Out of sight, out of mind: dear enemy effect in the rufous hornero, Furnarius rufus

Abstract

Dear enemy and nasty neighbour are both phenomena that depend on the ability to discriminate between neighbours and strangers. Although both mechanisms reduce costs in territorial maintenance, their occurrence has been poorly investigated in duetting animals. In this study, we sought to test neighbour–stranger discrimination via cues in duets by the rufous hornero, Furnarius rufus (Furnariidae), a suboscine bird with high territorial fidelity and in which both sexes cooperate to defend the territory against competitors. Specifically, we assessed whether territorial owner aggression (1) is higher against neighbours or strangers (dear enemy or nasty neighbour effects) in the breeding and nonbreeding seasons and (2) whether aggression varies according to the intruder's position in the territory (centre versus periphery), the distance between neighbouring territories and the number of neighbours. Owners were more tolerant of neighbours' duets than of strangers' duets in the breeding and nonbreeding seasons. Furthermore, females and males demonstrated similar social discrimination abilities. The number of neighbours significantly influenced only the flight latency of the owner, and aggression against competitors was more intense at the territory centre. Aggressiveness was higher in territories whose neighbouring territorial boundaries were farther apart. Our results demonstrate that rufous hornero pairs can discriminate duets based on familiarity with the sound producers, with stranger pairs being more threatening than neighbouring pairs to resource possession throughout the year. To our knowledge, this is the first study to reveal neighbour–stranger discrimination in a duetting species. Future evaluations may reveal whether more sophisticated recognition mechanisms, such as individual recognition, are also present in this as well as in other duetting animals.