SHORT COMMUNICATIONS 
609 
(Vireo solitarius) and Red-eyed Vireo (V. oliva¬ 
ceous). Both are socially monogamous, but differ 
in their relative contribution to care of offspring. 
Male Blue-headed Vireos participate in nest 
construction, incubation, and provisioning young 
and do not have a pre-nesting peak in testosterone. 
Male Red-eyed Vireos, however, exhibit parental 
care only after nestlings are present, and circulat¬ 
ing testosterone is elevated early in the nesting 
season, but declines by the nestling stage (Van 
Roo el al. 2003). 
We examined temporal profiles of plasma 
testosterone in male and female Tree Swallows 
(Tachycineta bicolor) during the breeding season. 
Tree Swallows are socially monogamous and 
secondary cavity nesters. There is competition for 
typically scarce natural nesting sites, which are 
defended by both males and females (Holroyd 
1975, Winkler 1992). Males contribute to nest 
building, do not incubate, but contribute about 
equally to provisioning nestlings (Quinncy 1986). 
If either the male or female parent is lost, the 
remaining parent will increase their nestling food 
provisioning rate. This increase neither fully 
compensates for loss of llie other parent nor can 
it be sustained for an extended period of time, 
resulting in reduced reproductive success (Leffe- 
laar and Robertson 1986). Tree Swallows also 
have unusually high rates of extra-pair fertiliza¬ 
tions for a socially monogamous species. Whereas 
extra-pair paternity rates for socially monoga¬ 
mous species in general average 18.7% of broods 
and 11.]% of offspring (reviewed in Griffith el al. 
2002); 50-87% of broods in nest box populations 
of Tree Swallows contain extra-pair young and 
extra-pair males father 38-53% of the offspring in 
those broods (Barber et al. 1996). Testosterone is 
thought to influence the tradeoff between behav¬ 
iors that facilitate obtaining additional mating 
opportunities versus investing in offspring care 
(Raouf et al. 1997). We predicted testosterone 
concentrations would be highest in males during 
•test establishment and would remain elevated 
during incubation when males can obtain extra- 
pair matings. Their testosterone concentrations 
should decrease when eggs hatch and males begin 
provisioning nestlings. 
Both male and female Tree Swallows defend 
•he nest, although males have a greater maximal 
alarm call-rale and longer call duration compared 
l o females. Several other defense measures 
including number of dives towards predators and 
time defending the nest do not differ between 
males and females (Winkler 1992). Elevated 
female aggression has been associated with peaks 
in female testosterone concentrations in other 
species (Cristol and Johnsen 1994, Woodley and 
Moore 1999). However, females displaying ag¬ 
gressive behavior do not always have a corre¬ 
sponding increase in testosterone (Elekonich and 
Wingfield 2000, Jawor et al. 2006). We predicted 
testosterone concentrations in temale Tree Swal¬ 
lows would be lower than those of males, but 
would be elevated during the territory and nest 
establishment phase and decline once incubation 
began. 
METHODS 
Field and Laboratory Procedures .—We col¬ 
lected blood samples to assay testosterone con¬ 
centrations from Tree Swallows using nest boxes 
near Ames, Iowa, USA, (42" 01' N, 93° 37' W) in 
May-June 2009. Boxes were spaced —20 m apart, 
and there was considerable interaction (nest box 
and mate guarding) among breeding birds. 
We captured 25 male and 30 female adult Tree 
Swallows during three time periods: pair bond and 
nest-box establishment (hereafter nest establish¬ 
ment), incubation, and the nestling stage. We 
caught individuals during nest establishment 
either with mist nets or upon entrance into a nest 
box during nest building. Not all females had 
brood patches at this stage, and we classified 
individuals based on the presence of a cloacal 
protuberance in males or the brown plumage of 
second-year females. We confirmed our classifi¬ 
cation for seven of 10 females and three of seven 
males through later recapture at a nest, data from 
previous years, or wing length in the predomi¬ 
nately female (<II3 mm) or male (>122 mm) 
ranges (Stutchbury and Robinson 1987). We 
caught all birds during the incubation and nestling 
stages using nest-box traps. Males and females 
were easily distinguished during these two stages 
based on presence of a brood patch in females. 
Each bird was sampled only once. 
We took a blood sample of —100 pi within 
5 min of capture from a wing vein, collecting 
blood into heparinized capillary tubes that were 
immediately cooled on ice. We separated plasma 
from cells by centrifugation within 4 hrs and 
stored the plasma at -80 C. We measured 
plasma testosterone in duplicate using a radioim¬ 
munoassay kit (DSL-4100) from Diagnostics 
Systems Laboratories (Brea. CA. USA) in Sep¬ 
tember 2009. This R1A kit measures testosterone 
