Parasites are organisms that rely on another species, known as the host, for their survival, often to the host’s detriment. Parasitism takes several forms in the natural world. In birds, the most familiar example is brood parasitism, as seen in cuckoos, which lay their eggs in the nests of other bird species, leaving the unsuspecting host to raise their young.
Another form of parasitism is displayed by oxpeckers, which feed on the blood of large mammals such as rhinos, zebras, and cattle, while appearing to groom them.
A more unusual type is kleptoparasitism, a feeding strategy in which one animal steals food directly from another. The term, first recorded in the 1950s, comes from the Greek κλέπτω (kléptō), meaning ‘to steal’, and literally translates as “parasitism by theft”. Though relatively rare among birds, a few species are specialist kleptoparasites, while others may engage in it opportunistically. Some orders of birds contain a disproportionately high number of kleptoparasitic species, and those that do often occupy a narrow range of ecological niches.
Like all survival strategies, kleptoparasitism, must be evolutionarily stable to be beneficial. This means that once the behaviour is common within a population, it can’t easily be replaced by a new strategy through natural selection. In other words, it must be robust and self-sustaining within that specific evolutionary setting.
A 1998 paper published in Behavioral Ecology explored this idea using a simple computer model to study how animals in groups find food. In the model, each animal could either search for food itself or steal it from another.
Stealing food can save a bird the energy it would spend hunting and may allow it to obtain food it couldn’t catch on its own. But there are trade-offs: challenging another bird takes time, which could instead be spent foraging, and carries the risk of injury if the other bird fights back.
The study found that depending on the situation, it may be more effective to always steal, or to never steal at all. For any given set of environmental conditions, there is usually one strategy that results in the highest food intake. This also means that even small changes in the environment, such as a drop in food availability, can lead to sudden shifts in behaviour, making animals more aggressive or competitive.
The researchers also examined what happens when animals must choose between searching for food and looking for opportunities to steal, since they can’t do both at the same time. Using game theory, they calculated the best balance between these two activities under different ecological conditions, based on how easy it is to find food and how rewarding or risky stealing might be.
Their findings showed that when food is plentiful, ordinary foraging is the best approach. But when food becomes scarce, kleptoparasitism can suddenly become the more effective strategy.
Nearly 200 bird species from 33 different families are known to practise kleptoparasitism, and this behaviour is especially common among seabirds.
A notable example are frigatebirds, a family of birds found across tropical and subtropical oceans. These highly skilled fliers spend most of their lives in the air and feed mainly at sea. Their diet includes small fish, squid, jellyfish, and plankton. They’re also known to prey on seabird eggs and chicks, particularly those of the sooty tern.
Frigatebirds are famous for their kleptoparasitic behaviour, a trait reflected in both their English name and common nicknames such as man-o’-war bird and pirate of the sea.
Using their speed and agility, they attack other birds such as boobies, tropicbirds, shearwaters, petrels, terns, gulls, and even ospreys as they return to their nests with food for their young, forcing them to drop or regurgitate their food mid-air. The frigatebird then snatches the meal for itself. However, despite their reputation, studies show they only get about 5% of their food this way.
Skuas display similar behaviour, aggressively stealing from other seabirds. However, unlike frigatebirds, which engage in piracy all year round, skuas mostly do so outside the breeding season.
This behaviour may be playing a role in the spread of avian flu. When a frigatebird or skua forces another bird to vomit, the stolen meal is often coated in saliva, which, if the victim bird is infected, can contain high levels of virus. Since both frigatebirds and skuas migrate long distances, crossing oceans and hemispheres, they could carry the virus far from the original source. Large gatherings at breeding or roosting sites create even more opportunity for transmission.
As well as food theft, both species also scavenge and hunt, which may further contribute to the spread of the disease.
Gulls are another well-known example of aerial pirates, stealing food not only from other birds but from humans as well. They can be both thieves and victims of opportunistic kleptoparasitism, especially during the breeding season. The targets are often members of the same species, though smaller gulls and terns are also at risk. For instance, Heermann’s gulls and laughing gulls have been observed snatching fish from brown pelicans as they surface from dives, while great black-backed gulls have even been seen stealing from birds of prey.
Raptors too engage in kleptoparasitism, with the bald eagle being one of the most notorious offenders. Benjamin Franklin once described the behaviour in a letter to his daughter, recounting how he had seen a bald eagle stealing a fish from an osprey.
You may have seen him perched on some dead tree, where, too lazy to fish for himself, he watches the labour of the fishing hawk; and when that diligent bird has at length taken a fish, and is bearing it to his nest for the support of his mate and young ones, the bald eagle pursues him, and takes it from him.
Ducks also take part in kleptoparasitism. One study in southern Spain recorded gadwalls stealing food from coots at a lagoon, but only during a year when water levels were low. In previous years, when the water was higher, this behaviour wasn’t seen. In the same study, female gadwalls were observed to steal more frequently than males.
After a coot came up from a dive with food, the gadwall would wait a few seconds before trying to steal it. This delay may help avoid triggering an aggressive response from the coot. Whether the coot reacted aggressively or not, they spent the same amount of time feeding on the surface. However, when coots did fight back, they took longer before diving again.
Gadwalls that stole food didn’t seem to get more food than those feeding normally. In fact, when a gadwall tried to steal from more than one coot at a time, it ended up getting less food. And coots that were being targeted by gadwalls actually dived more often, likely to make up for the food they were losing.
Researchers think this stealing happened because the low water levels made food harder to find. Although coots are usually aggressive birds, they didn’t often fight back against the gadwalls.
Mallards have also been observed trying to steal zebra mussels from both other mallards and coots.
The victims were most likely to be targeted when they had medium or large mussels in their beaks. If the prey was small, the chance of being attacked was almost cut in half. Attacks were also less likely to succeed when the prey was small, but more successful when the prey was medium or large.
Stealing from coots worked better than stealing from other mallards. These findings support the optimal foraging theory, which says animals try to get the most food for the least effort, backing up the idea that kleptoparasitism may be a strategy to maximise feeding efficiency.
Although far less common, some passerines have been known to show occasional kleptoparasitic behaviour.
When Desmond Morris, best known for his 1967 book The Naked Ape, was a postdoctoral student at Oxford University, he wrote a 61-page paper titled The Snail-Eating Behaviour of Thrushes and Blackbirds. In it, he revealed that blackbirds lack the coordination needed to crack open snails on anvils, a skill mastered by their relatives, the song thrushes. Instead of doing it themselves, blackbirds take a more opportunistic approach: they watch and wait for a thrush to break open a snail, then swoop in to steal the freshly exposed flesh.
Kleptoparasitism creates direct competition for food and can play a role in shaping how bird communities are structured.
To explore this, a team of Spanish researchers studied four types of corvids feeding at a rubbish dump; rooks, carrion crows, jackdaws, and magpies.
They found that carrion crows were particularly focused on stealing food rather than searching for it themselves. They spent most of their time watching from the edges of the dump and then swooping in to snatch food from other birds.
During winter, when the most birds were around, each species visited the dump at different times of the day. Rooks and jackdaws came early in the morning, carrion crows appeared mid-morning, and magpies arrived in the early afternoon.
The researchers believe that kleptoparasitism influenced these time patterns. Carrion crows, for example, arrived when starlings, one of their main targets were most active, giving them more chances to steal food. Meanwhile, magpies came later to avoid the crows, which helped them avoid conflict and feed more efficiently.
This kind of time-sharing among species, called temporal segregation, is not often seen in vertebrates, but this study suggests it might be an important way for different species to coexist and share limited food sources without constant conflict.
