ANNUAL MIGRATIONS 113 



year following the first stragglers are back; ten days 

 later the country is full of them. When one con- 

 siders the vagaries of the climate, their precision is 

 the more astonishing. Yet crows are typical of an 

 overwhelming majority of northern migrants. Ac- 

 curacy is particularly marked in many species of 

 shore-birds returning north from South America and 

 in the stronger flyers such as ducks, gulls, geese, and 

 hawks. Factors that may interfere with the prog- 

 ress of migration are well known and need hardly 

 be detailed. Here it is only wished to stress one of 

 the few facts on which there appears to be universal 

 agreement among migration writers, the remarkable 

 observance of dates shown by northern birds return- 

 ing to and departing from their breeding grounds. 

 With this fact in mind the hunt for a possible 

 stimulus involves a narrowed field. Any of the 

 factors already considered that make it impossible 

 for one species or another to survive the winter in 

 the North, might prove to be the key for the south- 

 w^ard migration but, with two exceptions, they are 

 all notably unstable, varying greatly from year to 

 year. That they have all been instrumental in 

 bringing migrations about there can be no doubt. 

 But there is no particular reason why any of them 

 should now be the stimulating factor. Let us con- 

 sider them individually. 



