They found that feather ruffling was more common when the birds were not in motion, such as during social interactions and resting periods.
Crown feather ruffling and blushing were both more common when the human caretaker was actively interacting with the parrot by talking and maintaining eye contact than when the keeper was in the room but ignoring and turning their backs to the bird.
Together, these results suggest that head feather ruffling is associated with states of lower arousal and positive social interactions, the authors concluded.
"How birds use facial displays and whether they communicate their inner subjective feelings is a question that is crucial to deepening our understanding of bird sentience," said Aline Bertin, one of the authors of the study published in the journal PLOS ONE.
"Although caution must be exercised when interpreting these data due to the small sample size, we argue that crown ruffling and skin colour variation may provide facial indicators of birds' inner subjective feelings," said Bertin.
"On a practical level, parrots are popular companion animals, with millions of parrots being kept as pets, and understanding visual communication in parrots may help to assess their well-being in captive conditions," she said.
"Blushing may not be a characteristic unique to humans: the featherless cheek of the blue-and-yellow macaw parrot reveals rapid skin color changes in situations associated with emotion," said Bertin.
"The macaw's particularly complex face may enable communication of emotion via colour and feather displays," she said.
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