Which bird features have been used to classify bird species and groups?

Systematists have historically relied on a variety of structural and morphological characteristics to classify bird species and groups, though modern taxonomy increasingly incorporates DNA sequence analysis to refine these relationships.12

External Morphological Features

• Plumage: Characteristics such as feather types, the presence of an aftershaft, and the presence or absence of down on feather tracts and the preen gland are key. The arrangement of feathers in specific tracts, known as pterylosis, and the number of flight feathers (remiges) and tail feathers (rectrices) are also used.13
• Bill and Feet: The morphology of the bill and the structure of the feet—such as the arrangement of toes and tendons—are traditional taxonomic markers.12
• Other External Traits: Features like rictal bristles, the coloration of fleshy mouth parts, and the presence of specialized feathers (e.g., iridescent plumes or crests) are often considered.23

Internal Anatomical Features

• Skeletal Structure: The arrangement of bones in the skull, particularly the palate structure, is highly significant. Other skeletal markers include the shape of the sternum (breastbone), the number of cervical vertebrae, and the structure of the hyoid apparatus.142
• Musculature and Organs: Taxonomists examine the arrangement of the syrinx (vocal organ), the presence of specific thigh muscles, jaw and tongue musculature, and the configuration of the carotid arteries.12
• Other Internal Traits: The condition of the young at hatching, the structure of spermatozoa, and the pneumaticity of the humerus are also utilized in classification.12