Thompson Bishop el a!. • SWAINSON'S WARBLER NESTING ECOLOGY 
733 
TABLE 4. Swainson's Warblers nest-site variables and 
measurements at Woodbury Wildlife Management Area. 
South Carolina. 1999-2001 and 2005-2006. Values are ,v ± 
SE: n in parentheses. 
Vegetaiion variables 
Measure menl 
Range 
Inner cup diameter, cm 
5.95 ± 0.10 (92) 
3.5-11 
Outer nest diameter, cm 
14.02 ± 0.36 (90) 
8.7 25.5 
Inner cup depth, cm 
4.29 ± 0.07 (91) 
2-6.9 
Entire nest depth, cm 
8.75 ± 0.27 (89) 
5 I't 
Distance from stem, m 
0.38 ±0.12 (63) 
0-2.55 
Nest height, m 
1.42 ± 0.06 (110) 
0.36-3.8 
Host height, m 
2.77 ±0.18 (83) 
0.95-10 
Host dbh. cm 
2.79 ± 0.72 (55) 
0.3-35 
overall daily nest survival rate was 0.968 (9597 Cl 
= 0.940. 0.983; n = 78). 
Nest-site Measurements. —Nest sites were mea¬ 
sured at 84 and 18 nests in 1999-2001 and 2005- 
2006. respectively. Nests were in the understory at 
a mean height of 1.4 ± 0.06 in (Table 4). and the 
plant host species tended to he small (a\jh, = 2.79 
~ °- 72 cm) and short Cv hosl heipht = 2.77 ± 
0.18 m) (Table 4). Nests were highly variable in 
M/.e as evidenced hy an outer cup diameter 
ranging from 8.7 to 25.5 cm. Nineteen species 
representing 18 genera were used as nesting 
substrate (typically more than one substrate used 
per nest; n = 163 substrates) with cane used most 
frequently (26%), followed hy greenhrier (Smi/a.x 
spp.) (13%). holly (12%), and grapevine ( Vitis 
spp.) (12%); 47% of nests were in vines. 26% in 
cane, 18% in shrubs, and 10% in trees. Seventy- 
one percent (72/102) of nests contained cane at 
the nest-site scale. Significantly more cane was 
found at the nest-site scale (x = 71.9 ± 12.2 
stems) than if another species was used for nest 
support (x = 18.4 ± 2.3 stems; t = 5.64. df = 30. 
P < 0.001). 
DISCUSSION 
Our earliest indication of multiple brooding in 
Swainson's Warblers was from our nesting 
phenology data (Holmes et al. 1992). On average, 
the first peak in nest initiations occurred the first 
week of May and the second 45 days later in June. 
The bimodal distribution implies a synchronicity 
of brooding as nest failures would be expected to 
■smooth out the peaks. This synchronicity could be 
caused by food availability and social behavior 
that would encourage pairs to nest at the same 
time. Our subsequent observations from color 
banding and telemetry work confirmed double 
brooding, up to three rcncsting attempts by 
females with earlier failed attempts, and several 
bigamous matings (JAG. unpubl. data). Twenty- 
one percent of nests were double-brooded (12/56) 
in 2000 and 2001 (Thompson 2005). Miller 
(2003) also observed an occurrence of double 
brooding by a pair of Swainson’s Warblers in 
South Carolina. Multiple brooding may be an 
important strategy to maximize Swainson's War¬ 
bler fecundity rangewidc. 
Many of our findings on nesting ecology are 
consistent with those reported by Meanley (1969. 
1971, 1982) and other researchers. Our estimates of 
incubation and nestling period lengths were within 
Meanley's (1969) estimated ranges of 13-15 and 
10 days, respectively. An average nesting cycle of 
23.3 days was required from start of incubation 
through Hedging. Clutch sizes (3.11 eggs) averaged 
slightly less than estimates from studies in 
Louisiana (3.30; Henry 2004). Arkansas (3.43; 
Benson et al. 2010b), and Missouri (3.65; Thomas 
et al. 1996). One of our nests had an exceptionally 
large clutch of seven, a potential sign of intraspe¬ 
cific brood parasitism; however, egg dumping is a 
rare behavior used by <2% of avian species and 
even fewer altriciul species (Payne 1977, Mac- 
Whirler 1989), Approxi mately one third of the eggs 
laid produced a Hedgling. and the number of 
fledglings produced per nest was slightly lower 
(2.50) than most observed in comparable studies 
(2.12; Thomas et al. 1996) (2.75; Benson et al. 
2010b) (3.0; Henry 2004) (3.25; Lanham et al. 
2006). Over 90% of nest losses were due to 
depredation. Many potential nest predators used 
the study site, but we observed only two instances 
of predation by black rat snakes, which are 
opportunists (Chalfoun et al. 2002). 
Most nesting studies report apparent nest success 
of Swainson’s Warbler, but relatively few examine 
Mayfield nesting success (Thomas et al. 1996. 
Henry 2004. Peters et al. 2005), and only one 
examined nest survival using logistic exposure 
(Benson el al. 2010b). Nesting success estimates 
were corrected for a 23-day nesting cycle, and ours 
was the second highest value (50.0%) when 
compared to populations in Missouri (62.8%; 
Thomas et al. 1996). South Carolina (40.0%; 
Peters ct al. 2005). Louisiana (30.0-40.0%; Henry 
2004). and Arkansas (32.2%; Benson 2008). The 
weighted mean for all five studies was 39.1%, well 
below the nesting success calculated in our study. 
The Peters et al. study (2005) is from the same 
population and it is unclear whether the difference 
