Yet, though but a beginner in nest building, she is a true tree bird, and her little ones are born naked and helpless, far out of reach of the ground, and must be fed and cared for till they can fly. So she feeds them with infant pap from her own mouth. The “crop” or bag in which the partridge or hen stores the grain she picks up is large and single; but the pigeon has two bags, one on each side of the throat, and when she is feeding her young these bags secrete a large quantity of milky fluid, which, mixing with the tender shoots she has pecked off in the spring, or with the oily seeds she has gathered for her autumn brood, makes a soft food, which she pours into the mouths of her nestlings till they fly and feed themselves. In the pigeons, then, we are gradually rising from the ground birds,--where the father generally has many wives[101] and the young ones run as soon as they are hatched,--to the tree birds, where father and mother, taught by the helplessness of their brood, share the cares of nest building and the pleasures of love. Even the pigeons did not all at once become tree birds, for we have them in all stages now from the ground to the air. Many years ago, in the island of Mauritius, there were heavy flat-breasted pigeons, the Dodos, which lived entirely on the ground without the power to rise, so that when the Dutch settled there, bringing rats with them in their ships, the Dodos soon fell victims to the intruders, and now there is not one left. Again, in New Guinea now, there are ground pigeons which fly heavily and slowly, and only go to the trees to roost. Then come our own tame pigeons, the rock-pigeons, and the stock-dove which builds in boles in the trees; and then our wood-dove and his relations, with their rude nests and their mixed food of grain and grass. And among these are the wonderful long-winged passenger pigeons[102] of America, which fly in flocks of hundreds of thousands through Ohio, Kentucky, and Indiana, in search of nuts and seeds, breaking down the boughs of the trees by their weight where they alight, and then darkening the whole sky as they start off again in a succession of vast multitudes to another forest where beech nuts, acorns, and chestnuts are plentiful, or to the rice-grounds of Carolina, to take their fill. And, lastly, we come to the beautiful green fruit-eating pigeons of India and the East--the feeders on nutmegs and palm-fruits and juicy berries of all kinds. These are true tree birds, difficult even to find, so like are they to the colour of the leaves; yet they still build the loose untidy nests of their kind.