Passeriformes

Small to moderately large modern land birds; aegithognathous palate; large brain size and intelligence; unique syringeal anatomy; unique insertion of forearm muscles; tarsi covered with small scales; large, reversed incumbent hallux; anisodactyl foot; hallux independently moveable; plantar tendons; bundled sperm with coiled head; metabolic rates up to 50% higher than comparable non-passarines of same size; complex nest-building behaviors; altricial young; vocal plasticity.