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The Wilson Journal of Ornithology 123(3):608-613, 2011 
Plasma Testosterone Concentrations in Adult Tree Swallows During the 
Breeding Season 
Molly Staley,'- 2 -’ Carol M. Vleck , 1 and David Vleck' 
ABSTRACT.—We studied seasonal profiles of 
circulating testosterone concentrations among male 
and female adult Tree Swallows (Tachycineta bicolor) 
breeding in nest-box colonies near Ames. Iowa, USA. 
Mean plasma testosterone in males was elevated during 
nest establishment (0.63 ± 0.86 ng/ml) and incubation 
stages (0.28 ± 0,26 ng/ml). and was significantly lower 
after hatching (0.03 ± 0.05 ng/ml) when males began 
provisioning nestlings. Male swallows do not incubate 
and high testosterone during the incubation stage may 
facilitate pursuit of extra-pair matings. Female testos¬ 
terone concentrations were an order of magnitude lower 
than those ol males (nest establishment, mean = 0.06 ± 
0.09 ng/ml) and did not change significantly over the 
breeding season. These testosterone profiles support the 
hypothesis that elevated testosterone in males is 
associated with defense behaviors and obtaining 
'Department of Ecology, Evolution, and Organismal 
lology, 253 Bessey Hall. Iowa State University, Ames, IA 
50010, USA. 
a 2 K CUrTe f, addrcss: 331 Fun chess Hall, Auburn University 
Auburn. AL 36849, USA. y ’ 
’Corresponding author; e-mail: mms0020@aubum.edu 
additional mating opportunities during the first pari of 
the breeding season, but is incompatible with parental 
care once the eggs have hatched. Received 8 September 
2010. Accepted 19 March 2011. 
Androgens such as testosterone promote sec¬ 
ondary sex characteristics, sperm production, and 
aggressive behavior in male birds. Elevated 
circulating testosterone increases song production, 
male guarding, and pursuit of extra-pair matins 
opportunities by males while also decreasing 
parental behavior (reviewed in Lynn 2008) 
Testosterone concentrations in temperate zone- 
breeding passerines generally are highest 01 c 
start ol the breeding season when males compete 
for territories and mates, but this pattern varies 
with mating system and whether or not the male 
participates in parental care (Wingfield et a! 
1990). For example, hormonal correlates of 
parental and sexual behavior have been studied 
in two closely related species: Blue-headed Vireo 
