Phenology modulates the top-down control of ants on bird ectoparasites: from mutualism to antagonism

Bird-ant interactions are diverse but rarely tested experimentally, limiting their integration into ecological theory. One hypothesized but unverified benefit is ant-mediated parasite control in bird nests. Here, we present the first experimental evidence supporting this hypothesis in a wild system involving house sparrows (Passer domesticus), arboreal ants (Crematogaster scutellaris), and blood-feeding mites (Pellonyssus reedi). Using field ant-exclusion experiments, we show that ant presence reduces mite abundance and enhances chick growth early in the breeding season, but has detrimental effects later. Nestlings in ant-excluded nests also show consistently higher H/L ratios, indicating greater physiological stress. Structural equation modeling reveals that ant effects on nestling condition are indirect and mediated by mite load. Our findings provide the first causal demonstration of ant-mediated parasite suppression in birds, revealing that the outcome of this interaction is highly context-dependent. This work underscores the dynamic nature of species interactions and highlights overlooked ecological roles of ants in avian systems.