Emory University's Donna Maney, who used white-throated sparrows for the study, says that the changes in hormone levels affect the perception by altering a system of brain nuclei, common to all vertebrates, called the 'social behaviour network.'
"Social behaviours such as courtship, parenting and aggression depend primarily on two factors: a social signal to trigger the behaviour, and a hormonal milieu that facilitates or permits it," said the study leader.
"Our results demonstrate a possible neural mechanism by which hormones may alter the processing of these signals and affect social decision-making," she added.
During the study, the research group treated female white-throated sparrows with estrogen, to mimic the levels seen during the breeding season, and compared them with females that had low, non-breeding levels of estrogen.
The birds listened to recordings of either male white-throated sparrow song (a courtship signal that should command the attention of breeding females) or synthetic beeps (which should be pretty boring for all the females).
The researchers then used a marker of new protein synthesis to map and quantify the activity in the social behaviour network that was induced specifically by song.
Across most of the network, song-specific neural responses were higher in the "breeding" females than the "non-breeding" ones. But the effects of estrogen were not identical in every region.
"If every node in the network just responded more in the presence of estrogen, then we'd conclude that estrogen acts as an on-off switch," Maney said.
"But what we're seeing is more complicated than that. Some activity goes up with estrogen, and some goes down. We are seeing how estrogen changes the big picture as the brain processes social information," she added.
The findings suggest that the perceived meaning of a stimulus may be related to the activity in the entire social behaviour network, rather than a single region of the brain.
"The same neural mechanism may be operating in humans. In women, preferences for male faces, voices, body odours and behaviour change over the course of the menstrual cycle as estrogen levels rise and fall. Our work with these songbirds shows a possible neural basis for those changes," Maney said.
The study will be published in the Nov. 10 edition of the Journal of Comparative Neurology.