You probably have to extract (with solvent) the steroids to do that; there are many small substances in serum, and other steroids, that could be interfering.  Do a lit search in the 1970's for testosterone measurement - you should find several articles that describe the purification steps often needed.  Also, be sure to look at the cross-reactivity profile of the antibody you have.  If it cross-reacts with (for example) aldosterone, and aldosterone is circulating in high concentrations in the crow, you may need column chromatography also.

Non-invasive (wet or dry fecal) hormone metabolite analysis. Research on methods of measuring the concentrations of fecal monoclonal antibody (in this case, anti-testosterone).

Also, attached a masters dissertation: Non-invasive measurements of gonadal hormones metabolites concentrations in ranfastides droppings (Tucano), that might be close to what you need. Just need to translate the extraction part.

And more article that might provide some more ideas: General and Comparative Endocrinology Volume: 168 Issue: 3 Page: 333-339 DOI: 10.1016/j.ygcen.2010.04.019 

Does anybody think that cortisol lower Testosterone during capturing or confinement? I need a plausible answer.