Résumé
Individuals can increase their conspicuousness to conspecifics while minimizing their probability of detection by predators by choosing the time and location of their display. Conspicuousness arises from the contrast between the light reflected by the colour patches, and the light reflected by the visual background in ambient light used for displays. Conspicuousness also depends on the discrimination abilities of both conspecifics and predators. We investigated whether different light environments are present in temperate woodlands, and whether male Blue Tits use this variability to optimize intraspecific conspicuousness while reducing the probability of detection by predators. We measured reflectance of plumage and Visual backgrounds, irradiance of available light environments during the breeding period. We used measures of photoreceptor sensitivity of the Blue Tit Parus caeruleus and of a bird approaching that of its main predator the European sparrowhawk Accipiter nisus. We found different light environments in space and time (during the day and throughout the year) that birds could exploit and select for sexual signalling. The blue crown colour appears to be more conspicuous in early/late light environments but during the day it appears to be least conspicuous in woodland shade for both conspecifics and predators. Male Blue Tits, by displaying more intensively at dawn during the breeding period, would optimize intraspecific communication without minimizing detectability by predators. Inversely, by choosing to sing mainly in woodland shade during the day, male Blue Tits reduce the probability of visual detection by predators but also their visibility to conspecifics.