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John Krebs investigated animal behaviour through experiments connecting signals with ecological outcomes. With Ruth Ashcroft and Mike Webber, he examined birdsong and territory maintenance in great tits. Their 1978 paper describes removing resident pairs and occupying the vacated areas with loudspeakers; new pairs settled faster in controls than in song-occupied areas.^[ref]

The experiment makes an invisible social boundary testable: physical space remains after the owners leave, while an acoustic cue can still influence newcomers. It supports a signalling role in territorial exclusion without requiring an observer to witness a fight at every boundary. The same paper tested the effectiveness of varied song repertoires, illustrating the use of controlled signal manipulation to study animal communication.

Institutional profile: www.biology.ox.ac.uk/people/john-krebs .

Mike Webber coauthored a 1978 experimental study with John Krebs and Ruth Ashcroft on birdsong and territory defence in the great tit. The paper compared aspects of song repertoires as signals used in maintaining territories.^[ref]

A central design feature was playback: a loudspeaker can reproduce an acoustic cue while separating it from the presence of a resident animal. This makes a sound signal experimentally manipulable. A difference in settlement after playback is evidence about the signal's effect in the tested situation; it does not by itself show how an arriving bird internally represents the singer. The contribution verified here is the joint experiment rather than a separately documented division of labour among its authors.

Ruth Ashcroft coauthored experimental research with John Krebs and Mike Webber on the role of birdsong in great tit territory defence. Their 1978 paper tested whether variation in a male's song repertoire improves its effectiveness as a territorial display.^[ref]

The methodological question is how to distinguish a song's possible functions. Sound associated with a territory could influence competitors, prospective mates, or both. A test measuring settlement by competitors addresses a different outcome from a test measuring mate attraction. The study's experimental framing therefore links an observable acoustic pattern to a specified behavioural response, rather than treating every elaborate signal as evidence for a single function.

By John Krebs, Ruth Ashcroft and Mike Webber. Nature 271, 539–542 (1978). DOI doi.org/10.1038/271539a0

Original publisher abstract: resident-pair removal, loudspeaker occupation and faster settlement in control areas; three experiments address song repertoire and territorial display. Published online 1 February; issue date 9 February 1978.

A territory is an area that an animal or group defends against others in a specified context, such as access to food, mates or a nest. A home range instead describes the area normally used. An animal can visit a large home range while defending only one part of it, and repeated use alone does not demonstrate defence.

Defence need not involve continuous fighting. John Krebs and collaborators studied territorial birdsong in the great tit. Their 1978 paper describes experiments that removed resident pairs and occupied their former territories with loudspeakers. New pairs settled more quickly in control areas than in areas occupied by playback, supporting a role for acoustic signals in maintaining exclusion.^[ref] The comparison changes an apparent occupancy cue while removing the original owners, helping separate the effect of song from the mere presence of a resident bird.

For an observational example, a cluster of animal radiotelemetry positions may establish concentrated space use. Calling that cluster a territory additionally requires evidence such as exclusion, boundary displays or a response to an intruder. A location map alone cannot supply that behavioural conclusion.