Interspecific associations between two globally Vulnerable grassland specialists: Alectrurus risora (Vieillot, 1824) and Alectrurus tricolor (Vieillot, 1816) (Passeriformes, Tyrannidae)

Neotropical grasslands are among the most threatened ecosystems worldwide, and their degradation has particularly affected grassland specialist birds. Alectrurus tricolor and Alectrurus risora are two globally Vulnerable tyrant flycatchers associated with these environments, whose ranges partially overlap in southern Paraguay. Despite their ecological similarity, interspecific interactions between these congeners have not previously been documented. Here, we report repeated interspecific associations between A. tricolor and A. risora based on field observations conducted between November 2023 and August 2025 at two localities in southern Paraguay. Eight independent association events were recorded, most involving one individual of each species, although some included two A. risora individuals. Birds remained in close spatial proximity, shared perches, moved in a coordinated manner, and engaged in simultaneous aerial foraging. In some cases, the movement of one individual appeared to flush arthropod prey subsequently captured by the other. Territorial behavior was also observed, including the active expulsion of another grassland bird species by A. risora while associated with A. tricolor . Most observations occurred during the austral autumn–winter period, when prey availability may be reduced. These behavioral patterns suggest that the association may provide ecological benefits related to foraging efficiency, prey detection, or vigilance. Additionally, ongoing habitat reduction affecting A. tricolor may contribute to increased heterospecific associations under changing environmental conditions. Our observations represent the first documented evidence of repeated syntopic association between these two Alectrurus species and provide new insights into the behavioral ecology of threatened Neotropical grassland birds.