But it is at the same time one of the most difficult, particularly in countries where man has not yet mastered the powers of vegetation, where numberless creepers and bush-ropes render the forest impenetrable, and the pathless wilderness obstructs the observer at every step. Thus it is by no means surprising that so many secrets still veil the life of the tropical birds--that comparatively so little is known as yet of their economy and mode of existence. Many families of birds have a wide range over the whole earth: falcons hover over the Siberian fir-woods, as over the forests of the Amazons; in every zone are found woodpeckers, owls, and long-beaked martin-fishers, while thrushes enliven with their song both the shades of the beech-woods and the twilight of the cocoa-nut groves. In the north and in the south, fly-catchers carry destruction among the numerous insect-tribes; in every latitude, crows cleanse the fields of vermin; and swallows, pigeons, ducks, gulls, petrels, divers, and plovers frequent the fields and lakes, the banks and shores in all parts of the world. Thus the class of birds shows us a great similarity in the distribution of its various forms all over the earth; and we find the same resemblance extending also to their mode of life, their manners, and their voice. The woodpeckers make everywhere the forest resound with the same clear note, and the birds of prey possess in every clime the same rough screech so consonant to their habits, while a soft cooing everywhere characterises the pigeon-tribes. But, notwithstanding this general uniformity and this wide range of many families of birds, each zone has at the same time its peculiar ornithological features, that blend harmoniously with the surrounding world of plants and animals, and, taking a prominent part in the aspect of nature, at once attract the attention of the stranger. In this respect, as in so many others, the warmer regions of the globe have a great advantage over those of the temperate and glacial zones, but nowhere do the feathered tribes find a richer or wider field for their development than in the forests and swamps of tropical America, where the vegetable world revels in luxuriant growth, and myriads of insects, peopling the woods, the waters and the fields, furnish each kind according to its wants with an inexhaustible supply of food. The circumstance that man but thinly inhabits these wilds is another reason which favours the multiplication of birds, for in Europe also they would no doubt be far more numerous, if the farmer, the sportsman, and so many other enemies were not continually thinning their ranks. To these elements of destruction they are far less exposed in tropical America, and being comparatively but little disturbed, they reign, as it were, over the forest and the field, over the mountain and the plain, over the river and the lake.