Canyon wrens alter their songs in response to territorial challenges

Many animals use low frequency and harsh sounds in aggressive contexts. Low frequencies are correlated with body size in some species, and thus can provide an indication of fighting ability. Because of this, low frequencies and harsh sounds may also indicate aggressive intent and motivation to attack. We tested for facultative adjustment of vocal behaviour in canyon wrens, Catherpes mexicanus, by using playbacks to simulate territorial intrusions. Thirteen territory-holding wrens were exposed to playback of typical conspecific songs which did not include harsh elements. Territory holders increased song rate in response to simulated intrusions, altered song type usage and altered some song sound frequency parameters. Birds singing in response to playbacks did not adjust the highest frequencies or the frequencies with maximum power in their songs, but they significantly lowered the lowest song frequencies and were more likely to append harsh notes to the ends of their songs. Results match predictions of the motivation-structural hypothesis, and suggest that canyon wrens alter their songs in multiple ways when faced with conspecific territorial intrusion.