The Tracheophonae among the Passeriformes, the possessors of this specialized although low type of syrinx, form a tolerably well-marked group, entirely neotropical.
The former is morphologically the more primitive condition, and is found in the overwhelming majority of birds, including many Passeriformes.
This way of using the characters of the syrinx for the classification of the Passeriformes seems simple, but it took a long time to accomplish.
Birds being of all animals most particularly adapted for extended and rapid locomotion, it became necessary for him to eliminate from his consideration those groups, be they small or large, which are of more or less universal occurrence, and to ground his results on what was at that time commonly known as the order Insessores or Passeres, comprehending the orders now differentiated as Passeriformes, Coraciiformes and Cuculiformes, in other words the mass of arboreal birds.
It is singular that only the first three of them belong to the order Passeriformes, a proportion which is not maintained in any other tropical region.