84 THE MIGRATIONS OF BIRDS 



viously to be sought near the center of the migration 

 path, and individuals will become increasingly rare 

 as the borders of the range are approached. In the 

 Plains region aquatic birds are abundant in autumn 

 migration if autumn rains fill ponds and marshes so 

 that suitable feeding grounds are available. In dry 

 seasons few or none are seen, as there is nothing to 

 attract them. The assumption is that in dry seasons 

 they pass overhead but, as there are no feeding 

 grounds do not descend. In this same region in 

 spring these water-birds abound, since the winter 

 snows and rains have provided a suitable habitat. 



Migration among supposedly 

 Resident Birds 



There are in all temperate regions certain birds 

 which, as a species, are resident, but yet among 

 whose individuals there is a certain amount of mi- 

 gratory movement. Bluejays and woodpeckers, nut- 

 hatches and chickadees, may be present constantly 

 through the year, yet the individuals seen in winter 

 are not always the same as those observed during 

 summer. 



The white-breasted nuthatch is an excellent illus- 

 tration of slight migratory movement among indi- 

 vidual birds of resident species. In the vicinity of 

 Washington the land shows two major topographic 



