296 



U. S. NATIONAL MUSEUM BULLETIN 217 



delayed observation adds a little lateness and consequently a slight 

 increase to the appearance of regularity in the plotted median dates. 

 Inspection of the arrival records indicates that species arrive at 

 various dates. In many cases the dates of first record of a species are 

 close together in succeeding years. I have shown in figure 13 the 

 number of days separating first recorded arrivals of a species in 3 

 years. It did not seem wise to assess commonness of occurrence or 

 our familiarity with a bird as a basis for selecting the most reliable 















10- 





 









UJ 













05. 



(0 





 















L _, , , , ^ , , p„ 



DAYS 5 



— r- 



— 1 1 



I 







SPAN OF FIRST RECORDS IN 











3 YEARS. 47 SPECIES 









10' 













SPECIES 



1 





 









































** 1; 1 1 1 k 1 1 1 

 DAYS 5 



' 



1 1 

 1 



~~1 





SPAN OF FIRST RECORDS IN 











4 YEARS. 37 SPECIES 









Figure 13, — Top: Span of first arrivals in the three closest annual arrivals among 47 species 

 Bottom: Span of first arrivals in the four closest annual arrivals among 37 species. 



