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]
Song repertoires and territory defence in the great tit by John Krebs and others. Byline and Abstract; three repertoire experiments
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.
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