Data and code for 'Morphometric Traits for Age and Sex Identification of the Galápagos Short-eared Owl (Asio flammeus galapagoensis)'

ABSTRACT.—The Galápagos Short-eared Owl (Asio flammeus galapagoensis) is a subspecies of the Short-eared Owl (A. flammeus) endemic to the Galápagos Islands, Ecuador. Little is known about Galápagos Short-eared Owl morphology or plumage characteristics, including potential sex and age differences. We captured 66 individuals, measured eleven morphological traits, and determined sex genetically to test whether individuals could be reliably sexed without molecular confirmation. We found the Galápagos Short-eared Owl exhibited stronger morphological sexual dimorphism than the nominate form (A. f. flammeus), with the combined measurements of body mass, bill length, and tail length enabling sex determination with over 90% accuracy. We also found that out of the commonly used plumage traits for sexing the nominate form only the size of the pale tip of the secondaries was applicable to the Galápagos subspecies. Similarly, the shape of the dark shaft-streak of the central tail feathers was also applicable as aging-characteristic. We also describe that the number of dark bars reaching the proximal edge of the inner secondaries were useful for categorizing individuals as non-adults (hatch year or 2nd year) and adults (3rd year and older), independent of molt patterns. This study provides the most complete morphological dataset for the Galápagos Short-eared Owl to date, establishes guidelines for sexing using external measurements alone, and identifies a novel characteristic for ageing. These findings will support future ecological and behavioral studies of this insular predator.