CHAPTER 5 



SEA-BIRD MOVEMENTS 



SINGE SEA-BIRDS cannot nest at sea their life must be a kind of 

 compromise, for during a substantial part of the year, in the 

 slow-breeding species about half, at least one member of the pair 

 has to be in residence at a nest on land which may be some distance — 

 perhaps some hundreds of miles — from the nearest place which provides 

 an assured supply of food. Other sea-birds operate for shorter distances 

 from their base, and the coastal species feed within sight of it. But the 

 pelagic or oceanic types may fly some hundreds of miles and possibly 

 even a thousand before returning to their nest, even when they are 

 incubating eggs; and these birds are very specially adapted to solve 

 the consequent problems, — chiefly those of mobility and endurance. 

 The oceanic sea-birds have solved these problems of mobility by 

 becoming sailplanes as well as power-craft. This fundamental fact, 

 although pointed out by Wynne- Edwards in 1935 and no doubt 

 familiar to others before him, has been widely overlooked in ornitholo- 

 gical circles. Supposing that the oceanic sea-birds relied entirely 

 on powered flight, they would only be able to operate to their goal in 

 calm weather, and only by the consumption of large amounts of energy 

 and, therefore, extra amounts of food. In fact the opposite is the case. 

 It is probable that the oceanic sea-bird would find it difficult to get 

 from land to the middle of the ocean in calm weather, for the motive 

 power of the pelagic sea-bird is provided by the natural winds of the 

 great oceans, deflected upwards from the waves. That is probably 

 why the evolution of the most important group of oceanic sea-birds, 

 the petrels and albatrosses, has taken place in the South Pacific 

 and the neighbourhood of the antarctic continent, for in those seas 

 nature's winds blow hardest and most continuously, and rollers, 

 swells and great waves are permanent features of the surface of the 

 waters. The absence of albatrosses in the North Atlantic may be due 

 to the fact that the equatorial waters of the Atlantic are, on the whole, 



