484 LAND ANIMALS 



carry the egg up with them. 20 The "broodspot" lies posteriorly on the 

 belly, near the legs; hence these birds hold themselves erect when 

 brooding, and not horizontally like birds with numerous eggs. The gulls 

 build a nest, in which three eggs are usually laid. The altricial young 

 birds, on account of their small numbers, are very amply provided for 

 by the parents. 



The birds of the islets have quite contrary characteristics, at least 

 in part. The ducks, especially divers, and the geese, have more nu- 

 merous eggs, but provide them with a nest on a base of plant material, 

 lined with down. The young are praecocial and very soon learn to 

 forage for themselves under the guidance of one or both parents. The 

 diving ducks and swans have relatively large eggs, so that the young 

 hatch with a correspondingly advanced development. 



Many northern seas birds breed inland, on fresh-water ponds and 

 lakes, at varying distances from the sea. This is true of the divers 

 (Colymbiformes) and for most ducks, though the eiders are excep- 

 tions. Among the divers and especially the sea divers {Urinator) the 

 eggs are placed at the water's edge or on floating islands of vegetation. 

 Such a location would be impossible on the seacoast on account of 

 tides, waves, and drift ice. 



With the advent of winter the bird cliffs and bird islands are de- 

 serted. Some of the birds, like the fulmar, scatter far and wide over 

 the arctic sea. One finds isolated specimens everywhere. The expert 

 divers such as the murres {Uria) follow the fish migrations. A large 

 number of the European arctic birds wander southward along the 

 Norwegian coast. 21 The shore birds are more completely migratory and 

 spend the winter in the tropics or in the southern hemisphere. 



The breeding colonies of birds in the Antarctic resemble those of 

 the Arctic in many ways. The numbers of birds are similar, but the 

 variety of species and the number of higher groups are smaller. The 

 environmental conditions are much less favorable. The winter is less 

 cold, but the summer is less warm, and the constant winds and long- 

 continued cloudiness of the sky make the temperature factor still more 

 unfavorable. The complete absence of predatory land mammals makes 

 it unnecessary for the birds to breed on islets or cliffs. Where glacier 

 ice does not form, the snow is largely carried away by the winds, and 

 the remainder melts in the spring. Such places are densely colonized 

 by breeding birds, which range on to high terrain, but avoid cliffs 

 (Fig. 125). 22 At such sites there will be an abundant bird life during 

 the summer. The principal element in the population is composed of 

 the penguins, which are primarily antarctic birds, found on the south- 

 ern coasts of the land masses of the southern hemisphere, and ranging 



