2 SEA-BIRDS 



comparison, the Indian Ocean is not quite as large (about twenty- 

 eight million square miles) ; but the Pacific (about sixty-four million 

 square miles) has nearly twice the area, and is ten thousand miles 

 across its widest part. The Arctic Ocean (about five and a half million 

 square miles) is small and nearly full of ice at all times of year; in 

 spite of this it is at times very full of life. Finally, it is usual to describe 

 the cold waters round the Antarctic Continent (itself the same size 

 as the Arctic Ocean) as the Antarctic Ocean. 



South of the normal steamship route from Britain to New York 

 the Atlantic is almost everywhere over two miles deep, and in large 

 areas more than three. But down mid-ocean, following the tropical 

 kink in the zig-zag, runs a very long submarine ridge, above which 

 is less than two miles of sea; it is only broken by deeps for a short 

 distance on the Equator, and it rises to the surface in places — in the 

 northern hemisphere at the Azores and St. Paul Rocks, and in the 

 south at the lonely isles of Ascension, Tristan da Cunha and Gough. 

 Other oceanic Atlantic islands, such as Bermuda in the north, and 

 South Trinidad and St. Helena in the south, rise abruptly from very 

 deep parts of the ocean. A sketch-chart will be found on p. 30 

 (Fig. 2c).^ 



It will be seen that there are prominent shallows along the east 

 coast of southern South America, north of the mouths of the Amazon 

 and along the Guianas, in parts of the Caribbean Sea and the 

 Gulf of Mexico (there are also marked deeps in these tropical 

 waters), off the New England States, Nova Scotia and (most particu- 

 larly) Newfoundland, and round Britain, the Channel and the North 

 Sea, and round Iceland. A submarine ridge, over which the sea is 

 five hundred fathoms or less, cuts the North Atlantic entirely from the 

 Norwegian Sea and the waters of the Polar Basin; Shetland, the 

 Faeroes and Iceland lie on this ridge. Davis Strait is shallow, and 

 the waters of Labrador and Hudson's Bay very shallow. Where the 

 waters are less than a hundred fathoms deep, what they cover is 

 usually described as the Continental shelf This has its own particular 

 community of birds. 



For practical purposes, and because all charts and maps mark the 

 Arctic Circle and the Tropics, we have classified the North Atlantic 

 and its birds into arctic, temperate and tropical areas based simply 

 on latitude. In our analysis of breeding-distribution, for instance 

 (p. 22), we regard birds nesting north of the Arctic Circle as arctic, 

 those nesting south of the Tropic of Cancer as tropical, and those 



