ing very different families, live chiefly, and for the most part exclusively, on the fruits; a thing that would not be possible in a temperate climate, where the development of the fruits is limited to a small part of the year. The immense northern pine-woods of the Old and New Worlds have their denizens from the bird-world living exclusively upon their seeds in the cross-bills, which are quite as much tree-birds as the tropical parrots.
The abundance of grains, fruits, insects, and even of mammalia, such as mice, is, on account of the varying local weather conditions on which they are dependent, much more variable in temperate than in tropical regions. One year may be rich in these products, and the next very poor; unusually favorable conditions for their growth may prevail for the time in one locality of an extensive territory, while in neighboring districts, from causes which it is often hard to explain, the contrary may be the case. Such circumstances must, of course, act with great effect upon the bird-world. At some times the birds of a region may, on account of the failure of their principal food and consequent famine, be driven from their hereditary quarters, and compelled, becoming wanderers, to resort to districts that are strange to them. When the pine-nuts fail in the north and on the mountains, the nut-crackers that feed upon them, obeying necessity, and not doing according to their custom, come south; and, when it is a bad year for birch-seed, multitudes of the northern linnets visit Europe. During long, snow-bound winters these northern and northeastern birds may be seen abundantly in Germany, and away down in Southwestern Europe, and the superstitious countrymen see in the unusual visitors the heralds of coming disasters.
On the other hand, an unusual abundance of food attracts birds into places to which they are not accustomed, and leads them into strange companies. Naumann relates that the thistles once got so firm a hold in the pasture-lands near his home that in many places the previously luxuriant grass wholly disappeared; then there came great flocks of green-finches, attracted by the abundance of their favorite food, and more in the next year at the time of the ripening of the seed, till, in a few years, all the thistles were exterminated through the agency of these birds. In another place he tells of a pine-woods of only thirty acres, to which hundreds of cuckoos resorted, attracted by the enormous number of caterpillars. It has also been frequently observed that horned owls are to be found by the thousand in localities that are plagued by an unusual visitation of mice.
Numerous birds attach themselves to droves of wandering animals. The passage-falcon travels regularly with the birds of passage; schools of herring are accompanied by throngs of fish-eaters, especially by the gannet, the presence of which is regarded by fishermen as the surest indication of the neighborhood of herring. So, swarms of locusts, pressing toward the West, are attended by birds to which they are
