Walk straight at a bird while staring at it and sooner or later it will leave. Biologists measure that moment, called flight initiation distance, and they use it as a rough gauge of fear. A team led by Federico Morelli wanted to know whether it matters who is doing the walking.
It does. As Morelli put it: "We have identified a phenomenon, but we really don't know why."
The experiment
The study, published in People and Nature, had pairs of researchers, one woman and one man of similar height, approach birds in seven cities in the Czech Republic, France, Germany, Poland and Spain. They wore similarly coloured clothes in white, grey or black, walked in a straight line at normal speed, and kept their heads and eyes fixed on the bird. Anyone whose hair was longer than their partner's hid it.
They collected 2,701 observations of 77 species, and analysed the 2,581 observations from the 37 species with at least ten records each: blackbirds, wood pigeons, magpies, great tits, sparrows and feral pigeons, among others.
On average, birds let men come to about 7.5 metres and women to about 8.5. That's a difference of around one metre, or 11% of the average distance, and it appeared in all five countries.
What's been ruled out, and what hasn't
The tempting explanations are height, clothes and hair. The design controlled for those in broad terms, but subtler differences remain possible. The authors mention details of hair, body shape, how people walk and how they smell, though they consider smell less likely. None of them has been tested, and the paper is explicit that the cause is unknown.
How much weight to put on this
It's a good study with some real limits, and they're worth spelling out:
- The observers were few. Four women and four men, all expert ornithologists. A small group of people might share a quirk that has nothing to do with sex, which is partly why the authors ask for replication elsewhere.
- The numbers are averages across species. Feral pigeons tolerate people far better, at around 3.5 metres, so what you see on a park bench may look different.
- "Same height" means similar. Pairs differed by an average of 5.5 centimetres, which wasn't significant. Calling them "the same height" would be slightly stronger than the paper.
- "Fear" is a shorthand. The authors call this distance a metric of fearfulness. It's a reasonable reading, but it's a measurement of leaving, and we can't ask the bird.
- It isn't new. The paper appeared online in December 2025. ScienceDaily ran the story in October 2026, which is when many people first saw it.
The other results
Two findings got less attention. Male birds let people come closer than female birds did, so boldness seems to differ on both sides of the encounter. And habitat made a difference: birds fled earlier with more trees around and later with more shrubs.
What we've skipped
The paper's discussion also wanders through waist-to-hip ratios and speculation about the role of women in hunting. We haven't gone into these because they're speculative. If you want to weigh them up yourself, the paper is linked below.
