Many territorial animals exhibit differences in their responses against intruders based on the level of threat that they pose. The dear-enemy and the nasty-neighbour effects refer to situations in which territorial aggressions are stronger against stranger and neighbour individuals, respectively. Using playback experiments during pre-breeding and post-breeding seasons in a songbird from Neotropical montane forests (Grey-browed Brushfinch, Arremon assimilis), we found that males exhibit the nasty-neighbour effect because they responded more aggressively towards neighbours than to strangers. However, territorial behaviour varied seasonally: (1) aggressions to all intrusions by neighbours were equally strong regardless of the location from which they were perceived prior to reproduction and (2) individuals were more aggressive towards neighbour males when perceived at a different border of their territory to the one they share during the post-breeding season. We conclude that territorial males respond to neighbours by assessing their threat to paternity and territoriality and thus modulate their aggressive response based on the season. In contrast, limited responses to strangers suggest that these individuals do not represent a serious threat to males of A. assimilis during the seasons we studied them. Thus, territorial aggressions against neighbours appear to be a mate-guarding mechanism in this species. Our results differ from those found in temperate zones, where strangers often elicit responses indicating they may represent a stronger threat than neighbours. Additional studies on the behavioral ecology of tropical birds are required to understand the generality of nasty-neighbour effects and the drivers of territorial behaviors.

All analyses and graphing were performed in the statistical software R. The necessary packages are mentioned in each script.