Thompson Bishop el al. • SWAINSON’S WARBLER NESTING ECOLOGY 
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1 2 
3 4 5 
■ 2001 
□ 2000 
□ 1999 
Week 
FIG. 1. Swainson's Warbler nesl initiations based on initiation week for 1999-2001 nests at Woodbury Wildlife 
Management Area. South Carolina. Week I began on 24 April and week 11 ended on 9 July. 
RESULTS 
Breeding Phenology and Cl ii roll Size .—We 
located 144 nests of which 78 were active: 18 in 
1999. 22 in 2000, 23 in 2001.5 in 2005, and 10 in 
2006. Most active nests were found during 
incubation with 28% discovered before egg 
laying, 12% during egg laying. 40% during 
incubation, and 9% during the nesl ling phase 
(11% had an unknown discovery lime). The 
excellent condition of 66 inactive nests indicated 
Ihey were built in the discovery year. 
Males arrived during the first 2 weeks of April 
followed by the females several days later. First 
nests were built in late April to early May, and the 
earliest egg (nest initiation) was laid on 28 April 
1999 and the latest on 12 July 2006. There were 
two average peaks in nesting during 1999-2001, 
one occurring the first week of May and the other 
the second week of June (Fig. I). There was 
substantial yearly variation in nest initiations. 
Nest initiations in 2001 and 1999 were bimodal. 
Nests remained empty after completion for 1 to 
6 days (x ± SE - 2.64 ± 0.99, n = 14 clutches). 
Females laid one egg per day and began incubation 
with the laying of the last egg ( A’incubatioii Icngdi 
13.85 ± 0.54 days; n - 14 clutches). One female 
continued to incubate three eggs during the 1999 
Hood with flood waters I m below her nest. This 
nest failed several days later, although she 
remained incubating above floodwaters the entire 
time. A clutch of seven eggs, the largest detected, 
hatched six eggs but most nestlings disappeared 
before the potential fledging period, leaving only 
two fledglings. Clutches averaged 3.19 ± 0.20 
eggs (// = 69 clutches. 220 eggs total). Hatching 
was synchronous unless Brown-headed Cowbird 
eggs were present, in which case the cowbird egg 
hatched at least I day in advance of the Swainson’s 
Warbler eggs ( v - 1,46 ± 0.23 days; range - 1- 
3). Eighty-one percent of the eggs that survived the 
incubation period hatched (J29/158). One hundred 
and twenty-nine of the 220 Swainson’s Warbler 
eggs laid produced nestlings of which 80 fledged. 
The nestling period lasted 9.94 ± 0.20 days (n = 
17 clutches) with 2,50 ± 0.33 fledglings per nest 
(n — 32 clutches, 80 fledglings). Seven nests were 
observed front start of incubation through fledging 
and were used to calculate a 23.3 ± 1.19 day 
nesting cycle. 
Fewer than 10% (7/78) of Swainson’s Warbler 
nests were singly parasitized by Brown-headed 
Cowbirds. Only 10 of 18 Swainson's Warbler 
eggs laid hatched in parasitized nests, a 26% 
reduction in hatching rate from 81 to 55% (/ = 
2.21. df = 6. P = 0.06). Parasitized nest outcomes 
were: three depredated during incubation; two 
Hedged cowbirds. but all host chicks disappeared; 
and two nests reached the potential Hedging 
interval, but no host fledglings were observed. 
Assuming these two nestlings Hedged, the average 
number of Swainson’s Warbler fledglings per 
parasitized nest was 0.28 ± 0.14, a significant 
reduction in fledgling production (/ = 8.87, df = 
18, P < 0.001). 
