page_title: Dear enemy effect
section_title: In birds
hierarchical_section_title: Dear enemy effect / Occurrence / In birds
caption_attribution_description: Athene noctua English: Little owl in Teira, Galicia, Spain. Español: Habia dos muy cerca uno del otro. Teira, Galicia, España.
context_page_description: The dear enemy effect or dear enemy recognition is an ethological phenomenon in which two neighbouring territorial animals become less aggressive toward one another once territorial borders are well established. As territory owners become accustomed to their neighbours, they expend less time and energy on defensive behaviors directed toward one another. However, aggression toward unfamiliar neighbours remains the same. Some authors have suggested the dear enemy effect is territory residents displaying lower levels of aggression toward familiar neighbours compared to unfamiliar individuals who are non-territorial "floaters".
  The dear enemy effect has been observed in a wide range of animals including mammals, birds, reptiles, amphibians, fish and invertebrates. It can be modulated by factors such as the location of the familiar and unfamiliar animal, the season, and the presence of females.
  The effect is the converse of the nasty neighbour effect, in which some species are more aggressive towards their neighbours than towards unfamiliar strangers.
context_section_description: Audio playback studies are often used to test the dear enemy effect in birds.  These studies have demonstrated several bird species respond more aggressively to played back songs of strangers than to songs of neighbours; such species include the alder flycatcher (Empidonax alnorum), male blue grouse, European robin (Erithacus rubecula), and male banded wren (Thryothorus pleurostictus).
  Neighbouring male song sparrows (Melodia melospiza) differ individually in their aggressiveness. Increased aggression by residents towards intruders indicates that residents not only respond to intrinsic aggressiveness of their neighbours, but also to short-term changes in aggression levels.
  Although neighbour–stranger discrimination has been reported in many passerine birds, it has seldom been investigated in territorial non-passerine species. The nocturnal raptor, the little owl (Athene noctua), hoots to defend its territory.  Male little owls respond less to their neighbour's hoots played back from the usual location. However, responses to playback of a neighbour from an unusual location are similar to responses to playback of a stranger's hoots from either location.
  During the breeding season of the skylark (Alauda arvensis), particular common sequences of syllables (phrases) are produced by all males established in the same location (neighbours), whereas males of different locations (strangers) share only few syllables. Playback experiments provided evidence for neighbour–stranger discrimination consistent with the dear enemy effect, indicating that shared sequences were recognized and identified as markers of the group identity.  Studies have shown that the dear enemy effect changes during the breeding season of the skylark.  Playbacks of neighbour and stranger songs at three periods of the breeding season show that neighbours are dear enemies in the middle of the season, when territories are stable, but not at the beginning of the breeding season, during settlement and pair formation, nor at the end, when bird density increases due to the presence of young birds becoming independent. In song sparrows, where neighbours are most often the sires of extra-pair offspring, males will alter their aggression toward neighbouring males with their female's fertility status. When presented with simulated stranger and neighbour intruders during their female's pre-fertile and post-fertile periods, males displayed the dear enemy effect. However, when presented with simulated stranger and neighbour intruders during their female's fertile period, males exhibited an equal response to both stimuli, likely in order to protect their paternity. Thus, the dear enemy relationship is not a fixed pattern but a flexible one likely to evolve with social and ecological circumstances.
caption: The little owl hoots less intensively at familiar neighbours than unfamiliar
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