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Birdsspecies group

Vultures

Accipitridae and Cathartidae

Two unrelated groups of birds arrived at the same body, because there is only one way to live on carrion.

The only vertebrates that live on carrion alone — and they can do it because soaring flight makes searching enormous areas nearly free. Two unrelated groups arrived at the same answer, and one of them was nearly wiped out by a drug given to cattle.

A vulture is the animal a model of search economics says must exist. Living on carrion alone requires covering huge areas to find things that appear unpredictably and vanish fast, and the only way to do that cheaply is to ride rising air. That is exactly what these birds do, and the fact that two entirely unrelated lineages — the Old World vultures, which are hawks, and the New World vultures and condors, which are not — arrived at the same body and the same habits makes the constraint clearer than any single case could. The convergence extends to details that look like family resemblance and are not: bare heads, enormous wings, the same soaring search pattern. It also stops at some details, which is why physiological findings from one group should not be assumed for the other. On what they eat: sequencing the microbial communities at the face and in the gut of New World vultures found a diverse community on the outside, reflecting the carcasses, and a drastically filtered one inside, dominated by bacteria — clostridia and fusobacteria among them — that would sicken most vertebrates. So the bird is not simply destroying everything it swallows. It is destroying most of it and then carrying a flora that matches its diet, which is a more interesting arrangement than pure resistance. And then the thing that happened to them. In the 1990s, Gyps vulture populations across South Asia fell by more than ninety per cent in about a decade, among the fastest declines recorded in any bird. The cause was diclofenac, a veterinary anti-inflammatory given to cattle, harmless to the cattle and fatal to vultures that fed on the carcasses. The mechanism is specifically a scavenging mechanism: one carcass feeds many birds, so only a small fraction of livestock needed treating for the effect to reach a whole population.

Early coverage · 34% complete · reviewed 2026-09-03

What this page covers

Two groups that are not closely related. Old World vultures are hawks and eagles; New World vultures and condors are a separate lineage. Their shared appearance and habits are convergence, which is why physiological findings from one should not be carried to the other.

Often confused with: One natural group, when they are two unrelated ones that came to resemble each other; Birds that circle over dying animals, which is not what soaring over a landscape is; The North American use of "buzzard" for a vulture, where elsewhere it means a hawk

Quick facts

Two groups, not one
Old World vultures are hawks; New World vultures are a separate lineage
Flight is why the diet works
Soaring makes searching huge areas nearly free
A gut that filters hard
Diverse flora on the face, a drastically reduced one inside
Over ninety per cent, in a decade
A veterinary drug in livestock carcasses

Two unrelated groups, the same bird

Convergence, and what it tells you about the constraint.

Old World vultures sit among the hawks and eagles. New World vultures and condors are a separate lineage entirely. They share bare heads, enormous wings, a soaring search and a diet of carrion — none of it inherited from a common vulture ancestor, because there was not one. When two lineages independently arrive at the same solution, the solution is usually being forced by something, and here the forcing is the search problem.

The practical consequence is that findings do not transfer automatically. The microbiome work below was done on New World species; whether Old World vultures share the arrangement is a separate question, not a detail. Convergent animals can differ substantially in physiology while looking and behaving alike.

Related

What survives the journey

Most of it does not — and what does would poison something else.

Sequencing found a diverse microbial community on a vulture’s face and a drastically filtered one in its gut, dominated by bacteria that would sicken most vertebrates. Not resistance alone: a flora to match the diet.

Well supported

Good evidence backs this, though some details remain open.

Facial microbial communities in New World vultures are diverse and reflect carcass contact, while hindgut communities are markedly less diverse and dominated by clostridia and fusobacteria, indicating severe filtering during passage together with tolerance of the surviving taxa.

Who this applies to
Two New World vulture species; Old World vultures are not closely related and were not sampled.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Coragyps atratus, Cathartes aura
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

The sequencing result is clear. What the surviving bacteria do for the bird is inferred rather than demonstrated.

How far it can be extended

New and Old World vultures resemble each other by convergence, not descent, and their physiology need not match.

Caveats

  • Sequencing shows which taxa are present, not what they contribute.
  • Two species, both New World; the convergent Old World vultures may differ.

Still unanswered

  • Whether the surviving gut community assists digestion of decayed tissue, or is merely what survives.

Last reviewed 2026-09-03

The evidence (1 study)

The design of the study is what makes the result legible: sample the outside of the bird and the inside, and compare. The face carries whatever the carcass carried, in all its diversity. The hindgut carries far less, and what it does carry is dominated by groups that are dangerous to most vertebrates. Both halves matter. A gut that only destroyed things would leave a sparse and unremarkable community; this one leaves a distinctive one.

A drug in the cattle

One of the fastest declines recorded in any bird, by a scavenging route.

A veterinary anti-inflammatory given to cattle, harmless to the cattle, killed the vultures that fed on their carcasses. Populations fell by more than ninety per cent within about a decade.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Residues of the veterinary NSAID diclofenac in livestock carcasses caused renal failure and visceral gout in Gyps vultures across South Asia, driving population declines exceeding ninety per cent over roughly a decade. The association was established by post-mortem residue analysis and supported by experimental exposure.

Who this applies to
Gyps vultures in South Asia; sensitivity to the drug differs between vulture lineages.
Studied in
Gyps
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

Pathology, residue analysis, experimental exposure and the population response to subsequent restrictions all point the same way.

How far it can be extended

Demonstrated across multiple Gyps species and colonies.

Caveats

  • Field association plus targeted experiment, not a randomised design — which the situation did not permit.
  • Other pressures on these populations existed; the drug is the established principal cause, not the only one.

Still unanswered

  • How far populations can recover where the drug is restricted but alternatives remain in partial use.

Last reviewed 2026-09-03

The evidence (1 study)

When the vultures went, carcasses lay longer and feral dog numbers rose. An appraisal attributed substantial public-health costs, chiefly rabies exposure, to the loss — an estimate, but a careful one.

Emerging evidence

Real findings exist, but too few or too recent to be settled.

National-scale appraisal linking the period of vulture decline in India to increased undisposed carcasses, increased feral dog populations and associated rabies-related health costs. The direction of effect is supported; the magnitude depends on modelling assumptions.

Who this applies to
India during the period of vulture collapse.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Gyps, Canis familiaris
Why we rate it this way, and what the caveats are
Emerging evidenceLow confidence

Correlational at national scale over a period of substantial social and economic change, with cost attribution resting on stated assumptions. The direction is credible; the figures should be read as estimates.

How far it can be extended

The consequences depend on local carcass-disposal practice, which differs greatly between countries.

Caveats

  • Many things changed in India over this period; isolating the vulture contribution is not straightforward.
  • Published cost figures vary substantially depending on the assumptions used.

Still unanswered

  • How much of the increase in feral dog numbers is attributable to carcass availability rather than to other changes.

Last reviewed 2026-09-03

The evidence (1 study)

The arithmetic of the exposure is the part worth understanding. A carcass is not eaten by one animal. Dozens of vultures may feed on a single cow, so a drug present in a small percentage of livestock reaches a large percentage of the vultures in a region. That amplification is a property of scavenging itself, and it is why a substance can be unremarkable in the treated animal and catastrophic one step along.

The cause was established from pathology and tissue residues in birds found dead, supported by experimental exposure — and the populations’ response after the drug was restricted is consistent with the diagnosis. It is about as strong as a causal case can be when a randomised design is out of the question.

The research behind this page

4 studies, newest first. Each one has a page explaining what it found and what it could not show.

2014Nature Communications

The microbiome of New World vultures

Facial communities were diverse and reflected the carcasses fed on.

2008Ecological Economics

Counting the cost of vulture decline — an appraisal of the human health and other benefits of vultures in India

The decline of vultures coincided with increases in carcasses left undisposed and in feral dog numbers, with substantial associated public-health costs attributed to rabies exposure.

2004Journal of Theoretical Biology

Obligate vertebrate scavengers must be large soaring fliers

Living entirely on carrion is energetically viable only for animals that can cover large areas at very low cost — that is, large soaring fliers.

2004Nature

Diclofenac residues as the cause of vulture population decline in Pakistan

Dead vultures showed visceral gout and renal failure, and carried residues of the veterinary anti-inflammatory diclofenac, acquired by feeding on the carcasses of treated livestock.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 34% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 4 claims and answers 0 mapped search questions.

  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • no popular claim about this subject has been checked yet
  • Old World vulture physiology is not covered; the microbiome work is New World only.
  • Soaring flight mechanics are described qualitatively and deserve their own treatment.
  • Vulture declines outside South Asia, including in Africa, are not covered here.