Balancing conflict and coexistence: Interactions between invasive monk parakeets and native urban birds.

Biological invasions often generate complex ecological paradoxes, particularly when invasive species act as ecosystem engineers that simultaneously compete with and benefit native communities. Understanding these dual dynamics is critical for managing urban biodiversity. Here, we investigated the interactions between invasive monk parakeets (Myiopsitta monachus) and native avifauna to assess the balance between behavioral competition and structural commensalism. We assessed competition through (1) agonistic interactions and (2) correlations between parakeet abundance and that of native species. Commensalism was evaluated by analyzing tenant species in parakeet nests and the drivers of their occurrence. Agonistic interactions manifested through highly species-specific conflicts: Density-dependent aggression with rock pigeons (Columba livia) was strictly reciprocal, while parakeets directed targeted intimidation toward Eurasian magpies (Pica pica). Conversely, direct agonistic encounters involving either Eurasian tree sparrows (Passer montanus) or house sparrows (Passer domesticus) were negligible. However, spatially, Passer spp. abundance correlated negatively with the number of parakeet nest chambers (a proxy for parakeet abundance), whereas common blackbirds (Turdus merula) showed a positive correlation. Furthermore, parakeets provided a massive structural subsidy. We recorded 11 native species breeding in 48% of surveyed parakeet nests (N = 252). Tree sparrows and stock doves (Columba oenas) dominated this tenant community, accounting for 86% of native breeding pairs. Native breeding abundance-including tree sparrows, stock doves, and rock pigeons-as well as total species richness scaled positively with nest chamber density. Yet, active parakeet presence limited nest use for stock doves but did not deter tree sparrows or rock pigeons from successful co-nesting. Our findings reveal a dual ecological dynamic: Parakeets show a negative spatial correlation with declining urban sparrows, yet simultaneously act as ecosystem engineers by providing valuable breeding habitats for local biodiversity. However, without data on tenant reproductive success and pathogen transmission, these novel subsidies risk functioning as ecological traps. Consequently, while indiscriminate nest removal could inadvertently harm native tenants, current evidence does not confirm the long-term safety of this commensalism. Effective management must transcend simple eradication, adopting a holistic framework that weighs the loss of nesting resources against the competitive and sanitary risks of retaining these invasive populations.