DELAWARE VALLEY ORNITHOLOGICAL CLUB. 9 



watching the birds most closely and when his notes have most 

 value. It is fortunate that this is the part of the migration 

 season when birds are little influenced by the variations in the 

 seasons. The following table gives Barton's dates of arrival for 

 the twenty-one species he noted from April 20th to May 1st, 

 and compares these dates with the average dates of arrival for 

 Philadelphia computed from the data of the Biological Survey. 



Barton's date of Average date of 



Species. arrival in arrival during 



1791. late years. 



Green Heron April 20 April 19 



House Wren "23 "21 



Chimney Swift "23 "17 



Whip-poor-will "23 "21 



Kingbird "23 "29 



Baltimore Oriole .... "23 May 3 



Orchard Oriole "23 " 2 



Catbird "23 April 26 



Black and White Warbler. . " 23 "17 



Summer Warbler . ..." 27 '' 23 



Northern Water-Thrush . . " 28 "28 



Warbling Vireo "28 "30 



Hummingbird "30 May 1 



Maryland Yellow-throat . . "30 April 24 



Ovenbird "30 "23 



Scarlet Tanager "30 "30 



Red-eyed Vireo . . ..." 30 "27 



Redstart ....... May 1 "24 



Wood Thrush "1 "27 



Great-Crested Flycatcher . . " 1 "30 



Yellow-breasted Chat ..." 1 "30 



Average April 27 April 26 



The average dates of arrival of these 21 species agree quite 

 closely with the dates when they were first seen by Barton. 

 Only one-quarter of the dates vary more than three days from 

 the average. The differences vary from nine days earlier than- 



