How and when the Rufous Hornero (Furnarius rufus) treats neighbors as dear enemies

Abstract

Social discrimination is crucial for mitigating territorial costs in animals and has been widely studied across different contexts. Among birds, the "dear enemy effect" (greater tolerance for familiar neighbors) and the "nasty neighbor effect" (increased aggression toward neighbors) are central phenomena. The rufous hornero (Furnarius rufus), known for its territorial fidelity and coordinated vocal duets, is an ideal model for investigating these effects. The rufous hornero consistently demonstrates the dear enemy effect, showing lower aggression toward neighbors compared to strangers. From an acoustic perspective, duets encode pair identity, individual identity, and sex differences, playing a central role in social discrimination. Despite their effective propagation in the environment, these vocalizations allow accurate identification of only adjacent neighbors. Acoustic degradation and limited close interactions hinder the precise identification of distant neighbors, despite the potential benefits of such ability. The rufous hornero's ability to discriminate between neighbors and intruders is similar across populations with different densities (Brasília and Juiz de Fora). Additionally, territorial centrality does not influence aggressive responses, indicating that territorial location offers no clear advantages in mitigating territorial costs. In urban environments, noise can complicate the distinction between neighbors and strangers. However, rufous horneros maintain consistent discrimination, even in areas with high human activity, demonstrating resilience in identifying competitors. Thus, the rufous hornero serves as a robust model for studying social discrimination and territorial behavior in birds, highlighting how ecological, territorial, and acoustic factors shape cost-mitigation strategies. Future studies could expand the understanding of these strategies across different contexts and species.