The intrinsic muscles of the syrinx fixed to the ends of the bronchial semi-rings ; the edges of both mandibles smooth, or the upper one simply notched; hinder aspect of tarsus bilaminated, the laminae entire and smooth; wing with ten primaries ; tongue non-tubular; nostrils always clear of the line of forehead, the space between the nostril and the edge of the maudibles less than that between the nostril and the culm en; plumage of the nestling mottled or squamated. There is only one moult in the year, in the autumn, but the fringes to the leathers of the winter plumage wear off in summer, causing a great alteration in colour in many species. Rectrices usually twelve, rarely fourteen.
The Turdidae includes the Short-wings, Chats, Forktails, Robins, Thrushes and Accentors and forms a large family of the Passeres. The only character which links all these species together is the squamated plumage of the young. The Dippers also have this characteristic but in other structural characters they show their affinity to the Wrens and therefore constitute a satisfactory family between the Troglodytidae, or Wrens, and the Turdidae.
From the Muscicapidae, or Flycatchers, the young of which also have mottled or squamated plumage, the Thrushes are easily separated by their having no hairs over the nostrils and base of the bill and also by their long, strong tarsi. The only exception to this is the genus Zoothera, which has frontal hairs somewhat developed and overlying the nostrils. The exceptionally powerful and long tarsi of this bird and its general Thrush-like appearance more than suffice, however, to show its proper position.
In the Brachypteryginae and in some other genera, especially Phaenicurus and Saxicola, the shafts of the feathers of the forehead are long and the webs somewhat disintegrated but these cannot be considered hairs, nor do they lie over the nostrils as is always the case with the Muscicapidae.
I have divided the Turdidae into six Subfamilies, which form groups easily distinguishable from one another by the field-naturalist, although there are really no structural differences between the Chats, Redstarts and true Thrushes.
As already noted, the Cinclinae have been raised to the status of a family, Cinclidae. The Short-wings, Brachypteryginae, have been brought from the Timaliidae, or Crateropodidae of Oates, to the present family, their position in which is determined by their squamated juvenile plumage and, finally, I have separated the Forktails from the Redstarts, the peculiar structure of the tails of the former clearly differentiating them from any other of the Turdidae.
Key to Subfamilies.
A. Wing very short and rounded; tail very
short Brachypteryginae, p. 9.
B. Wing pointed and not very short; tail not very short.
a. Tarsus smooth.
a1. Habits Muscicapine; food principally
captured by sallies from a perch .. Saxicolinae, p. 22.
b1. Habits terrestrial; insect-food sought for on the ground.
a2. Tail deeply forked; middle pair of feathers shortest; penultimate pair
longest Enicurinae, p. 56.
b2. Tail normal; square or rounded...... Phaenicurinae, p. 67.
c2. Habits terrestrial and arboreal; they are
both insectivorous and frugivorous .. Turdinae, p. 120.
b. Tarsus scutellated Prunellinae, p. 186.