Black vultures nested within a metre or two of two different predatory raptors in Argentina. In one of the two cases the adults of both species were carrying food to their own young in the same room.
Diego Gallego-García and colleagues describe both cases in the Journal of Raptor Research, and call them the first documentation of black vultures nesting that close to raptors that hunt. Gallego-García works at the Centro para el Estudio y Conservación de las Aves Rapaces in Santa Rosa, La Pampa, and at Hawk Mountain Sanctuary’s Acopian Center in Pennsylvania.
The two events ended very differently. At the first site, in La Pampa province, a Chaco eagle’s egg disappeared after a camera frame showed a vulture at its nest, and by the end of that season neither pair had raised anything. At the second, in Santa Fe province, a vulture chick fledged out of one water tank while barn owl nestlings sat in another half a metre away, and both attempts worked out. Both cases played out in buildings people had walked away from.
The tower in La Pampa
The first structure was an abandoned hunting tower, 25 metres high, on a private field in the Espinal ecoregion of central La Pampa. The surrounding open woodland had been partly cleared by livestock grazing and was dominated by caldén trees, piquillín shrubs and grasses including olivillo and desert salt-grass. The top of the tower held a small cabin, 1.5 metres long by 1.5 metres wide and 2 metres high, and a square terrace less than a metre across.
On 17 October 2019 the researchers found a Chaco eagle incubating in a nest on the terrace, and a black vulture nest holding two eggs inside the cabin. They installed a trail camera, and it confirmed that both species were present in their respective nests at the same time with no evident interaction between them.
Five days later the camera produced the frame the paper turns on. On 22 October it recorded an adult black vulture at the Chaco eagle’s nest while the adult eagle was away. The figure caption for that frame notes the eagle’s egg is still in the nest, with the vulture nearby. After that date, the paper says, the egg was no longer there.
The authors are careful about what that sequence establishes. Black vultures scavenge reptile eggs, and the same research group has described a pair eating an unincubated second clutch at their own nest, in a paper that says it cannot rule out another female having laid those eggs. On that basis they write that they cannot discard the possibility that this pair took the eagle’s egg. They do not go further than that. Eggs fail for many reasons that a camera pointed at a terrace will never record.
Nothing came of the tower for either bird that year. The vultures stayed on near the failed eagle nest for the following two months, and on 22 December the researchers found that the vulture nest had failed as well. Unpublished data from two of the authors has vultures still occupying that site through 2023, which the paper says may be noteworthy given that both species failed there. Cornell’s species account notes that black vulture pairs reuse successful sites for many years.
The Chaco eagle has more to lose on paper, because it is the threatened one. BirdLife International, the Red List authority for birds, lists it, under the name crowned solitary eagle, as Endangered, with a global population estimated at 800 to 2,000 mature individuals and a decreasing trend.
The tanks in the Santa Fe house
The second event ran through December 2024 and January 2025, in an abandoned house on a private field in northern Santa Fe province. This is Dry Chaco country, cleared for agriculture and used mainly for growing soybeans, with rows of introduced eucalyptus threaded through stands of native mistol and garabato.
One room of the ruined house contained two empty open-topped water tanks, set half a metre apart. A pair of black vultures nested in one of them, probably laying in mid-October, and a single fledgling was recorded on 6 January. A pair of American barn owls nested in the other, with their eggs hatching in early December.
The timing meant the two broods overlapped. When the young vulture emerged from its tank and began moving around the room, the barn owl nestlings were still in theirs. During a week when the young vulture was mobile, the researchers report observing no interactions between the two nests, though they did watch the adult vultures and the adult owls each carry food to their own offspring. The paper’s wording there is “intraspecific interactions”, which in context reads as a slip for interspecific.
Body size does not appear to have set the terms. The black vultures were smaller than the Chaco eagle and larger than the barn owls, and the paper reports that the coexistence did not seem to be affected by the difference either way.
Vultures build no nest of their own
What makes a ruined building worth this much trouble is that black vultures build nothing to lay in. According to the Cornell Lab of Ornithology, they usually nest in dark cavities such as caves, hollow trees, abandoned buildings, brush piles, thickets and stumps, and their eggs go straight onto whatever floor they find: in these two cases, the floor of a cabin and the bottom of an empty tank. The paper calls nest-site availability a key limiting factor for raptors that do not build their own nests, and empty rural structures may offer sheltered spaces where, the paper suggests, alternatives may be scarce.
Scarcity is also the argument for tolerating a dangerous neighbour. The paper suggests that the quality of a site, and perhaps the scarcity of alternatives, may balance the disadvantages of competition. It raises the further possibility that proximity cuts both ways, with alarm calls and protection from the neighbouring pair diluting the risk from predators.
Set against that is the outcome at the tower, where both nesting attempts failed and the vulture pair may have caused the eagle’s. The paper’s own framing for the wider question is whether rewilding human-modified land produces safe habitat or an ecological trap, a place that looks attractive and then does not pay off. Two nests cannot answer that, though they give the question a specific and observable form.
This is two cases, five years apart in two provinces, reported out of ongoing nest monitoring rather than a sampling design that could establish how often this happens. Nothing in the paper supports a claim about how often raptors and vultures nest this close, and the authors do not make one. One pairing ended in two failures and the other in two successes, and with a sample of two there is no way to tell whether proximity had anything to do with either outcome. The reasons the paper offers for why the birds chose these buildings, avoidance of predators and better food nearby, come from earlier work elsewhere, and this paper tested none of them.
The “first documentation” line takes the same handling. It is the authors’ reading of the published record for black vultures, not an independent audit of every unpublished observation.
The buildings people leave behind
The ongoing abandonment of rural areas worldwide is the frame the paper opens on, and in the north-west Mediterranean it has already been linked to shifts in bird communities. What it leaves behind is an odd category of habitat: structures built for human purposes, walked away from, and then occupied by whatever can use a cavity somebody else built. What the structures supply is a sheltered platform or a cavity with walls around it, which the paper suggests may be scarce in the land around them, and the birds arrive without anyone deciding that they should.
Nobody designed any of that. The paper’s contribution is small and concrete: two documented examples of these neighbours nesting within a metre or two of each other, and two dated records of what it looked like.