What avian features specifically place the sapsucker within Class Aves?

The presence of feathers, adaptation of forelimbs into wings, production of hard-shelled eggs, and a high metabolic rate via a four-chambered heart

The Class Aves is defined by a unique constellation of traits that collectively distinguish birds from other vertebrates. Key morphological indicators include the development of feathers, which are essential for insulation and flight, and the modification of the anterior limbs into wings, even if those wings are not used for primary locomotion in all species. Furthermore, birds are characterized by reproductive strategies involving the laying of hard-shelled eggs. Physiologically, they maintain a high metabolic rate, which necessitates a highly efficient circulatory system, exemplified by the four-chambered heart. The Yellow-bellied Sapsucker displays all these classic avian characteristics.