Evolutionary history reveals information on the functionality of ear tufts in owls (family: Strigidae)

Ear tufts are plumage features which have particularly high prevalence in owls (Strigidae). Several hypotheses have been developed to explain their function, mostly including camouflage, species recognition, deterring predators/mobbers, and visual signaling among conspecifics. In the present study, we used phylogenetically-informed comparative approach to reconstruct evolutionary history of ear tufts across the entire Strigidae family (184 species). Specifically, data on the occurrence and relative size of ear tufts compiled from color plates and photographs were analyzed in relation to life history and ecological traits.

We found that ear tuft occurrence coevolved with circadian activity rhythm and predominated in species with strictly nocturnal activity. The highest evolutionary rate was found for transitions from nocturnal to mixed activity in species without ear tufts and from mixed towards nocturnal activity in species with ear tufts. Consistently, strictly nocturnal owl species showed larger ear tufts (controlling for differences in body size) compared to species with mixed activity. We also found that owls preying upon birds had relatively larger ear tufts. Finally, a strong phylogenetic signal in tuft occurrence provided evidence for high evolutionary conservedness of this trait.

Our results suggest that ear tufts may enhance camouflage of nocturnal owls during the daylight rest, when they might be threatened by visually oriented predators or mobbed by their potential prey. Our results lay foundations for further experimental research required to determine the ultimate function of ear tufts in owls.