ECOLOGICAL FACTORS 411 



during the fall. 123 Certain colonial insects, such as honeybees and 

 harvester ants {Messor) , gather supplies, the former honey and pollen, 

 the latter seeds. 



Seasonal migration.— Finally, seasonal changes cause more or 

 less distant migrations of many homoiothermal animals, which enable 

 them to escape the unfavorable seasons. This is true for regions with 

 dry seasons as well as for those with cold winters. The best known of 

 these phenomena are the annual bird migrations, in which a great 

 number of birds of the temperate and cold zones seek warmer regions 

 in the late summer and autumn. Only a few migratory birds breed in 

 the tropical regions also, and these may be thought of as native to the 

 tropics, with stragglers that breed in temperate regions. We usually 

 consider the breeding place the natural home of our summer birds. 

 Among the many mooted questions of bird migration, there is one of 

 distinct geographical significance: the question of migratory routes. 

 All observations show that every avian species has its own peculiar- 

 ities in the development of migration, 124 despite which there are com- 

 mon routes which are followed by many species. These are not narrow 

 paths, but wide courses; they generally follow the seacoasts, borders 

 of inland lakes, or river courses, entirely or in great part. The asso- 

 ciation of so many species of birds, differing in habits, habitat, and 

 power of flight, on the same course at different times makes it apparent 

 that it is specially differentiated belts in the atmosphere, and probably 

 places with frequent ascending air currents, that the birds choose, 

 rather than visual landmarks, although birds can migrate without the 

 aid of such facilities, as is shown by those whose migration has a wide 

 front, such as finches, swallows, and others. 



Birds that feed on flying insects, swallows, swifts, and flycatchers, 

 are migratory in Europe and North America. Migrants also include 

 grain- and grass-eaters, most of the water birds, and many birds of 

 prey. In the north polar zone all the birds are migratory excepting 

 ptarmigans, northern ravens, and the snowy owls. The number of 

 permanent residents among birds increases towards the middle lati- 

 tudes. Certain species leave in autumn but are replaced by related 

 forms from the north, which remain during the winter, as for example, 

 the junco. On the other hand, birds that are permanent residents in 

 one region may migrate in other localities; the partridge {Perdix 

 perdix) , a strictly non-migratory bird in Germany, migrates in eastern 

 and southeastern Russia, southwards to the Volga delta; the starling 

 migrates in northern Germany and in the eastern United States; on 

 the Faeroes with oceanic climate they are non-migratory. The migrat- 

 ing habit does not depend wholly on the peculiar nature or on the 



