good scientists a hundred and fifty years ago, as to whether swallows migrate 

 or descend into the ponds to hibernate, but we still do not know why they 

 migrate. Several logical theories have been suggested, but no one fits all 

 cases. One of the oldest ideas is that the migration is to keep near a good 

 food supply. However, some birds migrate at a time when food is abundant 

 in the place they leave. Another theory is that the development of the sexual 

 organs initiates the migratory urge. This is supported by the fact that the 

 permanent residents show little seasonal change in the condition of the gonads, 

 while the migratory birds show marked changes. In some forms, as in the 

 bobolink, this seasonal change is correlated with changes in plumage. The 

 argument against the sexual-cycle theory of migration is that some forms which 

 take more than one year to mature, such as the Trumpeter Swan, Blue Goose 

 and Whooping Crane, migrate in their first year. Whatever the cause of 

 migration, it remains one of the greatest wonders of anmial life. The longest 

 migration is made by the Arctic Tern, which nests within eight degrees of 

 the north pole and after about fourteen weeks starts a trip of over eleven 

 thousand miles to the edge of Antarctica. The longest continuous flight is 

 that made by the Golden Plover, which, in its autumn trip from the Arctic to 

 South America, makes a non-stop flight of 2,500 miles from Nova Scotia to 

 Argentina. It returns by a difl^erent route, traveling up the Mississippi Valley, 

 two thousand miles west of its autumn trip. This use of different routes for 

 spring and fall migration is not confined to the Golden Plover, but has also 

 been shown for the Connecticut Warbler, which starts its fall migration by 

 travelling eastward from southern Canada to New England, and flies down 

 along the coast to Florida, returning in spring by the Mississippi Valley. The 

 Pacific Golden Plover has the most remarkable migration path, since it navi- 

 gates the Pacific Ocean, from Alaska to Australia and southeastern Asia, 

 making stops in the Midway, Hawaiian and South Sea Islands. Banding 

 has also revealed that some birds, including the bobolink, are gradually changing 

 their migration routes. 



In addition to their undoubted value to the farmer and gardener as de- 

 stroyers of unwanted insects, birds play a much more important part than is 

 generally realized in helping to maintain the balance of plant life in fields and 

 forests. Now that we are gradually learning the value of diversity in forest 

 plantings and the value of cover and food plants, we are becoming aware that 

 the birds, which share the benefits of these with other forms of wildlife, are 

 responsible for much planting. Many berries and other fruits eaten by birds 

 have hard, resistant seeds, which pass unharmed through the birds' digestive 

 tracts and are spread far and wide. This spreading of plants usually occurs 

 in waste places and, with the exception of poison ivy, does little harm to man 

 and much good to animals. 



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