For two consecutive seasons or more we monitored the vocal activity of populations located at thermal extremes of the distribution in the Iberian Peninsula of three species of midwife toads. We used automated recording stations with a fixed time-based recording protocol (3 min per hour, 24 hrs per day), coupled with dataloggers measuring water, soil and air temperature as well as air relative humidity. The correlation between body size and call dominant frequency allows to infer information about the distribution of male body size (age) in the chorus. Calling activity was detected in the digital files using X-Bat software. The phenology of vocal activity varies greatly between sites and species both in date of initiation and in duration of calling season. Chorus attendance and composition (size distribution) varies with time within population and also between populations. The results confirm the value of acoustic monitoring as a valid methodology for census and demographic comparisons of populations. The results about thermal correlation of calling conditions show a moderate amount of within-species variability and more marked between-species differences.