The Collaborative International Dictionary
Insessor
Insessor \In*ses"sor\, n.; pl. Insessores. [See Insessores.] (Zo["o]l.) One of the Insessores. The group includes most of the common singing birds.
Wiktionary
insessor
n. (context zoology English) Any of the Insessores, which includes many common songbirds.