Data from: Nasty neighbours in the Neotropics: seasonal variation in physical and vocal aggressions in a montane forest songbird, the Grey-browed Brushfinch

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.