Complex begging display by bird offspring has predominantly been investigated in diurnal species, which have conspicuous gape colours or plumage features. In nocturnal species, in contrast, such visual communication has received little attention because the assumption is that they exclusively rely on vocal communication. Here, we use a field experiment to investigate whether eagle owls, Bubo bubo, com- municate through visual signals at night. We artificially decreased the brightness of the white feathers surrounding fledgling eagle owls’ mouths during the post-fledging dependence period, and investigated the effect of this treatment by comparing the condition of these birds to that of birds who received a control treatment. Several physiologi- cal parameters considered in our analyses indicate that control owlets were in better condition than owlets with brightness-reduced mouth feathers, which suggests that they received more or better food from feeding parents who discriminated between those young. Brightness- dependent reactions of parent owls suggest that visual signalling may be more widely employed than previously thought, and studying birds at night may reveal sophisticated strategies of animal communi- cation.
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