SHORT COMMUNICATIONS 
611 
not differ significantly among breeding stages (P 
> 0.05). 
DISCUSSION 
Concentrations of circulating testosterone of 
male Tree Swallows were much higher than those 
of females, as expected, and particularly during 
the nest establishment and incubation stages. 
Testosterone did not decrease significantly in 
males until they began provisioning nestlings, 
when it was similar to that of females. Contrary to 
our predictions, we did not find evidence that 
females have elevated testosterone during nest 
esiablishment despite their active participation in 
nest defense (Winkler 1992). Rather, testosterone 
concentrations in females were generally low 
throughout the breeding season. 
Bishop et al. (1998) measured testosterone in 
Tree Swallows in Ontario. Canada, during incu¬ 
bation (females only) or chick rearing (males 
only). They used a different assay and reported 
average values that were significantly higher than 
ours (~2,5 ng/ml in males and 0.9 ng/ml in 
females). Whether these differences are due to 
methodology (Bishop et al. 1998 measured 
testosterone after ether extraction whereas we 
measured it directly in plasma) or whether 
population differences exist is not known, Differ¬ 
ences in seasonal testosterone patterns have been 
reported between populations of other species. 
Male Orange-crowned Warblers {Oreothlypis 
celata) in Alaska have a pre-incubation peak in 
testosterone and subsequent decline during incu¬ 
bation and nestling stages. In contrast, males on 
Catalina Island, California have elevated testos¬ 
terone throughout the breeding season (Horton et 
al- 2010). Variation in testosterone profiles also 
°°curs among Cliff Swallow (Petrochelidan 
Pyrrhonota) populations, depending on breeding 
density (Smith et al. 2005). 
We found testosterone was always low in 
female Tree Swallows. Elevated testosterone 
during nest stages that require parental behavior 
is frequently associated with negative conse¬ 
quences for reproductive success or other costs. 
p or example, artificially elevating testosterone in 
female Dark-eyed Juncos (Junt o hyematis) re¬ 
sulted in delayed onset of egg laying after nest 
completion as well as lower body mass and 
delayed molt (Clotfeher et al. 2004. Zysling et al. 
2006). Additionally. Zysling et al. (2006) found a 
negative correlation between cell-mediated im¬ 
mune function and total testosterone in female 
Dark-eyed Juncos. Veiga and Polo (2008) report¬ 
ed similar results in female Spotless Starlings 
(Stunuts unicolor), in which testosterone-treated 
females delayed egg laying, laid fewer eggs, and 
provisioned nestlings at a reduced rate in 
comparison to controls. Similarly, testosterone 
supplementation in males is known to disrupt or 
decrease parental behavior in many species 
(reviewed in VIeck and Vleck 2010). For 
example, male House Sparrows (Passer domes- 
licus) supplemented with testosterone during 
chick rearing have reduced feeding rates and 
lower breeding success (Hegner and Wingfield 
1987). 
The low circulating testosterone concentrations 
we measured in both male and female Tree 
Swallows during the nestling stage likely reflects 
the importance of parental care by both members of 
the pair for optimizing reproductive success (Leffe- 
laar and Robertson 1986). However, low testoster¬ 
one is not an absolute requirement for the exhibition 
of parental care in some species, indicating some 
behavioral insensitivity to testosterone is possible 
(Van Duyse et al. 2000. Lynn et al. 2002). Hau 
(2007) suggested the negative effects often seen 
with elevated testosterone may be avoided if 
selection acts on the responsiveness of target tissues 
to testosterone. Additionally, McGlothlin et al. 
(2007) suggested the ability to produce short-term 
increases in testosterone may allow for temporary 
shifts in territorial and sexual behaviors without 
compromising overall parental care. 
Decline in testosterone concentrations after egg 
laying may not be as critical for male Tree 
Swallows, which guard the nest but do not 
incubate, as for species in which the male 
incubates. There is a negative correlation between 
incubation behavior and testosterone concentra¬ 
tions in male European Starlings (Sturnus vul¬ 
garis) (Pinxten et al. 2007). Elevated circulating 
testosterone during the incubation stage may be 
beneficial to male Tree Swallows to facilitate 
extra-pair copulations (Raout ct al. 1997). The 
dale of egg laying can vary by as much as 7 weeks 
in our colonies because females that have lost 
their nest often lay a second clutch. Consequently 
other receptive females may be present after a 
male’s mate has begun incubation and may 
influence his testosterone level. A positive 
correlation between circulating testosterone in 
males and the presence of receptive females has 
been documented in other passerines such as 
European Starlings (Pinxten et al. 2003). Male 
