wings that result in lighter wing loading, a feature that facilitates sustained soaring flight and permits low-cost searching for carcasses.

Relatively large body masses. In both groups, body mass ranges from slightly less than 2.2 lb (1 kg) to more than 33 lb (15 kg). Large body mass permits alternating episodes of gorging and prolonged periods of non-feeding when carcasses are consistently available.

Distensible and visible crops. Representatives of both groups have visibly distensible crops that signal recent feeding, which serve to share useful information regarding successful feeding among individuals at communal roosts.

Physiological similarities

Controlled torpor. Representatives of both groups engage in controlled nocturnal torpor (a daily reduction in core body temperature) to save energy.

Low basal metabolic rate. Representatives of both groups exhibit low basal metabolic rates for their size.

Behavioral and ecological similarities

Communal feeding. This occurs particularly at large carcasses given that food availability exceeds the storage capacity of individual birds.

Communal roosting. This enhances the likelihood of information exchange among departing individuals regarding the location of carcasses.

Social networking when searching for food. Individuals in both groups often rely on cues from conspecifics to help locate carcasses.

Niche partitioning among species in feeding behavior in both groups. Species of both groups demonstrate species-specific behavioral and anatomical separation in feeding types in the shapes of their skulls, beaks, and mandibular dimensions that result in distinctive feeding types, which reduces competition at carcasses.

Plumage convergence between species

King Vultures and Egyptian Vultures. Although these two species differ considerably in the structure and color of their head and neck, as well as in the color of their tail, adults of both have distinctive, largely white or cream-white body feathers, and upper and lower wing coverts, with black primary and secondaries.

Turkey Vultures and Red-headed Vultures. Although the two species differ considerably in the structure of their head and neck, adults in both species have largely bare-skinned red or reddish heads and largely black or blackish body feathers and upper and lower wing coverts, and silvery or whitish gray bases to their wing feathers.

None of the convergences highlighted above negate the fact that New Word and Old World vultures differ in many significant ways. New World vultures, for example, have proportionately and notably smaller toes than do Old World vultures, and three species of New World vultures smell for as well as look for carcasses. Finally, New World vultures are decidedly less specific in habitat use.

Although initial assessments of the origins of Cathartidae and Aegypiinae-Gypaetinae placed the first in the New World and the latter two in the Old World, more recent fossil evidence suggests that the New World family Cathartidae most likely originated in the Old World Eocene more than 50 million years ago, and that both Old World subfamilies occurred in the New World Miocene more than 15 million years ago. Regardless of what we now know about their apparently convoluted geographic origins, anatomical differences between New and Old World vultures are such that Loye Miller, the California museum curator who unearthed the first fossils of Old World vultures in California, USA, in the early 20th century, delayed announcing his finding for several years because he thought his colleagues might not believe him. Also, although the terms New World and Old Word vultures remain widely accepted, it is important to keep in mind that they refer only to current species distributions of the two groups and not their geographical origins.

New World Vultures. New World, Cathartid, vultures first appear in the Old World fossil record dating from Lower Eocene and Oligocene of Europe, as far back as 50 million years ago, and in the lower Oligocene of Colorado 30–40 million years ago. Intriguingly, two of the earliest Cathartid fossils, Plesiocathartes and Diatropornis, were relatively small raptorial-like birds, leading some to suggest they were not specialized carrion feeders. The group’s appearance in Old World Eocene sediments suggests Cathartidae most likely originated and radiated in warmer parts of Eurasia sometime in the late Cretaceous or early Eocene, before emigrating to and thereafter radiating in the western hemisphere, where numerous Pleistocene fossils occur. Indeed, all seven living Cathartidae occur in the Pleistocene as well, as do several close relatives including a slightly larger version of the California Condor; a more massive, but shorter and bulkier-footed version of the Black Vulture; and a King Vulture type that was intermediate in size between its living counterpart and the Andean Condor.

As yet unexplained is the family’s ancestral extirpation in Europe. Whether that elimination resulted from changing climates, competition with evolving Old World vultures, or something else entirely, is largely undiscussed in the literature.

Old World Vultures. As mentioned above, Old World, or Accipitrid, vultures evolved into two separate and distinct subfamilies, the Gypaetinae and the Aegypiinae. The former, which is more ancient as well as more ecologically diverse, is particularly fascinating in that its lengthy history provides useful insight into origins of the vulturine lifestyle, at least within the subfamily. Gypaetinae currently includes the Palm-nut Vulture, a species once thought to be closely related to sea eagles, along with the Egyptian Vulture, the Bearded Vulture, and curiously enough, the non-vulturine Madagascar Serpent Eagle, all of which differ considerably in diet. The Palm-nut Vulture is mostly vegetarian and secondarily predatory, as well as carrion eating. The Egyptian Vulture is an opportunistic obligate scavenger whose eclectic diet includes insects, human rubbish, scraps from larger carrion, vulnerable young and injured small vertebrates, and ostrich and other large-bird eggs. The Bearded Vulture is a dietary specialist that feeds on large bones, which it breaks into small pieces, and on both non-bone carrion and living prey. The Madagascar Serpent Eagle is a predatory raptor that largely takes lizards and tree frogs. (Note: I have described the Palm-nut Vulture in this book because of its current common name, ancestry, and part-time carrion-eating behavior, but I have not described the Madagascar Serpent Eagle both because of its name and because, ecologically, it is not very vulture-like.) Although ecologically and somewhat genetically divergent from other vultures, these four species are related, genetically, more closely to each other than to other raptors including, most notably, the somewhat distantly related Aegypiinae. Both molecular and anatomical data indicate that the subfamily Gypaetinae is closely related to the Perninae, a predatory subfamily that includes honey-buzzards and several other medium-sized, broad-winged, and largely tropical raptors.

The more speciose Aegypiinae includes all eight living species of Gyps vultures, a group of large, sandy to brownish, long-necked vultures of southern Europe, Africa, and southern and central Asia. Gyps are believed to have diversified in response to a global grassland transition that created vast regions of megafaunal-filled landscapes during the early Miocene and late Pliocene epochs 23 to 2.5 million years ago, together with four larger, thicker-billed, and darker Old World species, including the Red-headed Vulture, White-headed Vulture, Lappet-faced Vulture, and Cinereous Vulture. The exact relationship of the much smaller and thinner-billed Hooded Vulture within the subfamily remains