Horse-hair fungus used in bird nests alters the behaviour of acacia ants

Organisms that nest in ant-defended trees presumably benefit from ants deterring potential enemies. How birds build a nest while surrounded by ant workers and how the ants are kept away from the fledglings are unknown. In Costa Rican dry forests, two bird species (streak-backed oriole, Icterus pustulatus, and yellow-olive flycatcher, Tolmomyias sulphurescens) preferentially nest in Pseudomyrmex ant-defended swollen-thorn acacias (Vachellia) and build the entire nest with fungal rhizomorphs. We tested whether rhizomorphs alter acacia ants' behaviour as a potential mechanism to deter aggressive ants from the birds’ nests. We also tested whether water increased the properties of the fungus, because birds nest during the rainy season. Hence, we recorded detailed observations of ant behaviour after contacting plant fibres (control) and dry and wet rhizomorphs. After touching the rhizomorphs, the probability of grooming and speeding up was higher (>65%) than after contacting plant fibres (<30%). Some ant behaviours were never observed with the plant fibres but had a higher probability of occurring (∼16–67%) after the ants touched rhizomorphs: gaster curling, biting a nestmate, spinning and wandering erratically. Water did not enhance putative rhizomorph properties. The results indicate signs of alarm, repellence and intoxication after interacting with rhizomorphs. Therefore, birds may benefit from using fungal rhizomorphs as it may deter the ants from entering the nest. This finding opens a venue for research questions, including whether the fungus effects are specific to acacia ants or can be generalized to other social hymenopterans that the birds choose to live nearby (e.g. wasps) or need to keep away from the nest (e.g. army ants).