Plasma concentrations of corticosterone and thyroid hormones in laying fowls from different housing systems

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Abstract
1. Plasma concentrations of corticosterone, thyroxine (T4) and triiodothyronine (T3) were measured in representative mature female domestic fowls from three housing systems: cages, covered strawyard and range. 2. There were no between‐system differences for T4 but T3 was lower in birds from range than in those from cages. Corticosterone concentrations were lower in birds from strawyards than in those from range and cages. 3. In the strawyards there were no differences in corticosterone concentrations between normal and low‐ranking hens. The latter were selected on the basis that they remained apart from the flock, were under‐weight, poorly feathered and unable to feed at will. 4. Low‐ranking hens had significantly higher plasma concentrations of T3 and T4 than normal birds. This was attributed mainly to the poor feather covering of most low‐ranking birds. 5. It was concluded that, in the assessment of different housing systems, plasma corticosterone and thyroid hormones were not useful measures of long‐term stress or welfare. Different factors relevant to welfare may have contrary effects on the plasma concentrations of these hormones.