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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 123, No. 3, September 2011 
White-crowned Sparrows (Zonotrichia leu- 
cophrys) placed with sexually receptive females 
had higher plasma testosterone and exhibited 
greater copulatory behavior than those paired 
with non-receptive females (Moore 1983). The 
wide range of testosterone concentrations we 
found in male Tree Swallows (Fig. I) may reflect 
variation in access to receptive females. 
Our breeding season testosterone profile for 
male Tree Swallows supports the role of testos¬ 
terone in behaviors associated w ith nest and mate 
defense, and obtaining additional mating oppor¬ 
tunities through extra-pair copulations. The de¬ 
crease in testosterone in males after eggs hatch, 
along with the continuously low testosterone 
profile in females, lends support to the predicted 
costs associated with possessing elevated testos¬ 
terone, particularly those associated with simul¬ 
taneously displaying parental behavior. 
ACKNOWLEDGMENTS 
This research was conducted under 1ACUC protocol 01- 
08-6498-Q and funded by a University Honors Program 
Grant to Molly Staley through the Iowa State University 
Foundation and a National Science Foundation Grant (IOS- 
0745156) to Carol Vleck and David Vleck. Wc thank Maria 
Palacios. Mike Shultz. Lindsey Wyckoff. and Stephanie 
Bittner for invaluable help in the field. 
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