The Collaborative International Dictionary of English v.0.48:

Insessor \In*ses"sor\, n.; pl. Insessores. [See Insessores.]
   (Zool.)
   One of the Insessores. The group includes most of the
   common singing birds.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Insessores \In`ses*so"res\, n. pl. [NL., fr. L. insessor, lit.,
   one who sits down, fr. incidere. See Insession.] (Zool.)
   An order of birds, formerly established to include the
   perching birds, but now generally regarded as an artificial
   group.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Insessores
    n 1: a bird with feet adapted for perching (as on tree
         branches); this order is now generally abandoned by
         taxonomists [syn: Insessores, order Insessores,
         perching bird, percher]