Bird nests made with a toxic fungus seem to fend off attacking ants

When building a bird nest in ant territory, the best defense could be an offensive fungus.

Swollen-thorn acacia trees are aggressively defended by multiple species of ants. And yet, several species of birds across Central America and Africa choose to nest in these trees. It seems that fungal fibers in the nests deter the ants who encounter them and alter their behavior, making them apparently alarmed and intoxicated, researchers report in the October Animal Behaviour.

“It seemed very strange to me that the ants did not harm the chicks,” says Rhayza Cortés-Romay, an ecologist at the Universidad Mayor de San Andrés in La Paz, Bolivia. “So I started to think from the bird’s perspective: How does it achieve this?”

The relationship between swollen-thorn acacia trees (Vachellia collinsii) and ants is a symbiotic one. The ants collect nectar from the acacia and shelter in its thorns while patrolling the trees like possessive bouncers, biting and stinging invaders and even trimming back or killing encroaching plants. The ants may provide protection from other predators, but it was unclear to researchers how the birds kept them away from their young.

Throughout the tropics and subtropics, many birds use fungal organs known as rhizomorphs to build their nests. These cordlike structures are formed from millions of densely woven fungal filaments, which the fungi use to search for nutrients. In tropical rainforests, rhizomorphs grow in the forest canopy, forming complex networks that catch falling litter from above. Previous research suggests the birds may use rhizomorphs for their strong support, antimicrobial features and water repellency.

In Costa Rica’s Palo Verde National Park, two bird species that live often on swollen-thorn acacias build their nests almost entirely using rhizomorphs of horsehair fungus (Marasmius) — and appear to successfully keep out ants. So Cortés-Romay and behavioral ecologist Sabrina Amador Vargas of the Smithsonian Tropical Research Institute in Balboa, Panama, placed strands of horsehair fungus onto the branches of 30 acacia trees inhabited by one species of symbiotic ant (Pseudomyrmex spinicola), along with fibers of a nonfungal plant of similar thickness and type that birds also use to build nests. Then the researchers filmed the ants to record their reactions to the different strands.

Ants who touched the fungus groomed themselves and sped up more often than those who touched only the plant fibers, actions indicating repellence and alarm. The researchers also spotted some extreme behaviors seen almost exclusively after contact with the fungus, including biting other ants, spinning and wandering around aimlessly and unpredictably. This “drunkard’s walk” is reminiscent of ant workers infected by the zombifying fungus Ophiocordyceps and suggests the ants may have been disoriented or ingested toxins (SN: 2/21/23).

“The first time we saw the behaviors, we were amazed,” Cortés-Romay says. “These ants are very aggressive. To see something that affected them really was a discovery.”    

Some of the results offer compelling support to the idea that these fungi are noxious to the ants, says Fran Bonier, a behavioral ecologist at Queen’s University in Kingston, Canada. But birds use the same rhizomorphs on trees that are free of ants as well, she says, suggesting that they could also benefit nests in other ways.

At least 176 bird species use fungal rhizomorphs in their nests. Given that frequency, it’s not surprising the researchers found evidence that the rhizomorphs may provide some chemical defenses, says Todd Elliott, a biologist at the University of New England in Armidale, Australia. “This raises many questions about the chemistry of what is being released, and if it is unique to this ant and this fungus or whether analogous situations are playing out in other ecosystems around the world.”