City birds let men walk about a meter closer than women before flying off, according to a study of 37 species in five European countries published in the British Ecological Society journal People and Nature and reported Oct. 22 by ScienceDaily.
The pattern held in every country and for every species tested. The researchers have no explanation.
“We have identified a phenomenon, but we really don’t know why,” said Federico Morelli of the University of Turin, one of the authors.
How they tested it
The team worked in city parks and green spaces in the Czech Republic, France, Germany, Poland and Spain. Male and female volunteers, matched for height and clothing, walked straight toward a bird. The researchers recorded how close each person got before the bird took off, a measure biologists call flight initiation distance.
They logged 2,701 approaches. The birds were familiar backyard and park species, including great tits, house sparrows, blackbirds, magpies and pigeons.
On average, men got about one meter, a little more than three feet, closer than women did.
A surprise to the researchers
The result caught the team off guard, including the women doing the walking.
“As a woman in the field, I was surprised that birds reacted to us differently,” said Yanina Benedetti of the Czech University of Life Sciences Prague.
Daniel Blumstein of the University of California, Los Angeles, a co-author, said the team used “bleeding-edge comparative analysis techniques” and still came up empty. “I can’t explain them right now,” he said of the results.
What might be going on
The researchers believe the birds can tell men and women apart, but they don’t know which cue the birds are using. Possibilities mentioned in the report include scent signals such as pheromones, differences in body shape, and differences in the way men and women walk. The authors stress that all of these remain speculation.
The study looked only at European city birds, so it isn’t known yet whether American backyard birds behave the same way.
Try it yourself
You don’t need a grant to test this. The next time you walk through a park with a friend or spouse of the opposite sex, take turns walking slowly toward the same robin or sparrow and note who gets closer. One walk won’t prove anything, but it is a good excuse to pay attention to the birds.
The bottom line
Birds have lived alongside people for thousands of years, and they still notice things about us that we can’t see ourselves. A simple walk in the park, done 2,701 times, turned up a puzzle that the experts now have to solve.