Natal down is a layer of down feathers that cover the body of nestlings in varying areas and densities. However, the evolutionary pattern and function of these differences are still unclear. Here, we performed ancestral state reconstruction of natal down and examined the correlated evolution with several traits that may affect it. Pictures of nestling from 187 avian families were collected and the natal down characters from seven body parts were classified into three categories according to their feather density. Ancestral state reconstruction showed that there is a similar evolutionary pattern of all body parts. Nest and parental care are key factors affecting the protection of nestling. Functional interactions between these factors and natal down may affect the evolutionary pattern of natal down. We examined the correlated evolution between natal down and parental care, nest structure, attachment, and site. The results revealed that species with higher parental care and nest protection tend to lose their natal down. This study can provide important insights into the evolutionary history, behavioral ecology, and conservation needs of different bird species.
Evolution of natal down patterns across avian phylogeny and the functional correlations with nest characteristics