The "dear-enemy" relationship of territorial songbirds could be
mutually beneficial to neighbors, as males who recognize neighbors
and reduce their responses to these neighbors would require less
time and energy for territorial defense.
In order for this relationship to be evolutionarily stable, this
reduction in response to a neighbor must be conditional on
reciprocal restraint by that neighbor.
This study examined the possibility of such conditional responses in
hooded warblers (Wilsonia citrina). Responses of territorial
hooded warblers to playbacks of neighbors' songs from shared
boundaries were measured before and after playbacks that simulated
intrusions of those same neighbors (NNNN treatment) or strange birds
(NSSN treatment) into the subjects' territories. Each male received
both treatments separated by at least 8 days.
Males increased their responses to playbacks of a neighbor's songs
at the boundary after simulated intrusions of that same neighbor
(NNNN) but did not increase their responses to such playbacks after
simulated intrusions of strangers (NSSN).
This increased response to a "defecting" neighbor suggests that the
relationship between neighboring territorial hooded warblers is
based on a conditional strategy like tit-for-tat.