In. this Subfamily all the genera have twelve tail-feathers, the tails themselves varying greatly in length and shape in the various genera and species ; the ambiens muscle, intestinal caeca and oil-gland are all present. The feet vary somewhat and some have broader soles than others according to whether their habits are terrestrial or arboreal; all, however, are well adapted for walking and even the most arboreal species walk well and freely on the ground.
The division of the Columbinae into genera is one of considerable difficulty and especially is it difficult to decide what species should be included in Columba. The type of Columba has hitherto been held to be aenas but Mathews has recently discovered that Selby in 1852 designated palumbus as the type in his Ill. Brit. Birds. Those therefore who split the Wood-Pigeons and Bock-Pigeons into two genera must use Lithaenas (Reichenbach, A.v. Syst. Nat., p. 25, 1862) for the latter. I can, however, find no constant character either structural or in colour or colour-pattern which is satisfactory. The two prominent characteristics are the presence or absence of the neck-spot or ring and the comparative length of the tail but as these two contradict one another it seems better to discard them and to follow Hartert in keeping all these birds in the one genus. The Purple Wood-Pigeon and the Andaman Wood-Pigeon seem to be sufficiently well differentiated by their curious coloration and much larger naked area round the eyes; these I retain in the genera Alsocomus and Ianthaenas, removing Blanford's other species of this genus back to Columba.
It seems probable that a review of the Pigeons of the World would demonstrate that colour and colour-pattern are characters of greater importance in Pigeons than are slight structural differences, which, as we have shown in the Treroninae, seem to mean nothing.
Key to Genera.
A.Tail not longer than wing and not greatly graduated.
A.Neck-feathers not acuminate.
a1. Larger, wing over 180 mm.
a2. Plumage of various greys, black and white...............COLUMBA, p. 219.
b2. Plumage deep chestnut, crown white............... ALSOCOMUS, p. 231.
c2. Plumage purple-black, whole head grey..............IANTHAENAS, p. 233.
b1. Smaller, wing under 180 mm.
d2. Sexes alike; 2nd and 3rd quill longest..............STREPTOPELIA, p. 236.
e2. Sexes differing; 1st and 2nd quill longest ..............OENOPOPELIA, p. 249.
B.Neck-feathers acuminate and lengthened..............DENDROTRERON, p. 234.
B.Tail-feathers longer than wing and much graduated ..............MACROPYGIA, p. 253.