Brown and grey morphs differ in their nocturnality in the polymorphic scops owl (Otus scops)

Abstract

Detectability of morphs in different light environments is one of the mechanisms favoring the persistence of color polymorphism via disruptive selection. This mechanism predicts that different morphs should be more active and successful when light conditions make them more cryptic. In nocturnal birds, no study has analyzed yet whether morphs show a different degree of nocturnality, showing different circadian activity rhythms in relation to night and day light. We analyze activity patterns and hunting efficiency of color morphs in relation to day/night light in the nocturnal polymorphic scops owl (Otus scops). Forty-four percent of owl activity took place in the first hours after sunrise and before sunset. Also, we found that nocturnal activity decreased from grey to brown females and that grey males fed their chicks more than brown ones at night. Visual modeling revealed that grey individuals would be more conspicuous to their prey than brownish ones during the day. These findings suggest that part of the activity of nocturnal scops owls may occur during the day and that grey individuals would be better adapted to forage during night-light than brownish ones. However, brown morphs seemed not to be better adapted to forage at day, suggesting that other mechanisms than visual detection by predators or prey contribute to the persistence of color polymorphism in the species.Significance statementCircadian activity patterns can vary intraspecifically in response to changing environmental conditions. Color variants in polymorphic species could change their circadian activity rhythms differently in relation to diel luminal variation, switching from nocturnality to diurnality or vice versa, to achieve better background matching. We study for the first time whether different color variants of the nocturnal polymorphic scops owl (Otus scops) show different circadian activity rhythms with respect to night and day light conditions by relying on GPS-tracking data and nest video recordings. We also use perceptual visual modeling to study crypsis of morphs during the day. We found that nearly half of the activity of owls took place in the first hours after sunrise and before sunset. Also, we found that grey individuals would be better adapted to forage during night-light than more brownish ones probably due to their poor background matching during the day for the visual system of insects. These results support the idea that in scops owls, color morphs may show different degree of nocturnality to improve their crypsis.