Scavenger Birds: What Would Happen If They Went Extinct?

Vultures are underappreciated birds, possibly for two reasons: their appearance and behavior. Their appearance, a featherless head, intimidating gaze, and somber, funereal plumage (Figure 1), definitely does not fit the beauty standards we have idealized for birds. And if we add their habit of feeding on carrion (decaying flesh), it becomes difficult to feel empathy toward a bird that we find repulsive and that reminds us of death itself.

In this article, we will learn a bit more about these underappreciated birds, the adaptations that allow them to have such a specialized diet, the benefits they offer to the ecosystem, and finally, we will show what happens when they disappear.

Figure 1. Black vulture (Coragyps atratus) perched on a log in Ocotepeque. Credits: Enrique Mejía.

Adaptations

Feeding on decaying carcasses means exposure to highly dangerous pathogens, in fact, some of the pathogens vultures face would cause us major public health problems if not removed. So how do vultures manage to survive these pathogens? Well, allow me to tell you about the incredible adaptations of these birds.

Figure 2. Two turkey vultures (Cathartes aura) feeding on a carcass in the middle of a highway in the United States. Credits: Matt Mason.

Let’s start with how they detect their prey. While their sight plays a fundamental role, smell can be crucial. Unlike most birds, which have a limited sense of smell, the turkey vulture (Cathartes aura) (Figure 2) possesses one of the most sensitive senses of smell among animals. This allows it to find carrion that could not be detected at plain sight¹. When a turkey vulture descends to search for some hidden treasure, other vultures, including black vultures, may approach to claim part of the spoils. When several individuals of different species gather at a mortuary feast, vultures maintain a strict hierarchy determined by their size and beak strength. In Honduras, the king vulture (Sarcoramphus papa) occupies the highest rank (Figure 3).

Figure 3. Black vultures and two king vultures (white-plumaged birds) gathered around a carcass in Peru. Credits: Scott Watson.

To feed, vultures access the internal tissues of a carcass through body cavities; for this reason, their characteristic bald head is a very important adaptation. The absence of feathers prevents blood, tissues, and decaying remains from sticking to them. However, this does not protect them from direct contact with pathogenic bacteria. A vulture’s head can harbor up to 500 bacterial species, while only around 80 bacterial species are found in its stomach², some toxic enough to kill any other animal³. The reduction in the number of microorganisms inside their bodies is due to the extreme acidity of their stomach juices, and the bacteria that inhabit their digestive system likely help in digestion and nutrient absorption².

Ecosystem services

The cleaning service vultures provide to ecosystems (Figure 2) has an invaluable value for humans. If animal mortality depended on other artificial methods such as burning, burial, incineration, composting, or anaerobic decomposition (which uses microorganisms to decompose organic matter without oxygen), the economic costs would be enormous. Vultures, in addition to being among the most efficient terrestrial scavengers⁴, help reduce the emission of gases generated during decomposition, including greenhouse gases like carbon dioxide and methane⁵.

Vulture Species in Honduras

Four vulture species are recorded in Honduras: the king vulture, the turkey vulture, the lesser yellow-headed vulture (Cathartes burrovianus), and the black vulture (Figure 4). Among these, the turkey vulture has the widest global distribution, and the black vulture is the most abundant species. Reading this, you might ask: why did we ask in the title, what would happen if they went extinct? Well, this wasn’t just a sensational title meant to catch your attention; this already happened in South Asia, and the absence of scavenger birds had alarming consequences.

Figure 4. Vulture species (Family Cathartidae) of Honduras.

It is important to clarify that within the guild of scavenger birds (a guild refers to a group of different species that exploit the same resources), Old World vultures (Europe, Asia, and Africa) and New World vultures (the Americas) do not share a close evolutionary origin; however, they fulfill a similar ecological function in their respective ecosystems.

Decline of Scavenger Birds

Scavenger birds have evolved an impressive tolerance to deadly toxins and bacteria⁴. However, one drug has brought them to the brink of extinction in India: diclofenac, an analgesic and anti-inflammatory widely used on livestock since 1994. No one could have imagined that the carcasses of livestock treated with diclofenac would cause the deaths of millions of vultures.

The decline of scavenger birds broke the natural cleaning system and triggered a cascade of effects. Carcasses accumulated, contaminating water bodies and attracting rats and feral dogs. The abundance of food caused these opportunists to increase in number, but since they lacked the necessary adaptations to neutralize pathogens in their bodies, they became spreaders of diseases like rabies, anthrax, and even Ebola⁶.

The consequences of the near-extinction of vultures in India led the government to ban diclofenac in 2006⁶; unfortunately, not before leaving a devastating mark on the environment. It is estimated that three vulture species lost about 95% of their populations. Today, only a fraction of them still soar over South Asian skies, mainly in refuges and protected areas⁷ (Figure 5).

Between 1992 and 2003, around 39 million people suffered dog bites, resulting in 48,000 deaths. This forced the government to allocate resources to sterilization campaigns and increase purchases of rabies vaccines. Moreover, when mortality rates were analyzed between 2000 and 2005, they showed an increase of 4.3%⁶. This period of public health crisis represented millions of dollars in government spending, and to this day, rabies remains a serious public health problem.

Figure 5. Group of Old World vultures. Photo from JSTOR Daily.

Vultures are underappreciated birds but play a fundamental role in keeping ecosystems clean. Their appearance and behavior have made them objects of disgust for doing a dirty but very necessary job. I hope this article helps you appreciate a bit more those bald, dark-plumaged birds and the important role they play in the ecosystem.

By: Stefany Flores, Biologist

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References

¹Lisney, T.J.; Stecyk, K.; Kolominsky, J.; Graves, G.R.; Wylie, D.R.; Iwaniuk, A.N. (2013). Comparison of Eye Morphology and Retinal Topography in Two Species of New World Vultures (Aves: Cathartidae). Anat. Rec., 296:1954-1970.

²Roggenbuck, M., Bærholm Schnell, I., Blom, N., Bælum, J., Bertelsen, M.F., Sicheritz-Pontén, T., Sørensen, S.J., Gilber, M.T.P., Graves, G.R., & Hansen, L.H. (2014). The microbiome of New World vultures. Nat. Commun., 5:5498. 

³Sibley, D.A. 2020. What it’s like to be a bird? From flying to nesting, eating to singing – What birds are doing, and why? Knope, USA.

⁴Ruxton, G.D. & Houston, D.C. (2004). Obligate vertebrate scavengers must be large soaring fliers. J. Theor. Biol., 228:431-436

⁵Plaza, P.I., & Lambertucci, S.A. (2022). Mitigating GHG emissions: A global ecosystem service provided by obligate scavenging birds. Ecosystem Services, 56:101455.

⁶Frank, E.G. & Sudarshan, A. (2023). The Social Costs of Keystone Species Collapse: Evidence From The Decline of Vultures in India. American Economic Review, 114:3007-40.

⁷Buechley, E.R. & Şekercioğlu, C.H. (2016). The avian scavenger crisis: Looming extinctions, trophic cascades, and loss of critical ecosystem functions. Biological conservation, 198:220-228.