Variation in the alarm calls of Southern House Wrens (Troglodytes musculus)

We examined variation in the structure and characteristics of alarm calls of nesting Southern House Wrens (Troglodytes musculus) during the 2007 and 2008 breeding seasons. We took multiple measures of 221 calls of 21 males and 18 females and analysed them for individual and sexual differences. We recognised two distinctive alarm calls, named Type I (TI) and Type II (TII). Sexes did not differ in rates of calling or call structure. However, females produced TI calls more frequently and TII calls less frequently than males. Rate and acoustical structure of TI calls varied with the age of nestlings, but there was no relationship between this factor and TII calls. Individuals increased the emission of TI calls with nestling age, but these calls had lower frequencies when nestlings were older. Given the differences we found across call types, we propose that each call type has a different function, which remains to be tested.