Many species maintain territories, but the degree of overlap between territories and the level of aggression displayed in territorial conflicts can vary widely, even within species. Greater territorial overlap may occur when neighbouring territory holders are close relatives. Animals may also differentiate neighbours from strangers, with more-familiar neighbours eliciting less-aggressive responses during territorial conflicts (the “dear enemy” effect). However, research is lacking on how both kinship and overlap affect territorial conflicts, especially in group-living species.

Here we investigate kinship, territorial overlap, and territorial conflict in a habituated wild population of group-living cooperatively breeding birds, the southern pied babbler Turdoides bicolor. We find that close kin neighbours are beneficial. Territories overlap more when neighbouring groups are close kin, and these larger overlaps with kin confer larger territories (an effect not seen for overlaps with unrelated groups). Overall, territorial conflict is costly, causing significant decreases in body mass, but conflicts with kin are shorter than those conducted with non-kin. Conflicts with more-familiar unrelated neighbours are also shorter, indicating these neighbours are “dear enemies”. However, kinship modulates the “dear enemy” effect; even when kin are encountered less frequently, kin elicit less-aggressive responses, similar to the “dear enemy” effect. 

Kin selection appears to be a main influence on territorial behaviour in this species. Groups derive kin-selected benefits from decreased conflicts and maintain larger territories when overlapping with kin, though not when overlapping with non-kin. More generally, it is possible that kinship extends the “dear enemy” effect in animal societies.