Feather damaging behavior is a common problem in African grey parrots (Psittacus erithacus). To determine if differences in clinical test results could be detected in parrots with feather damaging behavior (FDB), we studied 51 parrots, including 24 with FDB, with a variety of diagnostic tests. A predictable stress hemogram was found after administration of thyroid-stimulating hormone in both normal birds and birds with FDB. Birds with FDB had significantly lower lymphocyte counts and higher heterophil:lymphocyte ratios after thyroid-stimulating hormone injection than did normal birds. Although resting thyroxine (T₄) levels were not different between the 2 groups, the magnitude of the post–thyroid-stimulating-hormone increase in T₄ was significantly less in the FDB group. Alpha-1 and alpha-2 globulin fractions were significantly higher and gamma globulin fractions were significantly lower in birds with FDB. Birds with FDB also had higher Aspergillus antibody titers. No significant differences were found in Aspergillus antigen levels, Candida antibody titers, or blood concentrations of histamine, serotonin, or corticosterone. The clinical significance of these differences is not known.