Crested auklets sniff necks when they meet. Credit: Ian Jones

You smell a group of crested auklets before you see them, says Julie Hagelin. "It's like someone is peeling a tangerine next to you," she says.

The citrus-scented seabirds are the first found to communicate using odour. The birds seem to use perfume to make themselves attractive, Hagelin's team has discovered.

Crested auklets (Aethia cristatella) live in Alaska; a breeding colony can be 100,000 strong. When birds meet, they press their bills against each other's necks - feathers here have the strongest smell.

Auklets prefer smelly feathers, or cotton wool soaked in the same chemicals, to other mixes, such as animal musk or an odourless control, the researchers found1.

It's not known what the perfume signifies. It could convey the bird's quality as a mate - well-fed birds might be able to produce more scent, speculates Hagelin, a biologist at Swarthmore College in Pennsylvania. "This is a completely new mode of communication in birds that we've overlooked," she says.

The perfume is a blend of oils, but how it's made is also a mystery. Oil from the auklets' preening gland is odourless, but bacteria on the feathers might break it down into something smellier. The birds only smell during the breeding season, with males and females becoming equally pungent.

Many birds, including vultures and pigeons, rely on scent to find food and navigate, but the use of chemicals to signal to others was unknown. The auklets open up a new field, says physiologist Bernice Wenzel of the University of California, Los Angeles.

"It's too soon to say whether [these signals] will turn out to be common," says Wenzel. "The important thing is that more research of this sort should be conducted."