CAUSE AND ORIGIN OF MIGRATION 23 



many weeks, but when, as often happens, the mast 

 crop fails, they become nomadic, and pass from 

 place to place in their hunt for food. They visit 

 fields top-dressed with manure, glean the refuse of 

 the harvest, frequent the farm-yards, and in early 

 spring, visit the budding larches to prey upon their 

 insect pests. On the other hand golden plovers 

 and lapwings are remarkably local in their winter 

 habits, and so long as the weather remains open 

 will frequent the same fields throughout the winter. 

 Severe weather, especially snow, which effectually 

 closes their chance of obtaining food, at once drives 

 them away. They will migrate to the unfrozen 

 mud-flats of the coast, or to those parts of England, 

 generally the south-west, and Ireland, where the 

 climate is normally milder, or they will even leave 

 our islands altogether under great stress. 



The wandering habit, except during the breeding 

 season, is confirmed in most birds, and experience 

 shows that the same species of birds visit the same 

 districts again and again when there is some par- 

 ticular food supply to attract them. Memory and 

 experience guide them from place to place. This 

 regular visitation of certain food bases, being of the 

 greatest importance to birds which have a long period 

 of travel or wandering before them, tends to originate 

 the so-called route by which they travel. The fact 

 that as a rule these stages are in consecutive steps 



