Alarm signals in birds: the role of testosterone.

In the laboratory and under semi-naturalistic conditions, male domestic fowl (Gallus gallus) produce more alarm calls in response to predatory birds flying overhead than females. This study tests the hypothesis that testosterone is a factor in the control of aerial alarm call production. Birds were castrated early in life and tested as adults in the laboratory. Aerial predators were simulated by cardboard silhouettes drawn overhead. Blood samples were taken at intervals and plasma testosterone levels were measured by radioimmunoassay. Each subject was tested in four conditions: with an empty Silastic implant, with a testosterone-filled implant, with an additional testosterone implant, and after removal of the implants. Results show that the presence of testosterone increases the production of aerial alarm calls, and that removal of the hormone supply results in a drop in calling rate. As a control a second vocal system, food calling, was considered. In contrast with alarm calling, there was no correlation between testosterone levels and variation in food-call production. We conclude that testosterone plays a specific role in activation of the production by cockerels of vocalizations that signal the presence of aerial predators.