Tonic immobility and heterophil/lymphocyte responses of the domestic fowl to corticosterone infusion. 
The tonic immobility (TI) fear reactions, plasma corticosterone concentrations and heterophil/lymphocyte (H/L) ratios of adults laying hens were measured before and at intervals of 4 and 11 days after the subcutaneous implantation of osmotic minipumps delivering either corticosterone solution (15 micrograms/hr) or only polyethylene glycol vehicle.  The dummy pumps exerted no apparent behavioral or endocrine effects, whereas tonic immobility was significantly prolonged and circulating corticosterone concentrations significantly elevated at 4 and 11 days after implantation of the corticosterone minipumps.  H/L ratios were significantly elevated from pre-treatment levels in both groups.  However, H/L ratios were considerably higher at both post-treatment points among birds receiving corticosterone rather than vehicle.  The present findings suggest that chronic elevations of plasma corticosterone not only alter the haematological profile but may also predispose birds to react more fearfully to alarming stimulation.