38 BIRDS OF THE NEW YORK CITY REGION 



all the insectivorous species, whether summer residents or 

 transients. In mild seasons a maximum of bird-life is reached 

 between the 4th and 13th; the normal transient host is re- 

 enforced by lingering summer residents and the earlier tran- 

 sients such as Warblers. Should a frost then ensue, a big 

 "wave" occurs, and 80 species can be recorded in a day. 

 Such a fortunate combination occurs about once in five 

 years. Just as in May, continued warm weather means that 

 birds are relatively scarce, and no marked flights occur. 

 Apparently the birds move south anyhow, and sudden cold 

 weather merely serves to hasten their departure in a body. 

 The Pipit, Winter Wren, Rusty Blackbird, Solitary Vireo, and 

 White-crowned Sparrow arrive regularly early in October. 

 The Fox Sparrow and Hermit Thrush always arrive on the 

 heels of the first hard frost, which in recent years at least has 

 never been later than the 20th. After this date the bird- 

 life of the month is practically the same as that of March, 

 plus those species of the earlier April groups, which are not 

 purely insectivorous. These latter linger until the end of the 

 month or the first days of November, when the Tree Sparrow 

 arrives. During this period the most abundant birds are the 

 Sparrows, which throng the countryside in countless numbers. 

 There is a second factor, however, which is obvious to 

 those who have carefully followed the migration both spring 

 and fall for a sufficient number of years. There can be no 

 question of the fact that an early or a late fall migration is 

 somewhat dependent upon that of the preceding spring, 

 provided that the summer has not been abnormal or that the 

 fall is not abnormally cold. Two excellent illustrations of 

 this are at hand. The spring of 1907 was the latest on record, 

 and many species were reported in October, never before or 

 since reported, while many remained later in the month than 

 ever before or since. The fact that the relative mildness of 

 the month that year had little or nothing to do with it, is 

 proved by the past fall (1922), in which the months of 



