Finding curious objects in bird nests is not uncommon. Birds may place them there intentionally – to attract mates, for example – but on other occasions their presence is an accident. Sometimes birds mistake these inanimate objects for eggs, treating them as their own and incubating them alone or alongside real eggs. This is known as pseudo-egg incubation.
In a new study published in the journal Ornithology Research, scientists document what they believe is the first case of pseudo-egg incubation by a black vulture (Coragyps atratus). And the pseudo-egg in question? An empty snail shell.
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The nesting behaviour was spotted by birdwatchers during a wildlife excursion in north-east Brazil.
“An unusual behaviour was observed wherein the vulture appeared to treat the shell as an egg,” say the authors in the study, “showing signs of nesting behaviour, specifically settling its brood patch over the shell and adjusting its position as if incubating; no other eggs were present.”
A month after the first encounter, a team member returned to the site, where “an adult vulture was observed at the same location near the shell, consistent with the species’ incubation period,” say the authors.
The team believe that, given the shell’s morphological similarity to a vulture egg, the most likely reason for this behaviour is that the bird mistook the shell for its own egg.
Although this is thought to be the first documented case of pseudo-egg incubation in black vultures, many other bird species have been seen exhibiting the behaviour.
In 1998, a study documented a case where a Canada goose was seen incubating a golf ball, and in 2012, a long-billed dowitcher (a type of shorebird) was observed incubating mammal bones. More recently, in 2017, researchers even recorded a grey-headed albatross ‘crafting’ its own pseudo-egg out of plant fibres.
This latest record improves knowledge of pseudo-egg incubation in birds of prey and shows the importance of citizen science in generating unusual ornithological discoveries.
Read more about the study.
Top image: Black vulture interacting with ground snail shell. Credit: André Barbosa Reis et al. Ornithology Research 2026
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