(sometimes less than half as long), the rectrices (usually 12, but sometimes only 10) usually rounded, never acuminate, terminally.
The Formicariidæ comprise one of the larger groups of birds peculiar to the Neotropical region, but, like several others of the same class, wholly absent from the Antillean Subregion. The group is well represented in the whole of the extensive territory extending from Costa Rica to southern Brazil, but reaches its greatest development in number of species and genera in the great valley of the Amazon and the Guianas. Dr. Sclater[1] recognizes 250 species belonging to 33 genera — numbers considerably less than those that are actually known at the present time.[2]
Among so great a number of species and genera there is, naturally, great variation in size and form. The smallest are no larger than a Gnatcatcher (Polioptila) , while the largest are fully equal to an average-sized Jay; some resemble Shrikes, others Thrushes, Wrens, Dippers, or other oscinine groups in their general appearance. They are nearly all birds of plain plumage, none having any brilliant colors, and most of the species are terrestrial, or nearly so, feeding, as their name implies, largely upon ants, though perhaps less extensively so than has been supposed; it having been stated by competent observers that some species subsist more upon various forms of insect life which the immense armies of ants, as they march across the forest floor, startle from their hiding place among the dead leaves, etc., over which they pass.
Notwithstanding their mesomyodian larynx, many of the Formicariidæ are good songsters, some of them being conspicuous for their vocal powers.
The classification of this group is very difficult, more so probably than in the case of any other American family of birds. Indeed it may be truly said that even the most recent attempts to present an orderly and natural sequence of the genera are very far from satisfactory. My effort to bring order out of chaos can be considered as only partially successful, but it is hoped that some improvement at least has been made in that direction. Undoubtedly better results would have been reached had a better representation of the genera and species been available; but unfortunately there are very many species and several recognized genera which I have not been able to examine in this connection. As in the case of many other groups, the segregation of the species into genera has been based far too much on general resemblance, and too little attention paid to structural characters. This may be said of nearly every group of birds; but in the present instance it is difficult to understand how certain associations could have been made.
