Factors Affecting the Population 



The Swainson's Warbler is the least abundant of southern war- 

 blers, except for Bachman's Warbler. There are several reasons 

 why the Swainson's Warbler is not more successful. From my 

 observations it would appear that it has a lower nesting success 

 than most other species of warblers. In a total of 16 nests of 

 which I am reasonably sure that my presence had nothing to do 

 with desertion, only three were successful. Some of these were 

 second attempts; others were initial attempts, in which case the 

 birds may have been successful on the second try. At three of the 

 nests, cowbirds removed all of the Swainson's Warbler eggs. A 

 mouse expropriated another nest during the laying period, and 

 two nests were abandoned with clutches intact. 



Some of the reasons for its low nesting success may be the 

 vulnerability of the large, bulky nest that is poorly concealed, is 

 located close to the ground, and contains white eggs. Other 

 species of warblers nesting in the same breeding range have 

 better-concealed nests, most of which are much smaller, and all 

 of which contain speckled eggs except the very rare Bachman's 

 Warbler, which also has white eggs. Furthermore, most Swain- 

 son's Warbler nests are lined with dark material, so that the 

 white eggs stand out against the dark background. 



In the Dismal Swamp, I found that whenever a Common Grackle 

 or a Blue Jay had a nest in or near a Swainson's Warbler nesting 

 territory, the warbler's nest was almost always robbed. However, 

 since the Grackle and Jay begin nesting before the Swainson's 

 Warbler, and their nesting seasons overlap the first nesting at- 

 tempt of these warblers, a second attempt can be made after the 

 two nest plunderers have completed nesting and left the area. 



Since the Swainson's Warbler places its not-too-well-hidden 

 nest close to the ground, it is well within the "cruising" range of 

 various snakes and mammals. C. E. Collier, Jr., (1941, p. 28) 

 discovered a milk snake (Lampropeltis triangulum) in the act of 

 robbing a Swainson's Warbler nest, near Clarksville, Tenn. The 

 snake had one of the warbler's eggs in its mouth at the time. 



Cowbird parasitism is becoming a more important limiting 

 factor. Friedmann (1929, p. 150) and Mayfield (1965, p. 13-18) 

 believe that the cowbird originated in the prairies and plains of 

 the West, and only in the last 100 years or so invaded the eastern 

 forest. As late as 1950 most of the southeastern Coastal Plain 



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