According to a new report in the journal Science, they are regulated by virtually identical neurochemicals in the brain, known as oxytocin in mammals and mesotocin in birds.

Neurobiologists at Indiana University showed that if the actions of mesotocin are blocked in the brains of zebra finches, a highly social songbird, the birds shift their social preferences. They spend significantly less time with familiar individuals and more time with unfamiliar individuals. The birds also become less social, preferring to spend less time with a large group of same-sex birds and more time with a smaller group. Conversely, if birds are administered mesotocin instead of the blocker, the finches become more social and prefer familiar partners.

Perhaps most striking is the fact that none of the treatments affect males -- only females.