Consistent dear enemy effect despite variation in territorial centrality and population density in Rufous Horneros

Social discrimination is used as a territorial cost-mitigation behaviour across several taxonomic groups. However, whether this phenomenon is similar across populations or varies due to local ecological traits remains an open field. Here, we tested whether social discrimination ability exhibited by pairs of Rufous Horneros (Furnarius rufus) varies between two spatially distinct Brazilian populations (Bras & iacute;lia and Juiz de Fora), with contrasting population densities. We predicted that Rufous Horneros would show the highest aggression towards strangers' duets in the low-density population (the 'dear enemy effect'), and towards neighbours' duets in the high-density population (the 'nasty neighbour effect'). We also examined the impact of territorial centrality (distance to the centre of the conspecific neighbourhood) on territorial responses, assuming that territorial harassment varies with centrality. We observed a consistent dear enemy effect expression (i.e. lower response to simulated intrusions by neighbours compared to strangers) across both populations, despite their differing densities. Additionally, we found no effect of territorial centrality on playback responses. Our findings show that the dear enemy effect is widely distributed in Rufous Horneros, at least in the breeding season, being observed even in a high-density population. Our findings also highlight the lack of advantages for central territories. Future studies should investigate whether territorial mitigation strategies exhibit consistency across different contexts and taxa.