Plumage ontogeny and color polymorphism in the forest-dwelling Crested Eagle inferred from citizen-science photographs

The Crested Eagle (Morphnus guianensis) is a near threatened forest-dwelling raptor whose elusive habits and low population density have long constrained knowledge of its delayed plumage maturation and color polymorphism. The number and sequence of molt cycles leading to the definitive basic plumage, as well as the phenotypic patterns underlying discrete morph-types, remain poorly understood. I analyzed a dataset of 152 photographed individuals from citizen science repositories (eBird and WikiAves Brasil) to investigate plumage ontogeny and color polymorphism. Photographic evidence suggests that the Crested Eagle (1) acquires its definitive basic plumage after four annual molt cycles, passing through four distinguishable non‑definitive basic plumages; (2) the first and second basic plumages are monomorphic, after which morphs begin to differentiate following the second annual molt; and (3) upon acquisition of definitive basic plumage, three discrete morph-types are present (brownish-barred, whitish-barred, and unbarred-black). By inferring the plumage sequences, phenotypic patterns within age classes, and the onset of morph differentiation, this study provides a framework that advances understanding of previously poorly known aspects of the biology of the Crested Eagle.