The males of songbirds, parrots and hummingbirds develop complex song through learning. Males of some species mimic the vocalizations of other species and make their song more complex through vocal mimicry. Females of several songbird species respond preferentially to more complex song. The sensory exploitation hypothesis is an explanation how female preferences for complex song historically came to exist in birds. Female response to song readily habituates to repeated presentation of simple (that is, monotonous) song, while complex song can reduce habituation in female response to song. Males singing complex song might have exploited such pre-existing property (or bias) in the female's response to song. This explanation is supported by experiments involving measurement of the expression of immediate early genes (IEGs). Analysis of IEG expression has been useful to reveal brain activation patterns associated with specific sensory stimuli. When exposed to male song, female songbirds and parrots show increased IEG expression in the auditory system in the caudal telencephalon, notably the caudomedial nidopallium (NCM) and the caudomedial mesopallium (CMM). Current data from female brains suggest that the NCM is related to song complexity. In addition, both of the NCM and the CMM may be involved in discriminating conspecific from heterospecific vocalizations.