For researchers working on the reproductive anatomy of birds, the standard
tool has been the scalpel. We demonstrate. that magnetic resonance imaging
(MRI) allows us to study non-invasively the testes and ovaries of small bir
ds. Thus, repeated measurements of the same individual can be obtained at d
ifferent life-history stages or before and after experimental treatment wit
h a minimum of harm done to the birds. We present cross-sectional images of
male and female Garden Warblers Sylvia borin at different stages of their
gonadal maturation cycle to illustrate the high resolution that can be achi
eved through MRI. Volumetric analyses enabled us to determine the total tes
ticular volume with high accuracy.