A test of a hierarchical signalling model in song sparrows

• Male song sparrows tested twice with playback were not consistent in whether they type matched.
• Individuals were consistent in measures of aggressive response such as soft song and approach.
• Males that type matched did not produce greater numbers of aggressive displays or show stronger approach responses.
• Results were not consistent with a hierarchical model of aggressive signalling in which matching is a low-level threat.

Hierarchical signalling may be a common adaptation for aggressive signalling. In this strategy an animal progresses through a series of discretely different signals of escalating level of threat before eventually proceeding to physical aggression. A model of such hierarchical aggressive signalling has been proposed for song sparrows, Melospiza melodia, in which a core part of the sequence is: song type match → soft song → attack. The model predicts that song type matching is a strong predictor of soft song, but only a weak predictor of attack. We used a two-part playback design to test these predictions, with an initial edge playback from just off the subject's territory using a song type that the subject could match, followed by a centre playback from a speaker placed well within the territory. Each male was tested twice with this design. We found that matching the edge playback did not predict soft song production at the centre. A second strong threat, wing waving, was actually negatively associated with matching. Matching the edge playback also was not associated with physical measures of aggression such as approach and latency to approach. Thus, this particular model of hierarchical aggressive signalling was not supported for our study population. Song type matching in our study population may have some function not associated with aggressive signalling to the matched individual, or no function at all.