City birds fly away sooner when a woman walks toward them

Birds in city parks often move away before a person gets close. A study across five European countries found they move away sooner when that person is a woman. On average, birds let men walk about one meter closer than women before taking off, and the researchers cannot explain why.

How the difference was measured

The team used a simple measure: the distance between a person and a bird at the moment the bird flies off. Volunteers walked straight toward birds in city parks and other green spaces. The walkers were men and women matched for height and clothing, so the birds were not simply reacting to a taller person or a differently dressed one. Even so, the birds left earlier when a woman was coming. The gap was about one meter — a small distance on a path, but a real one for a bird deciding whether to keep eating.

The same pattern in five countries and 37 species

The result held in Czechia, France, Germany, Poland and Spain. It also held across all 37 bird species the team looked at, including great tits, house sparrows and blackbirds, and even magpies, which are quick to startle, and pigeons, which usually let people come quite near. In total the researchers collected 2,701 observations, and the work was published in the journal People and Nature.

Professor Daniel Blumstein of the University of California, Los Angeles, who worked on the study, said the results convinced him even though the reason is missing: "I fully believe our results, that urban birds react differently based on the sex of the person approaching them, but I can't explain them right now."

Ideas, but no answer yet

What could birds be noticing? The team lists a few possibilities: chemical signals, differences in body shape, or differences in the way men and women walk. None has been tested enough to settle the question, and the researchers describe these ideas as speculative.

Dr. Federico Morelli of the University of Turin, a co-author, said the open question is part of what makes the finding interesting: "We have identified a phenomenon, but we really don't know why."

Dr. Yanina Benedetti of the Czech University of Life Sciences Prague said the discovery changed how she thinks about fieldwork. Many studies of animal behavior assume that a human observer is neutral, she said, but for the urban birds in this study that was not the case. She added: "Urban birds clearly react to subtle cues that humans do not easily notice."

The team stresses that the finding is preliminary. More work is needed to see whether the same difference shows up in other places and with other birds. Follow-up studies, Dr. Benedetti suggested, could test single factors — movement, scent, physical traits — one at a time instead of grouping them all under the sex of the person walking.

For now, the birds in city parks appear to be reading something about people that people themselves have not learned to read.