62 



NORTH AMERICAN FAUNA 69 



Cowbird parasitism 



In some parts of its breeding range the Swainson's Warbler 

 may be rather heavily parasitized by the Brown-headed Cowbird. 

 During the first week in May 1967 at Pendleton Ferry, Ark., I 

 located three Swainson^s Warbler nests, all of which were para- 

 sitized. At one of the nests the warbler was incubating three 

 cowbird eggs and one of its own. Three days later it was still 

 incubating, but one of the cowbird eggs and its own egg were 

 missing. At another one of the nests a warbler was incubating a 

 single cowbird egg, and at the third a warbler was incubating 

 three cowbird eggs ; evidently the warbler eggs had been removed 

 by the cowbirds. 



Kirn (1918, p. 97-98) reported several parasitized nests in 

 Copan County, Okla. ; Sims and DeGarmo (1948, p. 5), in the 

 course of 3 years, found three of 18 nests parasitized at Charles- 

 ton, W. Va. I found that none of 11 Dismal Swamp nests were 

 parasitized. Dismal Swamp is near the northern limit of the 

 southeastern breeding range hiatus of the cowbird (Webb and 

 Wetherbee 1960, p. 83-87), and I found only two or three cow- 

 birds during an entire day in the Swamp in the spring of 1968. 

 From 1944 to 1946 at Macon, none of six nests were parasitized, 

 since at that time the area was out of the cowbird breeding range. 

 By 1960, however, cowbirds were commonly breeding there. 



INCUBATION 



Information on behavior during the incubation period was ob- 

 tained from a nesting pair in the first week of May at Pendleton 

 Ferry, Ark., and from a pair in the Dismal Swamp in the middle 

 of June. The Pendleton Ferry pair was the one mentioned above 

 whose nest contained three cowbird eggs and one warbler egg. The 

 nest was located about 2 feet above the ground between two cane 

 poles. The height of the Dismal Swamp nest was also about 2 

 feet, and it was placed in a greenbrier vine. At each nest, incuba- 

 tion was performed only by the female. 



During incubation the Pendleton Ferry female spent about 78 

 percent of her daylight time on the nest. The average period on 

 the nest was 70 minutes; the average period off was 19 minutes. 

 The longest period on the nest was 110 minutes, and the shortest 

 was 30 minutes. The longest period away from the nest was 25 

 minutes; the shortest was 15 minutes. The Dismal Swamp female 

 averaged 54 minutes on and 15 minutes off the nest. Lawrence 

 (1953, p. 138), summarizing studies of six wood warblers, found 

 that the birds were at the nest 67 to 83 percent of the time. 



