In the spring the sea-birds assemble in large flocks, pair off, and proceed to deposit their eggs in nests constructed generally without skill, but always lined or carpeted with a fine down, which forms a soft warm bed for the embryo progeny. Certain localities are frequented by preference, which are occupied by innumerable flocks in the breeding season, all of which seem to live together in perfect harmony. Some of the families of the Natatores are valuable additions to the poultry-yard. Ducks and Geese furnish delicate and nourishing food for man; the Swan is gracefully ornamental on our lakes and ponds. The down of all the aquatic birds is of immense value to the commerce of northern countries. The eggs are good to eat, and in many countries the inhabitants consume them in great quantities. Nor does their usefulness end here. _Guano_, so eagerly sought for by the farmer, is the excrement of aquatic fowls--the accumulation of ages, until, in the South Pacific Ocean, it has formed whole islands, some of them being covered with this valuable agricultural assistant to the depth of ninety or a hundred yards. Nor is this so marvellous, if it is considered that twenty-five or thirty thousand sea-birds sleep in these islets night after night, and that each of them will yield half a pound of guano daily. Our lands receive valuable assistance to fertility from this unrivalled material, which owes its power to the ammoniacal salts, phosphate of lime, and fragments of feathers of which it is composed. The order of Natatores, or Palmipedes, consists of four families:--1. _Brevipennes_, or Divers; 2. _Longipennes_, or Skimmers; 3. _Totipalmates_, or _Pelicanidæ_; 4. _Lamellirostres_, including Geese, Ducks, Swans, and Flamingos. THE DIVERS (_Brevipennes_). Penguins, _Aptenodytes_; Auks, _Alca_; Grebes and Divers, _Colymbus_; Guillemots, _Uria_. The birds which constitute this family of the Natatores are characterised by wings so thin and short as to be totally useless for the purposes of ærial locomotion. They are also called _Brachypteres_, from the Greek compound +brachys+, short, and +ptera+, winged. These are all habitual divers and indefatigable swimmers, using their wings as fish do their fins. To raise these after making the down-stroke requires a considerably greater effort than a bird of flight makes in raising its wings in the air, for which reason the second pectoral muscle in this and other diving birds has an unusually large development to give further strength. Their plumage is smooth and silky, and impervious to water from its oily nature. They live chiefly on the sea, coming ashore in the breeding season.