Up to now, little is known about female vocal behaviour and about vocal communication between male and female songbirds in the northern temperate zone. However, raising a brood together requires successful communication between both members of a pair. By placing small microphones inside the nesting cavity of great and blue tits, we were able to record several novel female sounds that have not been described previously. These recordings also provided evidence of vocal interactions between the female inside and the male outside the nest hole. Females produced on average 2.7 different vocalisation categories during each recording session, ranging from one to seven. With only one exception, all females used the same vocalisation type when vocally interacting with their mate. The acoustical properties of these sounds suggest that they are designed for short range communication and sonographic representation revealed remarkable similarities in the vocal signals of blue and great tit females. We discuss the possible functions and selection pressures shaping female vocal signals and we provide a possible mechanism promoting vocal mimicry in blue tits. We present a recording approach that might help studying short range communication and vocal behaviour in female (song)birds which are otherwise difficult to observe.