Colour phases of the eastern screech owl: a comparison of biomechanical variables of body contour feathers

1. Melanin is a ubiquitous pigment in the animal kingdom and has been reported to increase biological tissue strength. Melanin is responsible for many observed cases of colour polymorphism, or existence of two or more distinct plumage colours in one species (regardless of age, sex, or mass). Researchers have postulated that the relative concentration of melanin pigments deposited in feathers may function to strengthen feathers but colour polymorphic species have been ignored. Here, we investigate whether different morphs of a polymorphic bird species exhibit differential plumage strength. 2. While colour polymorphism is widespread in birds, there is no consensus on the adaptive significance of polymorphism or the maintenance of clinal variation in colour morphs. The eastern screech owl (Megascops asio) is a polymorphic species; individuals exhibit rufous, intermediate, or grey coloration that is likely caused by relative amounts or concentration of black or rufous melanin subtype (eumelanin and pheomelanin respectively). This species exhibits clinal variation in morph prevalence; the rufous morph predominates in humid deciduous forests while the grey morph is more common in dry or cold environments. 3. Here, we test the hypothesis that differences in feather strength may select for darker plumage in drier climates. First, we use reflectance spectrometry to objectively verify differences in colour morphs of the eastern screech owl. Second, we measure mechanical variables and morphology of dorsal feather barbs. 4. While reflectance spectrometry objectively separated the three colour morphs, we found no significant differences in the mechanical variables of plumage morphs. These results fail to demonstrate differences in plumage strength among eastern screech owl colour variants. 5. This conclusion rejects the hypothesis that melanin subtype affects the strength of body contour feathers in the eastern screech owl and may not factor in the maintenance of the observed clinal variation.