Recent research has demonstrated the important role of visual communication in nocturnal birds. Achromatic plumage patches (i.e., pigment-free white feathers) with high levels of contrast against dark backgrounds are excellent candidates for visual signaling in the dark or at twilight, when differences in color may be less effective. In this study, with the goal of investigating the signaling role of certain achromatic plumage patches, we examined the characteristics and patterns of the brightness (i.e., total reflectance) of Eurasian Eagle-Owl (Bubo bubo) white feather patches for both young birds and adult individuals. Our results showed that (1) the total reflectance of young birds' white feathers differed significantly from that of adult owls' white feathers; (2) the brightness differed between the sexes in adults only, with females showing a significantly higher reflectance than males; (3) the total reflectance of the white patch around a young bird's mouth was positively correlated with brood size; (4) the total reflectance of the white badge on the throat of adults was positively correlated with their hematocrit values; (5) an assortative mating scenario based on the brightness of an individual's white badge was deemed possible; and (6) we did not detect any significant relationship in the levels of reflectance for related individuals: the young and their parents were not found to be distinguishable based on the brightness of their white feather patches.

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