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How can sounds trigger aggressive responses in territorial birds?

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red deer stags (Cervus elaphus) — Aggressive behaviour A pair of red deer stags (*Cervus elaphus*) competing for possession of a female in the rutting season.

In territorial birds, sounds serve as critical auditory cues that signal the presence of an intruder, often triggering aggressive responses as part of territorial behaviour. During the breeding season, males use birdsong to advertise their territory and warn off rivals of the same sex.12

When a territorial bird hears the song of a conspecific, it perceives this as an incursion into its defended area. This auditory stimulus can elicit a direct aggressive response; for example, researchers have demonstrated that robins will vigorously attack a tape recorder playing the song of another robin.234

However, aggression is not an automatic, reflexive response. The likelihood of an attack depends on the bird's internal state, which is influenced by the activity of the nervous system and circulating hormones. Consequently, a bird may choose to attack a rival on one occasion while ignoring the same auditory cue at another time. Beyond song, other vocalizations, such as specific threat calls or bill snapping in owls, also function to deter intruders and maintain boundaries.41