Acid rain may be harming not only the trees birds nest in, but also the eggs they lay, a new study suggests.

The haunting song of the wood thrush was heard in forests throughout eastern North America, but the bird's population has been slowly declining.

In the United States, research has focused on how development and habitat fragmentation have affected bird populations. European studies have linked acid rain to declining bird populations there.

Wood thrushCourtesy: Mont St-Hilaire Nature Center

American researchers studied whether acid rain played a role in the decline of the wood thrush in North America. Pollutants from power plants contribute to acid rain.

Ralph Hames and his colleagues at Cornell University's ornithology lab used statistical models to compare breeding data with environmental measurements.

When they factored out the canopy, the researchers found acid rain probably harmed breeding by depleting soil calcium. Females need to eat calcium-rich foods such as snail shells to lay eggs with hard shells.

The team concluded birds may have difficulty finding these foods in acidified regions. They say their results suggest acid rain should be considered in addition to habitat fragmentation when addressing bird populations declines.

The study appears in Tuesday's issue of the Proceedings of the National Academy of Sciences.