| Sumario: | 6 pages, 10 (supplementary) figures.-- Supplementary figures: S1. Pictures showing details of eagle owl faecal markings. 2. To increase the conspicuousness of faecal signaling, owls need to mark the most prominent rock surfaces. 3. An example of the spatial distribution of faecal markings within an eagle owl's home range. 4. Temporal pattern of the appearance of defecation and plucking sites, which generally become visible during the pre-laying season, and remain visible up until the fledgling period. 5. Some examples of preferential use of darkest substrates for eagle owl faecal marking. 6. Both faecal marks and plucking sites are located in positions with increased conspicuousness, such as dominant places and the highest points of valley slopes. Some marks also appear at the entrance of the valley in which the nest is located. 7. Examples of faecal marks being refreshed after we experimentally obscured them with spray paint. Generally, the eagle owls returned to re-mark within one to two nights of the experimental covering. In several cases, faeces were scattered at exactly the same position that had been previously marked. 8. Pictures showing the significant contrast between the bright feathers and the dark surface of the plucking site. All the prey species on plucking sites were birds with highly visible feathers. 9. Some faecal marks were only visible from the nest, not the surroundings. In such a context, they could act to signal reproductive status between the male and female of the breeding pair. 10. In the absence of dominant posts, eagle owls use different locations to signal their breeding status, such as trunks, fences, poles and human structures. Faecal marks and plucking sites could also function as visual signals in other avian species, such as the Little Owl, Athene noctua.
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