Birds can be tested for toxoplasma infection by taking a blood sample. I
assume if your doing a behavioral study that you will be mist-netting and
banding the birds anyway. Blood can be safely taken from the brachial or
jugular vein with tubercullin needles.
Antibody titres are an indirect measure of the intensity of infection. They
represent the dilution of the blood sera. A bird that is antibody positive
at 1:500 vs. a bird that is antibody positive at 1:25 will have sustained a
higher infection. If you are serious about testing for toxoplasmosis you
should contact Dr J. P. Dubey at the U.S. Dept. of Agriculture lab in
Beltsville, MD. He is the king of toxoplasmosis (over 400 publications).