Tropical Birds Show Repeated, Irreversible Evolution of Ant-Following
New research shows that dozens of Neotropical bird lineages have independently evolved to follow army ants — and almost never turn back.
The lure of the swarm
As rivers of army ants surge through Neotropical forests, they flush out insects and small creatures trying to escape. For hundreds of bird species, these chaotic hunting parties are a moving buffet. A new study by William Sweet and colleagues reveals that this remarkable behaviour – known as ant-following – has evolved repeatedly, irreversibly, and across distant branches of the avian family tree.
The team compiled data on 472 species across 41 Neotropical bird families and used evolutionary models to trace how and when they began following army ants. Their findings show that regular or ‘obligate’ ant-following evolved independently at least eight times, across four families: the Antbirds, Ovenbirds, Tanagers and Cuckoos. Once a lineage began down the path of specialisation, there was almost never a reversal.
Evolution in one direction
Phylogenetic analysis revealed an extraordinarily strong evolutionary signal for ant-following, suggesting the trait is deeply conserved once it arises. Ant-following birds seem to “ratchet up” their specialisation through evolutionary time – progressing from occasional attendance at ant swarms to full dependence on them for food. Obligate followers, such as the Ocellated Antbird or the Rufous-vented Ground-Cuckoo, have largely lost the flexibility to forage independently.
Across the studied clades, the transition towards specialisation was strikingly one-way. Few species showed evolutionary reversals back to less specialised forms. In the antbirds and woodcreepers, ant-following developed in three distinct clades apiece. Even where minor reversals occurred – for example, in certain woodcreepers and ground-cuckoos – the overall pattern was of ever-increasing reliance on the swarms.
When morphology doesn’t matter
Intriguingly, physical traits such as body size, bill length, or leg proportions – often considered key indicators of ecological role – did little to predict whether a species was a dedicated ant-follower. Random forest models found weak associations, and the birds’ trait spaces overlapped heavily between categories. This means that even though larger ant-followers often dominate the prime feeding positions at the swarm front, those size advantages haven’t driven the evolution of the behaviour itself.
Instead, Sweet and colleagues suggest that ant-following evolves primarily through evolutionary history and ecological opportunity rather than physical adaptation. Once a lineage acquires the behaviour, natural selection seems to favour its retention and gradual intensification, not its loss.
The ecology of dependence
Army ants themselves dictate much of the opportunity. The most influential species roams in the forest understory at low elevations – conditions that may help explain why obligate followers are mostly found in lowland tropical forests. Birds in higher or fragmented forests tend to remain occasional followers, taking advantage of swarms when they appear but not relying on them.
In the dense jungles of Central and South America, the sight and sound of a feeding frenzy – dozens of antbirds, woodcreepers, and tanagers swarming alongside predatory ants – encapsulates the intricate interdependence of tropical ecosystems. This study shows that such partnerships have deep evolutionary roots, and once formed, they become enduring features of the forest’s living web.
Beyond the Neotropics
The authors note that army ant-following of this kind is unique to the New World tropics. In Africa and Asia, ants and birds interact differently – with no equivalent assemblages of obligate ant-following species. The repeated, irreversible emergence of this behaviour across Neotropical bird lineages highlights both the ecological power of the ant swarms and the evolutionary pull of opportunity in a complex environment.
In evolutionary terms, once a bird joins the swarm, there seems to be no turning back.
October 2025
Read the full paper here
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