In vivo magnetic resonance imaging of the reproductive organs in a passerine bird species

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 birds. Thus, repeated measurements of the same individual can be obtained at different life‐history stages or before and after experimental treatment with 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 achieved through MRI. Volumetric analyses enabled us to determine the total testicular volume with high accuracy.