What a 20-month study of 9,318 contests and 572 hours of watching reveals about rank, friendship and the evolution of intelligence
In the first half hour of a typical feeding at Avifauna Bird Park in the Netherlands, a flock of Rüppell’s vultures (Gyps rueppellii) got into roughly 800 disputes over a carcass, of which about 230 had a clear winner. To a visitor it looks like a free-for-all. A new study from Utrecht University suggests there is far more order beneath the squabbling: a very stable pecking order, friendships that reach beyond mated pairs, and a social scene that shifts with the breeding season. In several respects, the vultures’ social structure looks comparable to that of some ravens and parrots.
Why Scientists Care
Researchers have long debated what makes animals clever. The Social Intelligence Hypothesis says big brains evolve because group life is demanding: you must cooperate with some individuals, compete with others and keep track of who is who. The Ecological Intelligence Hypothesis says brains evolve because finding food is demanding, especially when food is scattered or short-lived.
Most tests have focused on primates and, among birds, on crows, ravens and parrots. The lineages leading to birds and mammals split roughly 310 to 320 million years ago, so any similar cleverness in the two groups probably evolved independently. That makes birds a good test of whether social life shapes intelligence in general.
Vultures of the genus Gyps are a promising addition. Like corvids and parrots, they are long-lived, have fairly large brains, keep one partner for the long term and have a slow life history. Unlike them, they are specialists: they eat carrion and find it largely by watching each other across huge landscapes. They combine a hard-to-find food source with a busy social life. Yet little was known about how individual vultures actually relate to one another.
Ruppell’s Vulture Social Structure: How the Study Worked
The team studied 23 Rüppell’s vultures (nine females, five breeding pairs). Group size ranged from 16 to 23 birds as seven chicks hatched and two young birds were moved out. The birds lived in an aviary of about 14 × 34 × 4.5 metres, and were fed cow carcass pieces or whole rabbits every one to three days. Observations ran across 2023 and 2024, covering two breeding seasons (January to June) and two non-breeding seasons (July to December). Birds were classed as immatures (from about six months to 4 years), young adults (5 to 10) or adults (over 10). Exact ages were unknown for ten birds.
Two methods were used:
- Feeding contests, to measure dominance. Dominance here means how reliably a bird wins disputes over food. The first 30 minutes of 40 feedings were filmed, and every clear win, such as one bird making another retreat or submit, was logged. This gave 9,318 decided contests, which were converted into Elo scores, the rating system used in chess. Scores rise with wins and fall with losses.
- Follow-the-bird watches, to measure friendship. Individual birds were followed for 10 minutes at a time, 3,233 times in all (572 hours). The researchers recorded how long each pair spent within one wing-length of each other. Time on the nest was excluded, so sharing a nest would not be mistaken for choosing each other’s company.
Proximity is a proxy for friendship, since overt affection such as preening is rare in these birds.
Finding 1: A Very Stable Pecking Order
On a scale where 1.0 means nobody ever changes rank, the flock scored 0.9831. Rank reversals were rare over the 20 months. The hierarchy was also moderately steep (0.60 on average). Steepness measures how much the winning success of neighbouring birds differs, so a steep hierarchy means that a bird’s place in line really does predict its chances. Steepness was similar in breeding and non-breeding seasons.
Ravens and carrion crows have similarly stable hierarchies. Conflicts among wild Gyps vultures rarely escalate to injury, which may mean that an established order lets birds settle access to food through threats rather than fights.
Finding 2: Females Win More Contests Than Males
Male Elo scores were 28% lower. Males made 40% fewer threat displays and won 42% fewer contests. In birds of prey, females are often dominant because they are larger, but in Gyps vultures the size difference is minor, so size alone is unlikely to explain this. Females also dominate in Egyptian vultures. The result is not universal, though: a smaller captive study of mostly griffon vultures found no sex effect.
Finding 3: Pairing Boosts Rank, but Only in Breeding Season
During the breeding season, unpaired birds had Elo scores 45% lower than paired birds. Outside it, the gap closed, and unpaired birds’ scores rose 58% above their own breeding-season level. Having a chick made no detectable difference, so the benefit seems to come from being paired. Because ranks themselves hardly moved, what changed was birds’ likelihood of winning a contest, not the order of the ladder.
Age did not predict rank. In the wild, adults tend to win at carcasses, probably because they arrive first. The authors suggest that predictable feeding in captivity weakens this advantage, though this is untested.
Finding 4: Friendships Reach Beyond the Mate
Nearly every bird spent at least some time within a wing-length of every other bird, averaging about 2 minutes 16 seconds per pair for every hour of observation (nest time excluded). Some pairs spent far more, up to nearly 17 minutes. The researchers counted a pair as having a strong tie if they spent more than this group average together, which gave each bird about 6.6 strong ties. Because most pairs spend little time together and a few spend a lot, these ties point to real preferences rather than random mingling.
Mated pairs, birds in an established pair bond, were the closest of all. Outside breeding season, partners spent roughly eight times as long near each other as two birds that were not partners. During the breeding season the gap narrowed to about 2.4 times. Paired birds did not shut out the flock: even in breeding season they kept roughly 4.5 to 7.5 strong ties besides their mate.
Breeding changed who was sociable. During breeding season, paired birds spent less time near other birds off the nest and were less central, meaning less connected to the flock’s web of relationships. Unpaired birds spent more time near others, were more central and had 50% more strong ties than paired birds. Outside breeding season, paired birds became more sociable again.
Partners also spent less time near each other during breeding season. This may reflect the two taking turns at the nest, though the exclusion of nest time from the data could contribute to the dip.
The friendships were stable over the 20 months. Pairs that spent a lot of time near each other in one six-month period tended to do so in later periods too, with an average correlation of 0.52. A correlation of 1 would mean identical patterns and 0 no relationship, so this is a moderate but consistent carry-over. Most pairs were close to 0.5 (0.45 to 0.50), while the two periods of 2024 matched most closely (0.75). Because only five of the roughly 250 possible pairs were mated, the stability cannot be explained by mated pairs alone.
Finding 5: The Young Are the Most Sociable
Pairs of two immatures (birds from about six months to 4 years old) spent about 2.6 times as long near each other as pairs of two adults. Pairs of an immature and an adult also spent more time together than adult pairs, though the difference was smaller (about 1.3 times). Whether a pair was two males, two females or one of each made no difference, and neither did the gap in their ranks. This contrasts with non-breeding wild ravens, which seem to seek out higher-ranking companions. Because the study compared age groups over only two years rather than following birds through their lives, it cannot say whether individual birds become less social as they age.
What It Means for the Evolution of Intelligence
The authors assess social complexity along two lines. Relationship differentiation is how varied a bird’s relationships are. Social uncertainty is how unpredictable the social environment is. The vultures show signs of both: a stable rank order, a spread of friendships of different strengths, and seasonal swings in who is strong and who is sociable.
Tracking all this plausibly requires recognising individuals, remembering past encounters and adjusting to change, demands similar to those proposed for ravens, crows and parrots. Because vultures are specialist scavengers, the results are consistent with the idea that the pressures of finding scattered food and of living in dense groups may act together.
What the Study Cannot Tell Us
This was one captive flock in one aviary, with guaranteed food and managed breeding, and captivity clearly shapes behaviour, as the missing age effect shows. The study measured social structure, not thinking. It shows these birds live in a world that would demand sophisticated cognition, not that they use any particular mental skill. The authors call for studies of wild vultures and for experiments testing social reasoning, such as whether vultures understand relationships among other birds.
The Takeaway
Far from being simple scavengers, these vultures live in a social world with a steady pecking order, preferred companions and a calendar of shifting status. The study does not prove that they are clever, but it makes a strong case that vultures belong in the conversation about how social life shapes minds.
Source
Study: The social structure of a captive group of Rüppell’s vultures (Gyps rueppellii)
Authors: Eva S.J. van Dijk, Sophia Wolfswinkel, Jessica H. Smith, Jade A. Zanutto, Aimée Bakker, Zofia Trafas, Maartje H. L. Hooymans, Lisa-Claire Vanhooland, Jorg J.M. Massen (2026)
Read the full paper: https://www.biorxiv.org/content/10.64898/2026.10.09.757993v1
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