Female zebra finches that orally ingest the hormone estradiol benzoate as chicks develop “masculinized” brains and can sing if stimulated with testosterone as adults, UC Davis scientists have found.

Scientists discovered several decades ago that female finches exposed to estradiol — an estrogen commonly used in hormone replacement therapy — can sing like their male counterparts. Now UC scientists have found that giving doses of hormones to female finch chicks orally, a natural route of exposure to estrogenic chemicals in the environment, can induce “truly significant brain changes,” UC Davis animal science professor James Millam says.

Exposure to estradiol benzoate also caused infertility in male finches and hindered the songbirds’ ability to reproduce, according to studies by Millam and colleagues published in the December and April issues of Hormones and Behavior.

“Our results indicate that songbird populations may be at risk if they are exposed to estrogenic chemicals as chicks,” Millam says.

Male zebra finches exposed to estradiol benzoate, an estrogen, had a greater incidence of infertility and reduced reproductive ability.

Hormones are powerful chemicals that regulate sexual development and reproductive ability. Synthetic hormones are leaking into the environment and may be having important impacts on wildlife, Millam says. For example, millions of women take estrogen in birth control pills and menopause treatments; estrogen is not broken down by water treatment and remains in sewage wastewater. More relevant to birds is the presence of hormone mimics in pesticides and industrial chemicals, which can be ingested by developing chicks.

The finches received a variety of estradiol doses, primarily at levels higher than those encountered in the environment (except the most polluted areas), although some reproductive impacts — such as increased egg breakage and male infertility — were identified at environmental levels. The U.S. Environmental Protection Agency funded the research.