Lavender, yarrow, curry and mint keep bugs off Corsican chicks. Credit: © M. Lambrechts

Birds weave aromatic plants into their nests, apparently to keep their home clean and bug-free for raising chicks. Blue tits on the fragrant Mediterranean island of Corsica can even smell when it's time to refresh fading fragments, ecologists have shown1.

Female blue tits gather lavender, yarrow, curry, mint and other scented plants for their nests shortly after laying eggs, and continue to do so until the chicks leave home.

"They are real botanists and do a great job exploiting their environment to protect their chicks," says Marcel Lambrechts of the Centre for Functional Ecology and Evolution in Montpellier, France, one of the researchers.

The birds make a pot-pourri of 10 aromatic plants from the 250 species in their habitat. Many of the chemicals in these plants ward off bacteria, viruses, parasites, fungi and insects.

Lambrechts's team removed the aromatic plants from 64 nests and then placed a hidden box containing lavender and yarrow underneath half of the nests.

In the first 24 hours, only the birds with empty boxes replenished their herb supply. After 48 hours, the other half of the birds began to restock too, as the scent from the hidden herbs waned.

"This field test directly shows that birds are attending to odour cues," says Larry Clark, who studies similar behaviour in European starlings at the National Wildlife Research Center in Fort Collins, Colorado.

The blue tits select for chemical diversity as well as high concentrations of chemicals, he points out, underlining the importance of olfaction in avian behaviour.