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Using a commercially available testos- terone enzyme immunoassay (EIA), we developed and validated an assay procedure for determining testosterone levels in small-volume (20 mL) avian plasma samples. We evaluated this EIA's utility by measuring plasma testosterone levels in Mourning Doves (Zenaida macroura), White-eyed Vireos (Vireo griseus), Red-eyed Vireos (Vireo olivaceus), and Indigo Buntings (Passerina cyanea). Standard bio- chemical validations (e.g., parallelism, recovery of exogenous testosterone) demonstrated that the assay accurately and precisely measured testosterone in avian plasma. We compared plasma testosterone levels in males and females of all four species and Indigo Buntings in various reproductive stages to physiologically validate the assay's ability to de- termine biologically important changes in testoster- one levels. Plasma testosterone levels were higher in males compared to females in three of four species. Prebreeding and breeding male Indigo Buntings had higher circulating testosterone levels than postbreed- ing males. Testosterone levels in our study were similar to reported values for other passerine species using radioimmunoassay procedures. Our results suggest that this EIA procedure is very effective for determining testosterone levels in small-volume avian plasma samples and is sensitive enough to detect biologically important changes in the gonadal activ- ity of birds. Thus, this assay has considerable utility for measuring testosterone in small birds (,15 g), from which only small volumes of plasma (20 mL) can be collected.

USING A COMMERCIALLY AVAILABLE ENZYME IMMUNOASSAY TO QUANTIFY TESTOSTERONE IN AVIAN PLASMA