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To be fair, I don't think most birds have a good sense of smell (as dogs, cats, deer etc have) so that would discount odour. I suspect if there is a factor here it may be something that is obvious to the birds.

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Odor was an example given by the researchers. And I too doubt it's due to odor, but we can't dismiss odor outright just because it seems reasonable. I mean humans for example are known to have an overall worse sense of smell than dogs, but can actually detect certain specific odors/chemicals at much lower thresholds than dogs (such as amyl acetate). Which is to say, birds may possess an acute sense of smell to certain things that we may not know of yet, even if they're bad in general when it comes to smells.

Another hypothesis given is sunscreen. If women wear sunscreen more often (such as on their face) and sunscreen reflects UV (and birds can see in the UV spectrum), then that could also explain it.

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