Rival Flycatchers Team Up in Paraguay’s Vanishing Grasslands

Deep in the grasslands of southern Paraguay, two of South America’s most threatened tyrant flycatchers are doing something no ornithologist has formally documented before: repeatedly joining forces. The strange-tailed tyrant (Alectrurus risora) and the cock-tailed tyrant (Alectrurus tricolor), both classified as globally Vulnerable, have long been known to share the same patchy stretch of Mesopotamian grassland. But a new study published in Neotropical Biology and Conservation reports eight independent association events between the two congeners, offering the first documented evidence of repeated syntopy within this unusual genus of grassland specialists.

The observations, gathered between November 2023 and August 2025 at two sites in southern Paraguay, reveal birds that not only tolerated each other’s presence but actively coordinated their behavior. Individuals perched within a meter of one another, shared perches, moved together across the sward, and launched simultaneous aerial sallies after flying insects. In several instances, the movements of one bird appeared to flush arthropod prey that the other then captured, a pattern consistent with the kind of trophic facilitation well described in mixed-species foraging flocks elsewhere in the tropics.

Both species are aerial insectivores that hunt from perches, a foraging strategy that makes them ecologically similar and potentially competitive. That similarity makes their apparent cooperation all the more striking. Alectrurus tricolor is distributed across central South America, occurring in northern Bolivia, southern Brazil, and eastern Paraguay, while A. risora occupies a more restricted range largely confined to southern Paraguay and northeastern Argentina, where it favors wet grasslands, savannas, and marshy environments. Southern Paraguay may represent the only region where their distributions currently overlap, since A. tricolor has not been recorded in Argentina for several decades.

The fieldwork took place at Estancia Algarrobo, surveyed from November 2023 to September 2024, and at Guyra Retã Natural Reserve, monitored from March 2025 onward. Both sites lie within the Mesopotamian grasslands ecoregion, a heterogeneous mosaic of natural grasslands and savanna-like structures embedded within forest formations. The savanna is dominated by herbaceous vegetation on silt-clay soils, with grasses such as Paspalum, Andropogon, and Axonopus shaping the landscape, while rushes and sedges characterize the wetland patches. This structural complexity supports a distinctive bird community that includes some of the Neotropics’ most specialized grassland species.

Detecting these birds is far from straightforward. A. tricolor is notoriously difficult to find in Paraguay, a limitation that has previously hampered standard survey approaches. The researchers therefore combined systematic searches along accessible grassland patches with opportunistic movements across suitable habitat to maximize encounter rates. Over roughly 600 hours of field surveys spanning 22 sampling months, A. tricolor was recorded on 22 independent occasions and A. risora on 19. Remarkably, the two species were found together on eight separate days, representing 36.4 percent of all A. tricolor records and 42.1 percent of all A. risora records during the study.

Most association events involved a single individual of each species, though two events included two A. risora individuals alongside one A. tricolor. Inter-individual distances ranged from less than one meter to approximately six meters, and birds frequently maintained close proximity over extended periods, often re-approaching each other after brief separations. Behaviors recorded during these encounters included coordinated movement, preening, active foraging, and aerial prey capture. The very first documented association, on 14 December 2023, produced one of the study’s most intriguing observations: when an individual of the lesser grass finch (Emberizoides ypiranganus) approached the A. tricolor, the nearby A. risora promptly drove the newcomer away, effectively defending a bird of a different species.

That territorial episode adds a layer of complexity to the interpretation. If the association were purely opportunistic, one might expect aggression between ecologically similar congeners competing for the same insects. Instead, A. risora appeared to guard its heterospecific partner, suggesting the relationship may confer genuine benefits. Prior research from Argentina offers supporting context: A. risora has been observed following army ants and armadillos alongside other grassland birds, apparently exploiting prey flushed by those animals’ movements. The species clearly has a predisposition for facultative foraging associations, and the new observations extend that pattern to a congener.

Timing may be critical. Most of the recorded associations occurred during the austral autumn and winter, when cooler and drier conditions reduce insect biomass across tropical systems. Under such constraints, joining forces could improve foraging efficiency through better prey detection, or reduce predation risk through collective vigilance. Notably, A. risora was recorded only between March and August, with no observations from September through February, a pattern consistent with historical accounts describing the species as rare and possibly seasonal in the region. Seasonal movements documented in Argentina suggest that local abundance may fluctuate substantially throughout the year, meaning the concentration of winter records may reflect broader movements within the species’ range rather than purely local changes in detectability.

Habitat degradation may also be reshaping these interactions. Research on fragmented and disturbed environments has shown that birds often respond to reduced habitat quality by strengthening heterospecific associations, joining mixed-species flocks to enhance foraging efficiency, information sharing, and predator detection. Selective logging and fragmentation, for example, have been shown to alter the structure and strength of mixed-species flock networks. For A. tricolor, ongoing habitat reduction has been documented in Paraguay, and the authors suggest that shrinking territory quality may be pushing the species into closer contact with A. risora under changing environmental conditions.

Both species depend on natural grasslands, among the most heavily transformed ecosystems in the Neotropics, and both face mounting pressure from habitat loss and agricultural conversion. Documenting repeated associations between them therefore carries conservation weight beyond its behavioral novelty. If these interactions enhance survival during lean seasons or in degraded landscapes, protecting the grassland mosaics where the two species overlap becomes doubly important. The researchers caution that eight observations, however striking, cannot yet establish the frequency, drivers, or full ecological significance of the behavior. Long-term systematic monitoring will be needed to determine whether these associations are a stable feature of the community or an emerging response to environmental stress. Either way, the image of two Vulnerable flycatchers hunting side by side in one of South America’s last great grasslands offers a vivid reminder of how much remains to be learned about the behavioral ecology of threatened birds, and how quickly the opportunities to learn it may disappear.

Subject of Research: Interspecific foraging associations between two Vulnerable Neotropical grassland flycatchers in southern Paraguay

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

Article References: Interspecific associations between two globally Vulnerable grassland specialists: Alectrurus risora (Vieillot, 1824) and Alectrurus tricolor (Vieillot, 1816) ( Passeriformes , Tyrannidae ). (n.d.). https://doi.org/10.3897/neotropical.21.e199685

Image Credits: AI Generated

DOI: 10.3897/neotropical.21.e199685

Keywords: Alectrurus risora, Alectrurus tricolor, tyrant flycatchers, grassland birds, Paraguay, interspecific association, mixed-species flocks, foraging behavior, Vulnerable species, Neotropical grasslands, habitat loss, behavioral ecology