Blushing was once thought to be the “most peculiar and most human of all expressions,” as Charles Darwin put it, but a new study has found that hens share this peculiarity and can also express their fear or excitement in this way.

“Our research shows that domestic chickens are sensitive and have very subtle ways of expressing their emotions,” the study’s co-leader Aline Bertin, a researcher at the National Institute of Agricultural Research, told CNN.

Alongside researchers from several French institutes and the University of Tours, Bertin’s study found that hens fluffed their head feathers when they were content and calm, and that blushing for a few seconds indicated a reaction to excitement – in positive situations like waiting to eat mealworms – as well as fearful situations, like being captured.

“In humans, blushing is often associated with shame or embarrassment but it also appears in the expression of a range of emotions such as anger or joy,” Bertin added. “Although the emotions of a chicken are not directly comparable to those experienced by humans, we have shown that they also blush within seconds during strong emotions.”

An expression of low redness and fluffed head feathers suggests that hens are calm and secure, providing knowledge that could be used to assess their welfare, the study, published Wednesday in the journal Plos One, concluded.

While facial expressions have been investigated in several other mammals, such as dogs, horses, pigs and mice, it has not been as widely studied in birds.

To understand how hens visibly express emotion, researchers spent four weeks on a French farm observing 17 hens of two different breeds, Bertin said, filming their routine behaviors and their reactions to different stimuli.

Each one had their own quirks and personalities – some “would startle very easily at the slightest noise, while others reacted much less,” Bertin said, adding that these individual differences are an area for further study.

Researchers observed 17 hens on a French farm.
INRAE Arnould Bertin

To make their more general conclusions, researchers extracted images from every two seconds of film and selected ones that featured the hen in profile to best study them.

Although researchers weren’t able to explain the mechanism by which hens blush in this study, they concluded that the cheeks and ear lobes were more revealing of the birds’ emotions than their comb or wattles.

Researchers acknowledged the limitations of their conclusion – notably that filming hens in their natural habitat without a controlled light source could make it difficult to pinpoint specific color changes while changing temperatures could have influenced skin color change too.

However, to mitigate against this, researchers analyzed the images using infrared thermography which did not produce the same affect, suggesting that there was little change in temperature, and the colors on the images were relatively well-balanced.

Of course, there is a subjectivity to analyzing human, let alone animal, emotion.

“Without language, subjective experience remains inaccessible,” Bertin said. Instead, scientists define emotions as “behavioral, physiological, and cognitive responses to environmental stimuli,” she said and measure things such as heart rate or observe an animal’s behavior.

Building on the results of this study, Bertin hopes to investigate whether these displays of emotion are linked to the hens’ social interactions, as well the implications for animal welfare.