750 EVOLUTION OF ANIMALS Part V 



the seasons are reversed, migrations are less general and occur in opposite 

 directions. 



The times and general migration routes of North American birds are now 

 fairly well known. Most of the insect eating birds, flycatchers and warblers, 

 retire to the southern states, many of them to South America. The majority of 

 species either cross the Gulf of Mexico, or follow its western shore and settle 

 in Central and South America. Ducks and other waterfowl have definite routes, 

 several of them over the ocean. Certain birds migrate by day and others by 

 night. This was long ago discovered by pointing a telescope at the moon and 

 observing the silhouettes of the birds that cross it. 



Many migrants follow river valleys, mountain chains and coast lines; others 

 launch off over the ocean, or across country where there seem to be no guide 

 marks. The urge to migrate is to a considerable degree affected by changes in 

 amount of light and other features of the environment, also by the endocrine 

 secretions of the reproductive organs. 



Ancestors 



In 1860, on a slab of limestone taken from a quarry in Bavaria, an imprint 

 was discovered that appeared to be the fossilized imprint of a feather. Its 

 identity was established a year later when in the same locality another fossil 

 was found, an almost complete skeleton of a vertebrate animal with feathers. 

 About 1 6 years later, a still better fossil of a feathered animal was found in the 

 same locality. The fossil record of birds is sparse. No other similar fossils 



Fig. 36.20. Fossil remains of an ancient reptilian bird ( Archaeopteryx) embedded 

 in a slab of limestone — as they were discovered. Above the slab is a partial recon- 

 struction of the distal part of the wing and below the foot is represented. In life, 

 the bird was about the size of a crow. (After Zittel. Courtesy, Rand: The Chor- 

 dates. Philadelphia, The Blakiston Co., 1950.) 



