26 BIRDS OF THE 



with or without rain, accompanied by much 

 lower temperatures, succeed within a few 

 hours, the host of migrants remains with 

 nearly unbroken ranks until the untoward 

 conditions have passed and clear skies and 

 a rising temperature induce a further flight 

 northward. They are notable days when 

 there is such a combination of conditions, 

 for the number of migrants and the diver- 

 sity of species are likely to be large, and the 

 delay may continue for three or four days. 

 It has not been found that the migrant 

 birds arrive every night, and that, therefore, 

 the birds of yesterday are replaced with 

 other birds of to-day, limiting as a rule .the 

 stay of the visitants to a single day. The 

 indications all point to the opposite idea, 

 namely, that there are occasional large mi- 

 gratory flights ; that birds coming in these 

 large movements remain to a considerable 

 extent to a second day, and some of them 

 during several days; that a species may 

 have no representatives present after sev- 

 eral or many individuals have come and 

 passed ; that then a new representation ar- 



